Re: [PATCH v3] mm: Fix error handling in __filemap_get_folio() with FGP_NOWAIT

[Dave Chinner]

On Mon, Feb 24, 2025 at 11:37:00AM -0300, Raphael S. Carvalho wrote:
> original report:
> https://lore.kernel.org/all/CAKhLTr1UL3ePTpYjXOx2AJfNk8Ku2EdcEfu+CH1sf3Asr=B-Dw@xxxxxxxxxxxxxx/T/
> 
> When doing buffered writes with FGP_NOWAIT, under memory pressure, the system
> returned ENOMEM despite there being plenty of available memory, to be reclaimed
> from page cache. The user space used io_uring interface, which in turn submits
> I/O with FGP_NOWAIT (the fast path).
....

> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 804d7365680c..3e75dced0fd9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1986,8 +1986,19 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  
>  		if (err == -EEXIST)
>  			goto repeat;
> -		if (err)
> +		if (err) {
> +			/*
> +			 * 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.
> +			 */
> +			if ((fgp_flags & FGP_NOWAIT) && err == -ENOMEM)
> +				err = -EAGAIN;
>  			return ERR_PTR(err);
> +		}
>  		/*
>  		 * filemap_add_folio locks the page, and for mmap
>  		 * we expect an unlocked page.

Looks good to me.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

-- 
Dave Chinner
david@xxxxxxxxxxxxx