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

[Sergey Senozhatsky]

Hi Barry,

On (25/11/29 17:55), Barry Song wrote:
> On Sat, Nov 29, 2025 at 1:06 AM Sergey Senozhatsky
> <senozhatsky@xxxxxxxxxxxx> wrote:
> >
> > From: Richard Chang <richardycc@xxxxxxxxxx>
> >
> 
> 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).