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

Matthew Wilcox
2025-02-24T15:49:28Z

On Mon, Feb 24, 2025 at 12:45:21PM -0300, Raphael S. Carvalho wrote:
> On Mon, Feb 24, 2025 at 12:33 PM Matthew Wilcox <willy@infradead.org> 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?

I really don't think the comment is needed. This is a common mistake
when fixing a bug.