linux-kernel - Re: [PATCH v1] zram: add size class equals check into recompression

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Message-ID: <Y1c5/mSBc+ctWH6v@google.com>
Date:   Tue, 25 Oct 2022 10:21:02 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexey Romanov <avromanov@...rdevices.ru>, minchan@...nel.org,
        senozhatsky@...omium.org, ngupta@...are.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...rdevices.ru
Subject: Re: [PATCH v1] zram: add size class equals check into recompression

On (22/10/24 13:59), Andrew Morton wrote:
> > It makes no sense for us to recompress the object if it will
> > be in the same size class. We anyway don't get any memory gain.
> > But, at the same time, we get a CPU time overhead when inserting
> > this object into zspage and decompressing it afterwards.
> > 
> 
> Dumb question: is it ever possible for compression to result in an
> increase in size?

That's a good question. Re-compressed object can be bigger than the
original compressed one, but this should already be taken care of.
We do
        if (comp_len_next >= huge_class_size ||
            comp_len_next >= comp_len_prev ||

This checks whether recompressed object is above huge-size watermark and
whether recompressed size is larger than the original size.