Re: [PATCH v2] mm: Fix error handling in __filemap_get_folio() with FGP_NOWAIT

[Raphael S. Carvalho]

On Mon, Feb 24, 2025 at 12:33 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Feb 24, 2025 at 03:17:44PM +0100, Christoph Hellwig wrote:
> > On Mon, Feb 24, 2025 at 05:13:28AM -0300, Raphael S. Carvalho wrote:
> > > +           if (err) {
> > > +                   /* Prevents -ENOMEM from escaping to user space with FGP_NOWAIT */
> > > +                   if ((fgp_flags & FGP_NOWAIT) && err == -ENOMEM)
> > > +                           err = -EAGAIN;
> > >                     return ERR_PTR(err);
> >
> > I don't think the comment is all that useful.  It's also overly long.
> >
> > I'd suggest this instead:
> >
> >                       /*
> >                        * When NOWAIT I/O fails to allocate folios this could
> >                        * be due to a nonblocking memory allocation and not
> >                        * because the system actually is out of memory.
> >                        * Return -EAGAIN so that there caller retries in a
> >                        * blocking fashion instead of propagating -ENOMEM
> >                        * to the application.
> >                        */
>
> I don't think it needs a comment at all, but the memory allocation
> might be for something other than folios, so your suggested comment
> is misleading.

Isn't it all in the context of allocating or adding folio? The reason
behind a comment is to prevent movements in the future that could
cause a similar regression, and also to inform the poor reader that
might be left wondering why we're converting -ENOMEM into -EAGAIN with
FGP_NOWAIT. Can it be slightly adjusted to make it more correct? Or
you really think it's better to remove it completely?