linux-kernel - Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

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Message-Id: <20230209132846.122ad88e1c2bd0603a630e5c@linux-foundation.org>
Date:   Thu, 9 Feb 2023 13:28:46 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yang Shi <shy828301@...il.com>,
        Zach O'Keefe <zokeefe@...gle.com>
Subject: Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page
 refcount

On Wed, 8 Feb 2023 21:09:04 -0800 (PST) Hugh Dickins <hughd@...gle.com> wrote:

> On Tue, 24 Jan 2023, Zach O'Keefe wrote:
> 
> > During collapse, in a few places we check to see if a given small page
> > has any unaccounted references.  If the refcount on the page doesn't
> > match our expectations, it must be there is an unknown user concurrently
> > interested in the page, and so it's not safe to move the contents
> > elsewhere. However, the unaccounted pins are likely an ephemeral state.
> > 
> > In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
> > collapse may succeed on retry.
> > 
> > Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
> > Reported-by: Hugh Dickins <hughd@...gle.com>
> > Signed-off-by: Zach O'Keefe <zokeefe@...gle.com>
> 
> This was
> Reviewed-by: Yang Shi <shy828301@...il.com>
> and now I'll give it a nudge with
> Acked-by: Hugh Dickins <hughd@...gle.com>
> since it hasn't appeared in mm-unstable or linux-next yet:

Buildbot failed on [2/2] so I skipped the whole series in expectation
of a v2 series, which didn't happen.  Instead, Zach trickily sent what
was [2/2] as a standalone patch.  So [1/2] got lost.  Sigh, poor me.

Thanks, I'll merge [1/2] into mm-hotfixes.

> I think its Cc:stable sibling 2/2, already in 6.2-rc, got all the attention.

I'm not seeing anything in the [1/2] changelog which indicates that a
backport is needed.  IOW,

# cat .signature
When fixing a bug, please describe the end-user visible effects of that bug.