[RFC PATCH] mm: Stop filemap_read() from grabbing a superfluous page

[David Howells <dhowells@redhat.com>]

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>
cc: 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;