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

David Howells
2021-10-18T14:53:37Z

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;