From 846b1c240bec8ba1e84f8e92acb4de85e2533669 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Fri, 5 Nov 2021 13:37:07 -0700
Subject: [PATCH] mm: don't read i_size of inode unless we need it

References: bsc#1190208 (MM functional and performance backports)
Patch-mainline: v5.16-rc1
Git-commit: 61d0017e5a32d3b13ba3c7becc83a5a9e53cdbe9

We always go through i_size_read(), and we rarely end up needing it.
Push the read to down where we need to check it, which avoids it for
most cases.

It looks like we can even remove this check entirely, which might be
worth pursuing.  But at least this takes it out of the hot path.

Link: https://lkml.kernel.org/r/6b67981f-57d4-c80e-bc07-6020aa601381@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Chris Mason <clm@fb.com>
Cc: Josef Bacik <josef@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/filemap.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index ded1e8240fa7..8ad61538c0b8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2663,9 +2663,7 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		struct file *file = iocb->ki_filp;
 		struct address_space *mapping = file->f_mapping;
 		struct inode *inode = mapping->host;
-		loff_t size;
 
-		size = i_size_read(inode);
 		if (iocb->ki_flags & IOCB_NOWAIT) {
 			if (filemap_range_needs_writeback(mapping, iocb->ki_pos,
 						iocb->ki_pos + count - 1))
@@ -2697,8 +2695,9 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		 * the rest of the read.  Buffered reads will not work for
 		 * DAX files, so don't bother trying.
 		 */
-		if (retval < 0 || !count || iocb->ki_pos >= size ||
-		    IS_DAX(inode))
+		if (retval < 0 || !count || IS_DAX(inode))
+			return retval;
+		if (iocb->ki_pos >= i_size_read(inode))
 			return retval;
 	}