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

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Date: Wed, 7 Feb 2024 12:14:47 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Jens Axboe <axboe@...nel.dk>, Barry Song <21cnbao@...il.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	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 (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.