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

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Message-ID: <Z7vyEdJ3SqjFkE9q@casper.infradead.org>
Date: Mon, 24 Feb 2025 04:14:09 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Raphael S. Carvalho" <raphaelsc@...lladb.com>
Cc: 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>, hch@....de
Subject: Re: [PATCH] mm: Fix error handling in __filemap_get_folio() with
 FGP_NOWAIT

On Sun, Feb 23, 2025 at 08:57:19PM -0300, Raphael S. Carvalho wrote:
> This is likely a regression caused by 66dabbb65d67 ("mm: return an ERR_PTR
> from __filemap_get_folio"), which performed the following changes:
>     --- a/fs/iomap/buffered-io.c
>     +++ b/fs/iomap/buffered-io.c
>     @@ -468,19 +468,12 @@ EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
>     struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
>     {
>             unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
>     -       struct folio *folio;
> 
>             if (iter->flags & IOMAP_NOWAIT)
>                     fgp |= FGP_NOWAIT;
> 
>     -       folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
>     +       return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
>                             fgp, mapping_gfp_mask(iter->inode->i_mapping));
>     -       if (folio)
>     -               return folio;
>     -
>     -       if (iter->flags & IOMAP_NOWAIT)
>     -               return ERR_PTR(-EAGAIN);
>     -       return ERR_PTR(-ENOMEM);
>     }

We don't usually put this in the changelog ...

> Essentially, that patch is moving error picking decision to
> __filemap_get_folio, but it missed proper FGP_NOWAIT handling, so ENOMEM
> is being escaped to user space. Had it correctly returned -EAGAIN with NOWAIT,
> either io_uring or user space itself would be able to retry the request.
> It's not enough to patch io_uring since the iomap interface is the one
> responsible for it, and pwritev2(RWF_NOWAIT) and AIO interfaces must return
> the proper error too.
> 
> The patch was tested with scylladb test suite (its original reproducer), and
> the tests all pass now when memory is pressured.
> 
> Signed-off-by: Raphael S. Carvalho <raphaelsc@...lladb.com>

Instead, we add:

Fixes: 66dabbb65d67 (mm: return an ERR_PTR from __filemap_get_folio)

> ---
>  mm/filemap.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 804d7365680c..b06bd6eedaf7 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1986,8 +1986,15 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  
>  		if (err == -EEXIST)
>  			goto repeat;
> -		if (err)
> +		if (err) {
> +			/*
> +			 * Presumably ENOMEM, either from when allocating or
> +			 * adding folio (this one for xarray node)
> +			 */

I don't like the comment.  Better to do that in code:

			if ((fgp_flags & FGP_NOWAIT) && (err == -ENOMEM))

> +			if (fgp_flags & FGP_NOWAIT)
> +				err = -EAGAIN;
>  			return ERR_PTR(err);
> +		}
>  		/*
>  		 * filemap_add_folio locks the page, and for mmap
>  		 * we expect an unlocked page.
> -- 
> 2.48.1
>