linux-kernel - Re: [PATCH] zram: Add a huge

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Date:   Tue, 15 Mar 2022 20:01:51 -0400
From:   Brian Geffon <bgeffon@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        linux-block@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] zram: Add a huge_idle writeback mode

On Tue, Mar 15, 2022 at 1:44 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Mar 15, 2022 at 01:34:21PM -0400, Brian Geffon wrote:
> > On Tue, Mar 15, 2022 at 1:28 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Tue, Mar 15, 2022 at 10:22:21AM -0700, Brian Geffon wrote:
> > > > Today it's only possible to write back as a page, idle, or huge.
> > > > A user might want to writeback pages which are huge and idle first
> > > > as these idle pages do not require decompression and make a good
> > > > first pass for writeback.
> > >
> > > We're moving towards having many different sizes of page in play,
> > > not just PMD and PTE sizes.  Is this patch actually a good idea in
> > > a case where we have, eg, a 32kB anonymous page on a system with 4kB
> > > pages?  How should zram handle this case?  What's our cut-off for
> > > declaring a page to be "huge"?
> > >
> >
> > Huge isn't a great term IMO, but it is what it is. ZRAM_HUGE is used
> > to identify pages which are incompressible. Since zram is a block
> > device which presents PAGE_SIZED blocks, do these new changes which
> > involve many different page sizes matter as that seems orthogonal to
> > the block subsystem. Correct me if I'm misunderstanding.
>
> Oh, so ZRAM's concept of huge is not the same as the "huge" in
> "hugetlbfs" or "THP"?  That's not at all confusing ...

I do not disagree, but there isn't much that can be done about it at
this point given the sysfs file takes an argument called "huge"