[patch 041/262] mm: stop filemap_read() from grabbing a superfluous page

Andrew Morton
2021-11-05T20:36:52Z

From: David Howells <dhowells@redhat.com>
Subject: mm: stop filemap_read() from grabbing a superfluous page

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.

Link: https://lkml.kernel.org/r/163472463105.3126792.7056099385135786492.stgit@warthog.procyon.org.uk
Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk/
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/filemap.c | 3 +++
1 file changed, 3 insertions(+)

--- a/mm/filemap.c~mm-stop-filemap_read-from-grabbing-a-superfluous-page
+++ a/mm/filemap.c
@@ -2625,6 +2625,9 @@ ssize_t filemap_read(struct kiocb *iocb,
if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
iocb->ki_flags |= IOCB_NOWAIT;

+ if (unlikely(iocb->ki_pos >= i_size_read(inode)))
+ break;
+
error = filemap_get_pages(iocb, iter, &pvec);
if (error < 0)
break;
_