linux-kernel - Re: [PATCH 1/2] zram: introduce compressed data writeback

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Message-ID: <hgk3zp5hwlcxo6ufiqasvte3hoksy2wb2kta3fime5rprq4org@xaprrqdabvgh>
Date: Mon, 1 Dec 2025 12:56:25 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Barry Song <21cnbao@...il.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Richard Chang <richardycc@...gle.com>, 
	Brian Geffon <bgeffon@...gle.com>, Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, linux-block@...r.kernel.org, Minchan Kim <minchan@...gle.com>
Subject: Re: [PATCH 1/2] zram: introduce compressed data writeback

Hi Barry,

On (25/11/29 17:55), Barry Song wrote:
> On Sat, Nov 29, 2025 at 1:06 AM Sergey Senozhatsky
> <senozhatsky@...omium.org> wrote:
> >
> > From: Richard Chang <richardycc@...gle.com>
> >
> 
> Hi Richard, Sergey,
> 
> Thanks a lot for developing this. For years, people have been looking for
> compressed data writeback to reduce I/O, such as compacting multiple compressed
> blocks into a single page on block devices. I guess this patchset hasn’t reached
> that point yet, right?

Right.

> > zram stores all written back slots raw, which implies that
> > during writeback zram first has to decompress slots (except
> > for ZRAM_HUGE slots, which are raw already).  The problem
> > with this approach is that not every written back page gets
> > read back (either via read() or via page-fault), which means
> > that zram basically wastes CPU cycles and battery decompressing
> > such slots.  This changes with introduction of decompression
> 
> If a page is swapped out and never read again, does that actually indicate
> a memory leak in userspace?

No, it just means that there is no page-fault on that page.  E.g. we
swapped out an unused browser tab and never come back to it within the
session: e.g. user closed the tab/app, or logged out of session, or
rebooted the device, or simply powered off (desktop/laptop).