linux-kernel - [PATCH] mm/filemap: don't revert iter on -EIOCBQUEUED

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Date:   Sat,  9 Jan 2021 16:00:32 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: [PATCH] mm/filemap: don't revert iter on -EIOCBQUEUED

Currently, if I/O is enqueued for async execution direct paths of
generic_file_{read,write}_iter() will always revert the iter. There are
no users expecting that, and that is also costly. Leave iterators as is
on -EIOCBQUEUED.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 mm/filemap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 5bc672d30143..2e1686e09425 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2633,7 +2633,8 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			iocb->ki_pos += retval;
 			count -= retval;
 		}
-		iov_iter_revert(iter, count - iov_iter_count(iter));
+		if (retval != -EIOCBQUEUED)
+			iov_iter_revert(iter, count - iov_iter_count(iter));
 
 		/*
 		 * Btrfs can have a short DIO read if we encounter
@@ -3458,7 +3459,8 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 		}
 		iocb->ki_pos = pos;
 	}
-	iov_iter_revert(from, write_len - iov_iter_count(from));
+	if (written != -EIOCBQUEUED)
+		iov_iter_revert(from, write_len - iov_iter_count(from));
 out:
 	return written;
 }
-- 
2.24.0