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

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Message-Id: <20230209141221.b7881fb0783a58571f2bca53@linux-foundation.org>
Date:   Thu, 9 Feb 2023 14:12:21 -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 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.