linux-kernel - Re: [PATCH v2] zram: easy the allocation of zcomp

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Date: Tue, 6 Feb 2024 20:17:15 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Sergey Senozhatsky <senozhatsky@...omium.org>,
 Barry Song <21cnbao@...il.com>
Cc: Minchan Kim <minchan@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, zhengtangquan@...o.com,
 Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v2] zram: easy the allocation of zcomp_strm's buffers
 through vmalloc

On 2/6/24 8:14 PM, Sergey Senozhatsky wrote:
> On (24/02/06 19:40), Jens Axboe wrote:
>> On 2/6/24 6:44 PM, Sergey Senozhatsky wrote:
>>> On (24/02/07 09:25), Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>
>>>> Firstly, there is no need to keep zcomp_strm's buffers contiguous
>>>> physically.
>>>>
>>>> Secondly, The recent mTHP project has provided the possibility to
>>>> swapout and swapin large folios. Compressing/decompressing large
>>>> blocks can hugely decrease CPU consumption and improve compression
>>>> ratio. This requires us to make zRAM support the compression and
>>>> decompression for large objects.
>>>> With the support of large objects in zRAM of our out-of-tree code,
>>>> we have observed many allocation failures during CPU hotplug as
>>>> large objects need larger buffers. So this change is also more
>>>> future-proof once we begin to bring up multiple sizes in zRAM.
>>>>
>>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>>
>>> Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
>>>
>>> Note:
>>> Taking it in NOT because of the out-of-tree code (we don't really
>>> do that), but because this is executed from CPU offline/online
>>> paths, which can happen on devices with fragmented memory (a valid
>>> concern IMHO).
>>>
>>> Minchan, if you have any objections, please chime in.
>>
>> Not Minchan, but I do have an issue with the title of the commit, it
>> doesn't make any sense. Can the maintainer please re-write that to be
>> something that is appropriate and actually describes what the patch
>> does?
> 
> Thanks Jens. I fully agree, I requested a better commit message in
> v1 feedback, we probably still can improve on this.
> 
> 
> Something like this?
> 
> ---
> 
> zram: do not allocate physically contiguous strm buffers
> 
> Currently zram allocates 2 physically contigous pages per-CPU's
> compression stream (we may have up to 3 streams per-CPU). Since
> those buffers are per-CPU we allocate them from CPU hotplug path,
> which may have higher risks of failed allocations on devices with
> fragmented memory.
> 
> Switch to virtually contiguos allocations - crypto comp does not
> seem impose requirements on compression working buffers to be
> physically contiguous.

Yep, this is much better! Thanks.

-- 
Jens Axboe