[PATCH 3.2 41/60] mm: make sendfile(2) killable

[Ben Hutchings <ben@decadent.org.uk>]

3.2.73-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@suse.com>

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@suse.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 mm/filemap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2408,6 +2408,11 @@ 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))
@@ -2448,10 +2453,6 @@ 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;