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

Commit Message

Andrew Morton Nov. 5, 2021, 8:36 p.m. UTC

From: David Howells <[e-mail]>
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/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: David Howells <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Reviewed-by: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

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

Patch

--- 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;