iio, syfs, devres: devm_kmalloc not aligned to pow2 size argument

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

[1]: https://lore.kernel.org/all/[email protected]/
[2]: https://lore.kernel.org/all/[email protected]/

Quoted 51 lines · On Mon, 28 Oct 2024 13:04:10 +0100, [email protected] wrote:

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 2 lines

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

Adding some more context to better understand the impact of this.

With a trivial grep it looks like there are only few instances where devm_k*alloc() is used to allocate a PAGE_SIZE buffer:

$ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE'
block/badblocks.c:1584:         bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/iio/multiplexer/iio-mux.c:287:          page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/mtd/nand/raw/mxc_nand.c:1702:   host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL);
drivers/usb/gadget/udc/gr_udc.c:1987:           buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC);
sound/soc/sof/debug.c:277:              dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL);

What takes my attention is the bb->page in blocks/badblocks.c, being the buffer named "page" maybe it is supposed to be page aligned?

Also in [3] it was suggested to add the page alignment check for sysfs_emit() and sysfs_emit_at(), but I haven't found why that's necessary. My guess is for optimizations to avoid the buffer to spread in more than one page. Is this correct? Are there other reasons? Can anyone add more details? I think it would help to understand whether page alignment is necessary in the other instances of devm_k*alloc().

Beside page alignment, there are plenty of devm_k*alloc() around the code base, is there any way to spot whether any of those instances expect the allocated buffer to be aligned to the provided size?

If this is a limited use-case it can be worked around with just regular k*alloc() + devm_add_action_or_reset() as Jonathan suggested. However, I still think it can be easy to introduce some alignment related bug, especially when transitioning from k*alloc() to devm_k*alloc() in an old implementation since it can be assumed that they have the same alignment guarantees. Maybe some comment in the devres APIs or documentation would help in this case?

Quoted 70 lines · Matteo Martelli

Quoted 51 lines · On Mon, 28 Oct 2024 13:04:10 +0100, [email protected] wrote:

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 2 lines

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

Adding some more context to better understand the impact of this.

With a trivial grep it looks like there are only few instances where devm_k*alloc() is used to allocate a PAGE_SIZE buffer:

$ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE'
block/badblocks.c:1584:         bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/iio/multiplexer/iio-mux.c:287:          page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/mtd/nand/raw/mxc_nand.c:1702:   host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL);
drivers/usb/gadget/udc/gr_udc.c:1987:           buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC);
sound/soc/sof/debug.c:277:              dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL);

What takes my attention is the bb->page in blocks/badblocks.c, being the buffer named "page" maybe it is supposed to be page aligned?

Quoted 6 lines

Also in [3] it was suggested to add the page alignment check for sysfs_emit() and sysfs_emit_at(), but I haven't found why that's necessary. My guess is for optimizations to avoid the buffer to spread in more than one page. Is this correct? Are there other reasons? Can anyone add more details? I think it would help to understand whether page alignment is necessary in the other instances of devm_k*alloc().

sysfs_emit* functions should only be operating on the buffer that was passed to the show function callback, which is allocated by the sysfs core, so should not have any of these issues. So why would it need to be checked?

Quoted 4 lines

Beside page alignment, there are plenty of devm_k*alloc() around the code base, is there any way to spot whether any of those instances expect the allocated buffer to be aligned to the provided size?

That's a good question, and a worry about the devm_* calls. I know many busses (i.e. USB) require that the data passed to them are allocated from kmalloc buffers, but I don't know about the alignment issues required, as that is usually very hardware-specific.

Quoted 8 lines

If this is a limited use-case it can be worked around with just regular k*alloc() + devm_add_action_or_reset() as Jonathan suggested. However, I still think it can be easy to introduce some alignment related bug, especially when transitioning from k*alloc() to devm_k*alloc() in an old implementation since it can be assumed that they have the same alignment guarantees. Maybe some comment in the devres APIs or documentation would help in this case?

I think the general statement of "don't migrate drivers to devm_* calls unless you have the hardware and can test the changes" is good to follow here. That should resolve the problem going forward as new drivers are expected to be at least tested by the submitter :)

thanks,

greg k-h

Quoted 78 lines · On Sat, 9 Nov 2024 10:29:55 +0100 Greg Kroah-Hartman…

Quoted 70 lines · Matteo Martelli

Quoted 51 lines · On Mon, 28 Oct 2024 13:04:10 +0100, [email protected] wrote:

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 2 lines

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

Adding some more context to better understand the impact of this.

With a trivial grep it looks like there are only few instances where devm_k*alloc() is used to allocate a PAGE_SIZE buffer:

$ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE'
block/badblocks.c:1584:         bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/iio/multiplexer/iio-mux.c:287:          page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/mtd/nand/raw/mxc_nand.c:1702:   host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL);
drivers/usb/gadget/udc/gr_udc.c:1987:           buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC);
sound/soc/sof/debug.c:277:              dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL);

What takes my attention is the bb->page in blocks/badblocks.c, being the buffer named "page" maybe it is supposed to be page aligned?

Quoted 6 lines

Also in [3] it was suggested to add the page alignment check for sysfs_emit() and sysfs_emit_at(), but I haven't found why that's necessary. My guess is for optimizations to avoid the buffer to spread in more than one page. Is this correct? Are there other reasons? Can anyone add more details? I think it would help to understand whether page alignment is necessary in the other instances of devm_k*alloc().

Quoted 4 lines

sysfs_emit* functions should only be operating on the buffer that was passed to the show function callback, which is allocated by the sysfs core, so should not have any of these issues. So why would it need to be checked?

For the IIO case above:
This is a weird code evolution thing. The IIO callbacks in question were defined to only write into sysfs buffers, but then got repurposed to provide access to an in kernel consumer. Note they are pretty rarely used but we do have a couple of users. The providers of those calls are much more common and at time of writing assume sysfs buffers even if someone makes another use of the device later. So the issue occurs if an untested mix of a provider and consumer are used.

So documenting those functions as requiring aligned buffers seems a good start - probably even adding a runtime check on alignment so that if a consumer is tested with a different provider that doesn't use sysfs_emit() we still catch the problem.

Quoted 5 lines

Quoted 4 lines

Beside page alignment, there are plenty of devm_k*alloc() around the code base, is there any way to spot whether any of those instances expect the allocated buffer to be aligned to the provided size?

Quoted 4 lines

That's a good question, and a worry about the devm_* calls. I know many busses (i.e. USB) require that the data passed to them are allocated from kmalloc buffers, but I don't know about the alignment issues required, as that is usually very hardware-specific.

worse than DMA_MINALIGN? That is used in the devm_kzalloc to ensure the buffers still obey that restriction.

Quoted 44 lines · On Mon, 28 Oct 2024 13:04:10 +0100, [email protected] wrote:

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

Quoted 7 lines

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 2 lines

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

Adding some more context to better understand the impact of this.

With a trivial grep it looks like there are only few instances where devm_k*alloc() is used to allocate a PAGE_SIZE buffer:

$ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE'
block/badblocks.c:1584:         bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/iio/multiplexer/iio-mux.c:287:          page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/mtd/nand/raw/mxc_nand.c:1702:   host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL);
drivers/usb/gadget/udc/gr_udc.c:1987:           buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC);
sound/soc/sof/debug.c:277:              dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL);

What takes my attention is the bb->page in blocks/badblocks.c, being the buffer named "page" maybe it is supposed to be page aligned?

Also in [3] it was suggested to add the page alignment check for sysfs_emit() and sysfs_emit_at(), but I haven't found why that's necessary. My guess is for optimizations to avoid the buffer to spread in more than one page. Is this correct? Are there other reasons? Can anyone add more details? I think it would help to understand whether page alignment is necessary in the other instances of devm_k*alloc().

Beside page alignment, there are plenty of devm_k*alloc() around the code base, is there any way to spot whether any of those instances expect the allocated buffer to be aligned to the provided size?

If this is a limited use-case it can be worked around with just regular k*alloc() + devm_add_action_or_reset() as Jonathan suggested. However, I still think it can be easy to introduce some alignment related bug, especially when transitioning from k*alloc() to devm_k*alloc() in an old implementation since it can be assumed that they have the same alignment guarantees. Maybe some comment in the devres APIs or documentation would help in this case?

Any thoughts?

Quoted 3 lines

[1]: https://lore.kernel.org/all/[email protected]/
[2]: https://lore.kernel.org/all/[email protected]/

[3]: https://lore.kernel.org/all/[email protected]/

Best regards,
Matteo Martelli

Quoted 45 lines · Matteo Martelli

Quoted 44 lines · On Mon, 28 Oct 2024 13:04:10 +0100, [email protected] wrote:

Hi everyone,

I found an issue that might interest iio, sysfs and devres, about a particular usage of devm_kmalloc() for buffers that later pass through sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to a buffer located after the devres metadata and thus aligned to PAGE_SIZE+sizeof(struct devres).

Specifically, I came across this issue during some testing of the pac1921 iio driver together with the iio-mux iio consumer driver, which allocates a page sized buffer to copy the ext_info of the producer pac1921 iio producer driver. To fill the buffer, the latter calls iio_format_value(), and so sysfs_emit_at() which fails due to the buffer not being page aligned. This pattern seems common for many iio drivers which fill the ext_info attributes through sysfs_emit*() helpers, likely necessary as they are exposed on sysfs.

I could reproduce the same error behavior with a minimal dummy char device driver completely unrelated to iio. I will share the entire dummy driver code if needed but essentially this is the only interesting part:

	data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL);
	if (!data->info_buf)
		return -ENOMEM;

	if (offset_in_page(data->info_buf))
		pr_err("dummy_test: buf not page algined\n");

When running this, the error message is printed out for the reason above.

I am not sure whether this should be addressed in the users of devm_kmalloc() or in the devres implementation itself. I would say that it would be more clear if devm_kmalloc() would return the pointer to the size aligned buffer, as it would also comply to the following kmalloc requirement (introduced in [1]):

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

Quoted 7 lines

Quoted 7 lines

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

no, IMO, that is not good idea absolutely.

Quoted 37 lines

Quoted 2 lines

I am sending this to who I think might be interested based on previous related activity. Feel free to extend the cc list if needed.

Adding some more context to better understand the impact of this.

With a trivial grep it looks like there are only few instances where devm_k*alloc() is used to allocate a PAGE_SIZE buffer:

$ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE'
block/badblocks.c:1584:         bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/iio/multiplexer/iio-mux.c:287:          page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL);
drivers/mtd/nand/raw/mxc_nand.c:1702:   host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL);
drivers/usb/gadget/udc/gr_udc.c:1987:           buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC);
sound/soc/sof/debug.c:277:              dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL);

What takes my attention is the bb->page in blocks/badblocks.c, being the buffer named "page" maybe it is supposed to be page aligned?

Also in [3] it was suggested to add the page alignment check for sysfs_emit() and sysfs_emit_at(), but I haven't found why that's necessary. My guess is for optimizations to avoid the buffer to spread in more than one page. Is this correct? Are there other reasons? Can anyone add more details? I think it would help to understand whether page alignment is necessary in the other instances of devm_k*alloc().

Beside page alignment, there are plenty of devm_k*alloc() around the code base, is there any way to spot whether any of those instances expect the allocated buffer to be aligned to the provided size?

If this is a limited use-case it can be worked around with just regular k*alloc() + devm_add_action_or_reset() as Jonathan suggested. However, I still think it can be easy to introduce some alignment related bug, especially when transitioning from k*alloc() to devm_k*alloc() in an old implementation since it can be assumed that they have the same alignment guarantees. Maybe some comment in the devres APIs or documentation would help in this case?

Quoted 2 lines

Any thoughts?

why not to use existing APIs?

addr = devm_get_free_pages(dev, GFP_KERNEL|__GFP_ZERO, 0); devm_free_pages(dev,addr);

Quoted 9 lines

Quoted 3 lines

[1]: https://lore.kernel.org/all/[email protected]/
[2]: https://lore.kernel.org/all/[email protected]/

[3]: https://lore.kernel.org/all/[email protected]/

Best regards,
Matteo Martelli

Quoted 11 lines

Quoted 11 lines

Quoted 11 lines

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 1 line

no, IMO, that is not good idea absolutely.

It’s now quite clear to me that the issue is a rare corner case, and the potential impact of making such a change does not justify it. However, for completeness and future reference, are there any additional reasons why this change is a bad idea?

Quoted 11 lines · Matteo Martelli

Quoted 11 lines

Quoted 11 lines

Quoted 11 lines

The address of a chunk allocated with kmalloc is aligned to at least ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the alignment is also guaranteed to be at least to the respective size.

To do so I was thinking to try to move the devres metadata after the data buffer, so that the latter would directly correspond to pointer returned by kmalloc. I then found out that it had been already suggested previously to address a memory optimization [2]. Thus I am reporting the issue before submitting any patch as some discussions might be helpful first.

Quoted 5 lines

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no, IMO, that is not good idea absolutely.

It’s now quite clear to me that the issue is a rare corner case, and the potential impact of making such a change does not justify it. However, for completeness and future reference, are there any additional reasons why this change is a bad idea?

1)
as i ever commented, below existing APIs is very suitable for your requirements. right ?
addr = devm_get_free_pages(dev, GFP_KERNEL|__GFP_ZERO, 0); devm_free_pages(dev,addr);

2)
touching existing API which have been used frequently means high risk?

3) if you put the important metadata at the end of the memory block. 3.1) it is easy to be destroyed by out of memory access.

3.2) the API will be used to allocate memory with various sizes
how to seek the tail metadata ? is it easy to seek it?

3.3) if you allocate one page, the size to allocate is page size
+ meta size, it will waste memory align.

4) below simple alternative is better than your idea. it keep all attributes of original kmalloc(). right ?

static int devres_raw_kmalloc_match(struct device *dev, void *res, void *p)

{
	void **ptr = res;
	return *ptr == p;
}

static void devres_raw_kmalloc_release(struct device *dev, void *res)

{
	void **ptr = res;
	kfree(*ptr);
}

void *devm_raw_kmalloc(struct device *dev, size_t size, gfp_t gfp)

{
	void **ptr;
	
	ptr = devres_alloc(devres_raw_kmalloc_release, sizeof(*ptr), GFP_KERNEL);
	f (!ptr)
		return NULL;
	
	*ptr = kmalloc(size, gfp);
	if (!*ptr) {
		devres_free(ptr);
		return NULL;
	}
	devres_add(dev, ptr);
	return *ptr;
}

EXPORT(...)

void *devm_raw_kfree(struct device *dev, void *p) {
devres_release(dev, devres_raw_kmalloc_release,
devres_raw_kmalloc_match, p);
}
EXPORT(...)

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[2]: https://lore.kernel.org/all/[email protected]/