Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a superfluous page

Jeff Layton
2021-10-19T17:13:12Z

On Mon, 2021-10-18 at 15:50 +0100, David Howells wrote:
> Under some circumstances, filemap_read() will allocate sufficient pages to
> read to the end of the file, call readahead/readpages on them and copy the
> data over - and then it will allocate another page at the EOF and call
> readpage on that and then ignore it. This is unnecessary and a waste of
> time and resources.
>
> filemap_read() *does* check for this, but only after it has already done
> the allocation and I/O. Fix this by checking before calling
> filemap_get_pages() also.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Acked-by: Kent Overstreet <kent.overstreet@gmail.com>
> cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> cc: linux-mm@kvack.org
> cc: linux-fsdevel@vger.kernel.org
> Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/
> ---
>
> mm/filemap.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index dae481293b5d..c0cdc44c844e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
> if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
> iocb->ki_flags |= IOCB_NOWAIT;
>
> + isize = i_size_read(inode);
> + if (unlikely(iocb->ki_pos >= isize))
> + goto put_pages;
> +
> error = filemap_get_pages(iocb, iter, &pvec);
> if (error < 0)
> break;
>
>

I would wager that it's worth checking for this. I imagine read calls
beyond EOF are common enough that it's probably helpful to optimize that
case:

Acked-by: Jeff Layton <jlayton@redhat.com>