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

Zach O'Keefe
2023-02-09T22:30:28Z

On Thu, Feb 9, 2023 at 2:12 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 9 Feb 2023 13:50:30 -0800 (PST) Hugh Dickins <hughd@google.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