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Subject[PATCH v2] mm: Stop filemap_read() from grabbing a superfluous page
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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.

Changes:
v2) Break out of the loop immediately rather than going to put_pages (the
pvec is unoccupied). Setting isize is then unnecessary.

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

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

diff --git a/mm/filemap.c b/mm/filemap.c
index dae481293b5d..e50be519f6a4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2625,6 +2625,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
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;

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