linux-kernel - [RFC PATCH 20/76] mm: Stop generic_file_buffered

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Date:   Fri, 20 Nov 2020 15:06:53 +0000
From:   David Howells <dhowells@...hat.com>
To:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>
Cc:     dhowells@...hat.com, Jeff Layton <jlayton@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 20/76] mm: Stop generic_file_buffered_read() from grabbing
 a superfluous page

Under some circumstances, generic_file_buffered_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.

Catch the overallocation in the "no_cached_page:" part and prevent it from
happening.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 mm/filemap.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index cfb753955e36..5a63aa1dd71e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2447,6 +2447,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		 * Ok, it wasn't cached, so we need to create a new
 		 * page..
 		 */
+		if ((index << PAGE_SHIFT) >= i_size_read(inode))
+			goto out;
 		page = page_cache_alloc(mapping);
 		if (!page) {
 			error = -ENOMEM;