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

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Message-ID: <Z7yRSe-nkfMz4TS2@casper.infradead.org>
Date: Mon, 24 Feb 2025 15:33:29 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Christoph Hellwig <hch@....de>
Cc: "Raphael S. Carvalho" <raphaelsc@...lladb.com>,
	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 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.