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atmel-mci causes kernel panic when CONFIG_DEBUG_VM is set

DISCUSSION Ludovic Desroches 3 participants 6 messages May 19, 2011

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Ludovic Desroches

author
#0 lore raw

Hello,

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

Into the atmci_read_data_pio function we use flush_dcache_page (do we really need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page comes from a scatter list). How could I correct this problem?

Thanks for your help

Regards,

Ludovic

#1 lore raw

2011/5/19 Ludovic Desroches <[email protected]>:

Quoted 11 lines

Hello,

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

Into the atmci_read_data_pio function we use flush_dcache_page (do we really need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page comes from a scatter list). How could I correct this problem?

linux/include/linux/mmdebug.h:

#ifdef CONFIG_DEBUG_VM
#define VM_BUG_ON(cond) BUG_ON(cond)
#else
#define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif

it is something like "assert" in kernel.

Quoted 13 lines

Thanks for your help

Regards,

Ludovic

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author
#2 lore raw
Quoted 14 lines · Barry Song

2011/5/19 Ludovic Desroches<[email protected]>:

Quoted 11 lines

Hello,

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

Into the atmci_read_data_pio function we use flush_dcache_page (do we really need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page comes from a scatter list). How could I correct this problem?

linux/include/linux/mmdebug.h:

Quoted 7 lines

#ifdef CONFIG_DEBUG_VM
#define VM_BUG_ON(cond) BUG_ON(cond)
#else
#define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif

it is something like "assert" in kernel.

Thanks for your answer but I know that. My question is more focused on why there is this check.

Is flushing a page taken from a slab forbidden ? Is it risky ? Is it no sense ?

Regards,

Ludovic

author
#3 lore raw
Quoted 23 lines · Ludovic Desroches
Quoted 22 lines · Barry Song

2011/5/19 Ludovic Desroches<[email protected]>:

Quoted 5 lines

Hello,

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

Quoted 8 lines

Into the atmci_read_data_pio function we use flush_dcache_page (do we really
need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page
comes from a scatter list). How could I correct this problem?

linux/include/linux/mmdebug.h:

#ifdef CONFIG_DEBUG_VM
#define VM_BUG_ON(cond) BUG_ON(cond)
#else
#define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif

Quoted 4 lines
Quoted 1 line

it is something like "assert" in kernel.

Thanks for your answer but I know that. My question is more focused on why there is this check.

This the reason:

commit b5fab14e5d87df4d94161ae5f5e0c8625f9ffda2

Author: Christoph Lameter <[email protected]>
Date:   Tue Jul 17 04:03:33 2007 -0700

Add VM_BUG_ON in case someone uses page_mapping on a slab page

Detect slab objects being passed to the page oriented functions of
the VM.

It is not sufficient to simply return NULL because the functions
calling
page_mapping may depend on other items of the page_struct also to
be setup
properly. Moreover slab object may not be properly aligned. The page oriented functions of the VM expect to operate on page aligned,
page sized
objects. Operations on object straddling page boundaries may only
affect the
objects partially which may lead to surprising results.

It is better to detect eventually remaining uses and eliminate them.

Quoted 3 lines

Is flushing a page taken from a slab forbidden ? Is it risky ? Is it no sense ?

Other drivers do a flush_dcache_page on a page coming from a scatter list. So I think they will also have an assert when CONFIG_DEBUG_VM is set.

What is the proper way to do this flush (if needed) ?

Regards,

Ludovic

author
#4 lore raw

As suggested I forward the question to Christoph Lameter and linux-mm.

Thanks for your help.

Regards

Ludovic Desroches

Quoted 69 lines · Ludovic Desroches
Quoted 23 lines · Ludovic Desroches
Quoted 22 lines · Barry Song

2011/5/19 Ludovic Desroches<[email protected]>:

Quoted 5 lines

Hello,

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

Quoted 8 lines

Into the atmci_read_data_pio function we use flush_dcache_page (do we really
need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page
comes from a scatter list). How could I correct this problem?

linux/include/linux/mmdebug.h:

#ifdef CONFIG_DEBUG_VM
#define VM_BUG_ON(cond) BUG_ON(cond)
#else
#define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif

Quoted 4 lines
Quoted 1 line

it is something like "assert" in kernel.

Thanks for your answer but I know that. My question is more focused on why there is this check.

This the reason:

commit b5fab14e5d87df4d94161ae5f5e0c8625f9ffda2

Author: Christoph Lameter <[email protected]>
Date:   Tue Jul 17 04:03:33 2007 -0700

Add VM_BUG_ON in case someone uses page_mapping on a slab page

Detect slab objects being passed to the page oriented functions of
the VM.

It is not sufficient to simply return NULL because the functions
calling
page_mapping may depend on other items of the page_struct also to
be setup
properly. Moreover slab object may not be properly aligned. The
page
oriented functions of the VM expect to operate on page aligned,
page sized
objects. Operations on object straddling page boundaries may only
affect the
objects partially which may lead to surprising results.

It is better to detect eventually remaining uses and eliminate them.

Quoted 3 lines

Is flushing a page taken from a slab forbidden ? Is it risky ? Is it no sense ?

Other drivers do a flush_dcache_page on a page coming from a scatter list. So I think they will also have an assert when CONFIG_DEBUG_VM is set.

What is the proper way to do this flush (if needed) ?

Regards,

Ludovic

#5 lore raw
In reply to Ludovic Desroches

Hello,

Quoted 2 lines · Ludovic Desroches

There is a bug with the atmel-mci driver when the debug feature CONFIG_DEBUG_VM is set.

for the new audience: the driver does the following:

	flush_dcache_page(sg_page(sg));

with sg being a struct scatterlist * provided by the caller of the struct mmc_host_ops.request callback.

Quoted 6 lines

Into the atmci_read_data_pio function we use flush_dcache_page (do we really need it?) which call the page_mapping function where we can find VM_BUG_ON(PageSlab(Page)). Then a kernel panic happens.

I don't understand the purpose of the VM_BUG_ON(PageSlab(Page)) (the page comes from a scatter list). How could I correct this problem?

I discussed this problem with Steven and Peter on irc and Steven found two functions in the mmc code (mmc_send_cxd_data and mmc_send_bus_test) that use the following idiom:

	struct scatterlist sg;
	void *data_buf;

	data_buf = kmalloc(len, GFP_KERNEL);

	sg_init_one(&sg, data_buf, len);

Is that allowed (i.e. pass kmalloc'd memory to sg_init_one)? That might be the source of the slub page in the scatterlist, no?

Best regards
Uwe

Signature · 2 lines
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