linux-kernel - [PATCH 3.12 08/72] mm: make sendfile(2) killable

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Message-Id: <ef1a1f76f66aeadbf151b60d35f6e63109dd98f2.1448283890.git.jslaby@suse.cz>
Date:	Mon, 23 Nov 2015 14:07:11 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 08/72] mm: make sendfile(2) killable

From: Jan Kara <jack@...e.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 296291cdd1629c308114504b850dc343eabc2782 upstream.

Currently a simple program below issues a sendfile(2) system call which
takes about 62 days to complete in my test KVM instance.

        int fd;
        off_t off = 0;

        fd = open("file", O_RDWR | O_TRUNC | O_SYNC | O_CREAT, 0644);
        ftruncate(fd, 2);
        lseek(fd, 0, SEEK_END);
        sendfile(fd, fd, &off, 0xfffffff);

Now you should not ask kernel to do a stupid stuff like copying 256MB in
2-byte chunks and call fsync(2) after each chunk but if you do, sysadmin
should have a way to stop you.

We actually do have a check for fatal_signal_pending() in
generic_perform_write() which triggers in this path however because we
always succeed in writing something before the check is done, we return
value > 0 from generic_perform_write() and thus the information about
signal gets lost.

Fix the problem by doing the signal check before writing anything.  That
way generic_perform_write() returns -EINTR, the error gets propagated up
and the sendfile loop terminates early.

Signed-off-by: Jan Kara <jack@...e.com>
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Al Viro <viro@...IV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 mm/filemap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index bd08e9bbf347..af9e11ea4ecf 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2511,6 +2511,11 @@ again:
 			break;
 		}
 
+		if (fatal_signal_pending(current)) {
+			status = -EINTR;
+			break;
+		}
+
 		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
 						&page, &fsdata);
 		if (unlikely(status < 0))
@@ -2548,10 +2553,6 @@ again:
 		written += copied;
 
 		balance_dirty_pages_ratelimited(mapping);
-		if (fatal_signal_pending(current)) {
-			status = -EINTR;
-			break;
-		}
 	} while (iov_iter_count(i));
 
 	return written ? written : status;
-- 
2.6.3

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