Patch "mm: make sendfile(2) killable" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

mm: make sendfile(2) killable

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-make-sendfile-2-killable.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jan Kara <[email protected]>
Date: Thu, 22 Oct 2015 13:32:21 -0700
Subject: mm: make sendfile(2) killable

From: Jan Kara <[email protected]>

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.

---
 mm/filemap.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

@@ -2509,6 +2509,11 @@ 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))

@@ -2546,10 +2551,6 @@ 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;

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/mm-make-sendfile-2-killable.patch