mm: don't read i_size of inode unless we need it

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.dkSigned-off-by: NJens Axboe <axboe@kernel.dk>
Acked-by: NChris 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: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2740,9 +2740,7 @@ 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))
@@ -2774,8 +2772,9 @@ 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;
 	}