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

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Date:   Thu, 9 Feb 2023 14:29:45 -0800
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

On Thu, Feb 9, 2023 at 2:12 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Thu, 9 Feb 2023 13:50:30 -0800 (PST) Hugh Dickins <hughd@...gle.com> wrote:
>
> >
> > >
> > > I'm not seeing anything in the [1/2] changelog which indicates that a
> > > backport is needed.  IOW,
> >
> > Correct: it's just changing the errno for some racy cases from "you're
> > wrong, don't bother me again" to "it might be worth having another go":
> > not fixing an instability, as 2/2 was.
> >
> > >
> > > # cat .signature
> > > When fixing a bug, please describe the end-user visible effects of that bug.
> >
> > If whatever's being run by the end-user is coded to try again on -EAGAIN,
> > then the end-user will less often see occasional unexplained failures.
> >
>
> OK, thanks.  I redid the changelog's final paragraph thusly:
>
> : In this situation, MADV_COLLAPSE returns -EINVAL when it should return
> : -EAGAIN.  This could cause userspace to conclude that the syscall failed,
> : when it in fact could succeed by retrying.
>

This looks good to me. Thanks Andrew! Also thanks Hugh for being on
the lookout for this patch -- I hastily read through my emails
regarding which patches were merged where and had assumed this merged
with 2/2.

Also, apologies about the confusing v1 [1/2] and v2 [2/2] fiasco; in
hindsight that probably wasn't the most decipherable thing to do :)

Best,
Zach