linux-kernel - Re: [PATCH v2] mm: Fix error handling in __filemap_get_folio() with FGP

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Message-ID: <Z7yVB0w7YoY_DrNz@casper.infradead.org>
Date: Mon, 24 Feb 2025 15:49:27 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Raphael S. Carvalho" <raphaelsc@...lladb.com>
Cc: Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, djwong@...nel.org,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v2] mm: Fix error handling in __filemap_get_folio() with
 FGP_NOWAIT

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@...radead.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.