staging: zsmalloc: Ensure handle is never 0 on success

Quoted 14 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

There he provided arguments for zram inclusion, how it can help in situations where zswap can't and why generalizing /dev/ramX would not be a great idea. So, cannot say why it wasn't picked up for inclusion at that time.

Quoted 8 lines

Quoted 3 lines

Should I just remove them if no one is working on getting them merged "properly"?

Please refer the mail thread (link above) and see Minchan's justifications for zram.
If they don't sound convincing enough then please remove zram+zsmalloc from staging.

You don't need to be convincing me, you need to be convincing the maintainers of the area of the kernel you are working with.

And since the last time you all tried to get this merged was back in August, I'm feeling that you all have given up, so it needs to be deleted. I'll go do that for 3.14, and if someone wants to pick it up and merge it properly, they can easily revert it.

thanks,

greg k-h
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Please read the FAQ at http://www.tux.org/lkml/

Quoted 24 lines · Greg KH

Quoted 14 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

There he provided arguments for zram inclusion, how it can help in situations where zswap can't and why generalizing /dev/ramX would not be a great idea. So, cannot say why it wasn't picked up for inclusion at that time.

Quoted 8 lines

Quoted 3 lines

Should I just remove them if no one is working on getting them merged "properly"?

Please refer the mail thread (link above) and see Minchan's justifications for zram.
If they don't sound convincing enough then please remove zram+zsmalloc from staging.

Quoted 7 lines

You don't need to be convincing me, you need to be convincing the maintainers of the area of the kernel you are working with.

And since the last time you all tried to get this merged was back in August, I'm feeling that you all have given up, so it needs to be deleted. I'll go do that for 3.14, and if someone wants to pick it up and merge it properly, they can easily revert it.

I'm guilty and I have been busy by other stuff. Sorry for that. Fortunately, I discussed this issue with Hugh in this Linuxcon for a long time(Thanks Hugh!) he felt zram's block device abstraction is better design rather than frontswap backend stuff although it's a question where we put zsmalloc. I will CC Hugh because many of things is related to swap subsystem and his opinion is really important.
And I discussed it with Rik and he feel positive about zram.

Last impression Andrw gave me by private mail is he want to merge zram's functionality into zswap or vise versa.
If I misunderstood, please correct me.
I understand his concern but I guess he didn't have a time to read my long description due to a ton of works at that time. So, I will try one more time.
I hope I'd like to listen feedback than *silence* so that we can move forward than stall.

Recently, Bob tried to move zsmalloc under mm directory to unify zram and zswap with adding pseudo block device in zswap(It's very weired to me. I think it's horrible monster which is lying between mm and block in layering POV) but he was ignoring zram's block device (a.k.a zram-blk) feature and considered only swap usecase of zram, in turn, it lose zram's good concept. I already convered other topics Bob raised in this thread[1] and why I think zram is better in the thread.

Will repeat one more time and hope gray beards penguins grab a time in this time and they give a conclusion/direction to me so that we don't lose lots of user and functionality.

========== &< ===========

Mel raised an another issue in v6, "maintainance headache". He claimed zswap and zram has a similar goal that is to compresss swap pages so if we promote zram, maintainance headache happens sometime by diverging implementaion between zswap and zram so that he want to unify zram and zswap. For it, he want zswap to implement pseudo block device like Bob did to emulate zram so zswap can have an advantage of writeback as well as zram's benefit. But I wonder frontswap-based zswap's writeback is really good approach for writeback POV. I think that problem isn't only specific for zswap. If we want to configure multiple swap hierarchy with various speed device such as RAM, NVRAM, SSD, eMMC, NAS etc, it would be a general problem. So we should think of more general approach. At a glance, I can see two approach.

First, VM could be aware of heterogeneous swap configuration so it could aim for being able to configure cache hierarchy among swap devices. It may need indirction layer on swap, which was already talked about that way so VM can migrate a block from A to B easily. It will support various configuration with VM's hints, maybe, in future.
http://lkml.indiana.edu/hypermail/linux/kernel/1203.3/03812.html

Second, as more practical solution, we could use device mapper like dm-cache(https://lwn.net/Articles/540996/), which makes it very flexible. Now, it supports various configruation and cache policy (block size, writeback/writethrough, LRU, MFU although MQ is merged now) so it would be good fit for our purpose. Even, it can make zram support writeback. I tested it following as following scenario in KVM 4 CPU, 1G DRAM with background 800M memory hogger, which is allocates random data up to 800M.

1) zram swap disk 1G, untar kernel.tgz to tmpfs, build -j 4
Fail to untar due to shortage of memory space by tmpfs default size limit

2) zram swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 OOM happens while building the kernel but it untar successfully on ext2 based on zram-blk. The reason OOM happend is zram can not find free pages from main memory to store swap out pages although empty swap space is still enough.

3) dm-cache swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 dmcache consists of zram-meta 10M, zram-cache 1G and real swap storage 1G No OOM happens and successfully building done.

Above tests proves zram can support writeback into real swap storage so that zram-cache can always have a free space. If necessary, we could add new plugin in dm-cache. I see It's really flexible and well-layered architecure so zram-blk's concept is good for us and it has lots of potential to be enhanced by MM/FS/Block developers.

As other disadvantage of zswap writeback, frontswap's semantic is synchronous API so zswap should decompress in memory zpage right before writeback and even, it writes pages one by one, not a batch. If we extend frontswap API, we would enhance it but I belive we can do better in device mapper layer which is aware of block align, bandwidth, mapping table, asynchronous and lots of hints from the block layer. Nonetheless, if we should merge zram's functionality to zswap, I think zram should include zswap's functionaliy(But I hope it will never happen) because old age zram already has lots of real users rather than new young zswap so it's more handy to unify them with keeping changelog which is one of valuable things getting from staging stay for a long time.

The reason zram doesn't support writeback until now is just shortage of needs. The zram's main customers were embedded people so writeback into real swap storage is too bad for interactivity and wear-leveling on low falsh devices. But like above, zram has a potential to support writeback with other block drivers or more reasonable VM enhance so I'd like to claim zram's block concept is really good.

Another zram-blk's usecase is following as.
The admin can format /dev/zramX with any FS and mount on it. It could help small memory system, too. For exmaple, many embedded system don't have swap so although tmpfs can support swapout, it's pointless. Then, let's assume temp file growing up until half of system memory once in a while. We don't want to write it on flash by wear-leveing issue and response problem so we want to keep in-memory. But if we use tmpfs, it should evict half of working set to cover them when the size reach peak. In the case, zram-blk would be good fit, too.

I'd like to enhance zram with more features like zsmalloc-compaction, , async I/O, parallel decompression and so on but zram developers cannot do it now because Greg, staging maintainer, doesn't want to add new feature until promotion is done because zram have been in staging for a very long time. Acutally, some patches about enhance are pending for a long time.

[1] https://lkml.org/lkml/2013/8/21/141

Quoted 9 lines

thanks,

greg k-h
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Kind regards,
Minchan Kim

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I'm guilty and I have been busy by other stuff. Sorry for that. Fortunately, I discussed this issue with Hugh in this Linuxcon for a long time(Thanks Hugh!) he felt zram's block device abstraction is better design rather than frontswap backend stuff although it's a question where we put zsmalloc. I will CC Hugh because many of things is related to swap subsystem and his opinion is really important.
And I discussed it with Rik and he feel positive about zram.

If I may add my usual 2c (and some news):

zram is used by default on all Chrome OS devices. I can't say how many devices, but it's not a small number, google it, and it's an important market, low-end laptops for education and the less affluent.
It has been available experimentally for well over a year.

Android 4.4 KitKat is also using zram, to better support devices with less than 1 MB RAM. (That's the news.)

When comparing the relative advantages of the two subsystems (zram and zswap), let's not forget that considerable effort goes into in tuning and bug fixing for specific use cases---possibly even more than the initial development effort. Zram has not just been sitting around in drivers/staging, it's in serious use.

If we were to judge systems based merely on theoretical technical merit, then we should consider switching en masse to FreeBSD. (I said we should *consider* :).

I am very familiar with the limitations of zram, but it works well and I think it would be wise to keep supporting it. Besides, it's small and AFAICT it interfaces cleanly with the rest of the system, so I don't see what the big deal is.

Quoted 24 lines · Greg KH

Quoted 14 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

There he provided arguments for zram inclusion, how it can help in situations where zswap can't and why generalizing /dev/ramX would not be a great idea. So, cannot say why it wasn't picked up for inclusion at that time.

Quoted 8 lines

Quoted 3 lines

Should I just remove them if no one is working on getting them merged "properly"?

Please refer the mail thread (link above) and see Minchan's justifications for zram.
If they don't sound convincing enough then please remove zram+zsmalloc from staging.

Quoted 7 lines

You don't need to be convincing me, you need to be convincing the maintainers of the area of the kernel you are working with.

And since the last time you all tried to get this merged was back in August, I'm feeling that you all have given up, so it needs to be deleted. I'll go do that for 3.14, and if someone wants to pick it up and merge it properly, they can easily revert it.

I'm guilty and I have been busy by other stuff. Sorry for that. Fortunately, I discussed this issue with Hugh in this Linuxcon for a long time(Thanks Hugh!) he felt zram's block device abstraction is better design rather than frontswap backend stuff although it's a question where we put zsmalloc. I will CC Hugh because many of things is related to swap subsystem and his opinion is really important.
And I discussed it with Rik and he feel positive about zram.

Last impression Andrw gave me by private mail is he want to merge zram's functionality into zswap or vise versa.
If I misunderstood, please correct me.
I understand his concern but I guess he didn't have a time to read my long description due to a ton of works at that time. So, I will try one more time.
I hope I'd like to listen feedback than *silence* so that we can move forward than stall.

Recently, Bob tried to move zsmalloc under mm directory to unify zram and zswap with adding pseudo block device in zswap(It's very weired to me. I think it's horrible monster which is lying between mm and block in layering POV) but he was ignoring zram's block device (a.k.a zram-blk) feature and considered only swap usecase of zram, in turn, it lose zram's good concept. I already convered other topics Bob raised in this thread[1] and why I think zram is better in the thread.

Quoted 24 lines · Greg KH

Quoted 14 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

There he provided arguments for zram inclusion, how it can help in situations where zswap can't and why generalizing /dev/ramX would not be a great idea. So, cannot say why it wasn't picked up for inclusion at that time.

Quoted 8 lines

Quoted 3 lines

Should I just remove them if no one is working on getting them merged "properly"?

Please refer the mail thread (link above) and see Minchan's justifications for zram.
If they don't sound convincing enough then please remove zram+zsmalloc from staging.

Quoted 7 lines

You don't need to be convincing me, you need to be convincing the maintainers of the area of the kernel you are working with.

And since the last time you all tried to get this merged was back in August, I'm feeling that you all have given up, so it needs to be deleted. I'll go do that for 3.14, and if someone wants to pick it up and merge it properly, they can easily revert it.

I'm guilty and I have been busy by other stuff. Sorry for that. Fortunately, I discussed this issue with Hugh in this Linuxcon for a long time(Thanks Hugh!) he felt zram's block device abstraction is better design rather than frontswap backend stuff although it's a question where we put zsmalloc. I will CC Hugh because many of things is related to swap subsystem and his opinion is really important.
And I discussed it with Rik and he feel positive about zram.

Last impression Andrw gave me by private mail is he want to merge zram's functionality into zswap or vise versa.
If I misunderstood, please correct me.
I understand his concern but I guess he didn't have a time to read my long description due to a ton of works at that time. So, I will try one more time.
I hope I'd like to listen feedback than *silence* so that we can move forward than stall.

Recently, Bob tried to move zsmalloc under mm directory to unify zram and zswap with adding pseudo block device in zswap(It's very weired to me. I think it's horrible monster which is lying between mm and block in layering POV) but he was ignoring zram's block device (a.k.a zram-blk) feature and considered only swap usecase of zram, in turn, it lose zram's good concept. I already convered other topics Bob raised in this thread[1] and why I think zram is better in the thread.

Will repeat one more time and hope gray beards penguins grab a time in this time and they give a conclusion/direction to me so that we don't lose lots of user and functionality.

========== &< ===========

Mel raised an another issue in v6, "maintainance headache". He claimed zswap and zram has a similar goal that is to compresss swap pages so if we promote zram, maintainance headache happens sometime by diverging implementaion between zswap and zram so that he want to unify zram and zswap. For it, he want zswap to implement pseudo block device like Bob did to emulate zram so zswap can have an advantage of writeback as well as zram's benefit. But I wonder frontswap-based zswap's writeback is really good approach for writeback POV. I think that problem isn't only specific for zswap. If we want to configure multiple swap hierarchy with various speed device such as RAM, NVRAM, SSD, eMMC, NAS etc, it would be a general problem. So we should think of more general approach. At a glance, I can see two approach.

First, VM could be aware of heterogeneous swap configuration so it could aim for being able to configure cache hierarchy among swap devices. It may need indirction layer on swap, which was already talked about that way so VM can migrate a block from A to B easily. It will support various configuration with VM's hints, maybe, in future.
http://lkml.indiana.edu/hypermail/linux/kernel/1203.3/03812.html

Second, as more practical solution, we could use device mapper like dm-cache(https://lwn.net/Articles/540996/), which makes it very flexible. Now, it supports various configruation and cache policy (block size, writeback/writethrough, LRU, MFU although MQ is merged now) so it would be good fit for our purpose. Even, it can make zram support writeback. I tested it following as following scenario in KVM 4 CPU, 1G DRAM with background 800M memory hogger, which is allocates random data up to 800M.

1) zram swap disk 1G, untar kernel.tgz to tmpfs, build -j 4
Fail to untar due to shortage of memory space by tmpfs default size limit

2) zram swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 OOM happens while building the kernel but it untar successfully on ext2 based on zram-blk. The reason OOM happend is zram can not find free pages from main memory to store swap out pages although empty swap space is still enough.

3) dm-cache swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 dmcache consists of zram-meta 10M, zram-cache 1G and real swap storage 1G No OOM happens and successfully building done.

Above tests proves zram can support writeback into real swap storage so that zram-cache can always have a free space. If necessary, we could add new plugin in dm-cache. I see It's really flexible and well-layered architecure so zram-blk's concept is good for us and it has lots of potential to be enhanced by MM/FS/Block developers.

As other disadvantage of zswap writeback, frontswap's semantic is synchronous API so zswap should decompress in memory zpage right before writeback and even, it writes pages one by one, not a batch. If we extend frontswap API, we would enhance it but I belive we can do better in device mapper layer which is aware of block align, bandwidth, mapping table, asynchronous and lots of hints from the block layer. Nonetheless, if we should merge zram's functionality to zswap, I think zram should include zswap's functionaliy(But I hope it will never happen) because old age zram already has lots of real users rather than new young zswap so it's more handy to unify them with keeping changelog which is one of valuable things getting from staging stay for a long time.

The reason zram doesn't support writeback until now is just shortage of needs. The zram's main customers were embedded people so writeback into real swap storage is too bad for interactivity and wear-leveling on low falsh devices. But like above, zram has a potential to support writeback with other block drivers or more reasonable VM enhance so I'd like to claim zram's block concept is really good.

Another zram-blk's usecase is following as.
The admin can format /dev/zramX with any FS and mount on it. It could help small memory system, too. For exmaple, many embedded system don't have swap so although tmpfs can support swapout, it's pointless. Then, let's assume temp file growing up until half of system memory once in a while. We don't want to write it on flash by wear-leveing issue and response problem so we want to keep in-memory. But if we use tmpfs, it should evict half of working set to cover them when the size reach peak. In the case, zram-blk would be good fit, too.

I'd like to enhance zram with more features like zsmalloc-compaction, , async I/O, parallel decompression and so on but zram developers cannot do it now because Greg, staging maintainer, doesn't want to add new feature until promotion is done because zram have been in staging for a very long time. Acutally, some patches about enhance are pending for a long time.

[1] https://lkml.org/lkml/2013/8/21/141

We spent much time with preventing zram enhance since it have been in staging and Greg never want to improve without promotion.

Quoted 2 lines · Minchan Kim

We spent much time with preventing zram enhance since it have been in staging and Greg never want to improve without promotion.

It's not "improve", it's "Greg does not want you adding new features and functionality while the code is in staging." I want you to spend your time on getting it out of staging first.

Now if something needs to be done based on review and comments to the code, then that's fine to do and I'll accept that, but I've been seeing new functionality be added to the code, which I will not accept because it seems that you all have given up on getting it merged, which isn't ok.

Quoted 10 lines · Minchan Kim

Quoted 9 lines · Greg KH

Quoted 8 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

Hello Andrew,

I'd like to listen your opinion.

The zram promotion trial started since Aug 2012 and I already have get many Acked/Reviewed feedback and positive feedback from Rik and Bob in this thread. (ex, Jens Axboe[1], Konrad Rzeszutek Wilk[2], Nitin Gupta[3], Pekka Enberg[4]) In Linuxcon, Hugh gave positive feedback about zram(Hugh, If I misunderstood, please correct me!). And there are lots of users already in embedded industry ex, (most of TV in the world, Chromebook, CyanogenMod, Android Kitkat.) They are not idiot. Zram is really effective for embedded world.

We spent much time with preventing zram enhance since it have been in staging and Greg never want to improve without promotion.

Please consider promotion and let us improve it.
I think only remained thing is your decision.

1. https://lkml.org/lkml/2012/9/11/551
2. https://lkml.org/lkml/2012/8/9/636
3. https://lkml.org/lkml/2012/8/8/390
4. https://lkml.org/lkml/2012/9/26/126

Quoted 11 lines · Minchan Kim

Quoted 10 lines · Minchan Kim

Quoted 9 lines · Greg KH

Quoted 8 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

...

Quoted 5 lines

Hello Andrew,

I'd like to listen your opinion.

Quoted 7 lines

The zram promotion trial started since Aug 2012 and I already have get many Acked/Reviewed feedback and positive feedback from Rik and Bob in this thread. (ex, Jens Axboe[1], Konrad Rzeszutek Wilk[2], Nitin Gupta[3], Pekka Enberg[4]) In Linuxcon, Hugh gave positive feedback about zram(Hugh, If I misunderstood, please correct me!). And there are lots of users already in embedded industry ex, (most of TV in the world, Chromebook, CyanogenMod, Android Kitkat.) They are not idiot. Zram is really effective for embedded world.

Sorry for taking so long to respond, Minchan: no, you do not misrepresent me at all. Promotion of zram and zsmalloc from staging is way overdue:
they long ago proved their worth, look tidy, and have an active maintainer.

Putting them into drivers/staging was always a mistake, and I quite understand Greg's impatience with them by now; but please let's move them to where they belong instead of removing them.

I would not have lent support to zswap if I'd thought that was going to block zram. And I was not the only one surprised when zswap replaced its use of zsmalloc by zbud: we had rather expected a zbud option to be added, and I still assume that zsmalloc support will be added back to zswap later.

Quoted 11 lines · Minchan Kim

Quoted 10 lines · Minchan Kim

Quoted 9 lines · Greg KH

Quoted 8 lines · Nitin Gupta

I'm getting really tired of them hanging around in here for many years

Quoted 2 lines

now...

Minchan has tried many times to promote zram out of staging. This was his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

...

Quoted 5 lines

Hello Andrew,

I'd like to listen your opinion.

Quoted 7 lines

The zram promotion trial started since Aug 2012 and I already have get many Acked/Reviewed feedback and positive feedback from Rik and Bob in this thread. (ex, Jens Axboe[1], Konrad Rzeszutek Wilk[2], Nitin Gupta[3], Pekka Enberg[4]) In Linuxcon, Hugh gave positive feedback about zram(Hugh, If I misunderstood, please correct me!). And there are lots of users already in embedded industry ex, (most of TV in the world, Chromebook, CyanogenMod, Android Kitkat.) They are not idiot. Zram is really effective for embedded world.

Sorry for taking so long to respond, Minchan: no, you do not misrepresent me at all. Promotion of zram and zsmalloc from staging is way overdue:
they long ago proved their worth, look tidy, and have an active maintainer.

Putting them into drivers/staging was always a mistake, and I quite understand Greg's impatience with them by now; but please let's move them to where they belong instead of removing them.

I would not have lent support to zswap if I'd thought that was going to block zram. And I was not the only one surprised when zswap replaced its use of zsmalloc by zbud: we had rather expected a zbud option to be added, and I still assume that zsmalloc support will be added back to zswap later.

I think your August 2013 posting moved zsmalloc under zram and moved it all to drivers/block? That is the right place for zram, but I do think zsmalloc.c (I'm not very keen on _drvs and -mains myself) should be alongside zbud.c in mm, where we can better keep an eye on its struct-pageyness.

IMHO
Hugh

Quoted 4 lines

We spent much time with preventing zram enhance since it have been in staging and Greg never want to improve without promotion.

Quoted 8 lines

Please consider promotion and let us improve it.
I think only remained thing is your decision.

1. https://lkml.org/lkml/2012/9/11/551
2. https://lkml.org/lkml/2012/8/9/636
3. https://lkml.org/lkml/2012/8/8/390
4. https://lkml.org/lkml/2012/9/26/126

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>

Quoted 19 lines · Hugh Dickins

On Wed, 13 Nov 2013, Minchan Kim wrote:
...

Quoted 5 lines

Hello Andrew,

I'd like to listen your opinion.

Quoted 7 lines

The zram promotion trial started since Aug 2012 and I already have get many Acked/Reviewed feedback and positive feedback from Rik and Bob in this thread. (ex, Jens Axboe[1], Konrad Rzeszutek Wilk[2], Nitin Gupta[3], Pekka Enberg[4]) In Linuxcon, Hugh gave positive feedback about zram(Hugh, If I misunderstood, please correct me!). And there are lots of users already in embedded industry ex, (most of TV in the world, Chromebook, CyanogenMod, Android Kitkat.) They are not idiot. Zram is really effective for embedded world.

Sorry for taking so long to respond, Minchan: no, you do not misrepresent me at all. Promotion of zram and zsmalloc from staging is way overdue:
they long ago proved their worth, look tidy, and have an active maintainer.

Quoted 8 lines

Putting them into drivers/staging was always a mistake, and I quite understand Greg's impatience with them by now; but please let's move them to where they belong instead of removing them.

I would not have lent support to zswap if I'd thought that was going to block zram. And I was not the only one surprised when zswap replaced its use of zsmalloc by zbud: we had rather expected a zbud option to be added, and I still assume that zsmalloc support will be added back to zswap later.

Yes, it is still the plan to reintroduce zsmalloc as an option (possibly _the_ option) for zswap.

An idea being tossed around is making zswap writethrough instead of delayed writeback.

Doing this would be mean that zswap would no longer reduce swap out traffic, but would continue to reduce swap in latency by reading out of the compressed cache instead of the swap device.

For that loss, we gain a benefit: the compressed pages in the cache are clean, meaning we can reclaim them at any time with no writeback cost. This addresses Mel's initial concern (the one that led to zswap moving to zbud) about writeback latency when the zswap pool is full.

If there is no writeback cost for reclaiming space in the compressed pool, then we can use higher density packing like zsmalloc.

Making zswap writethough would also make the difference between zswap and zram, both in terms of operation and application, more apparent, demonstrating the need for both.