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Commit 6814d7a9 authored by Andrew Morton's avatar Andrew Morton Committed by Linus Torvalds
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Revert "[PATCH] generic_file_buffered_write(): deadlock on vectored write" 

This reverts commit 6527c2bd, which
fixed the following bug:

  When prefaulting in the pages in generic_file_buffered_write(), we only
  faulted in the pages for the firts segment of the iovec.  If the second of
  successive segment described a mmapping of the page into which we're
  write()ing, and that page is not up-to-date, the fault handler tries to lock
  the already-locked page (to bring it up to date) and deadlocks.

  An exploit for this bug is in writev-deadlock-demo.c, in
  http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz

.

  (These demos assume blocksize < PAGE_CACHE_SIZE).

The problem with this fix is that it takes the kernel back to doing a single
prepare_write()/commit_write() per iovec segment.  So in the worst case we'll
run prepare_write+commit_write 1024 times where we previously would have run
it once. The other problem with the fix is that it fix all the locking problems.

<insert numbers obtained via ext3-tools's writev-speed.c here>

And apparently this change killed NFS overwrite performance, because, I
suppose, it talks to the server for each prepare_write+commit_write.

So just back that patch out - we'll be fixing the deadlock by other means.

Nick says: also it only ever actually papered over the bug, because after
faulting in the pages, they might be unmapped or reclaimed.

Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4b49643f

mm/filemap.c

+7 −11
Original line number Diff line number Diff line
@@ -1865,21 +1865,14 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, 
	do {

		unsigned long index;

		unsigned long offset;

		unsigned long maxlen;

		size_t copied;



		offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */

		index = pos >> PAGE_CACHE_SHIFT;

		bytes = PAGE_CACHE_SIZE - offset;



		/* Limit the size of the copy to the caller's write size */

		bytes = min(bytes, count);



		/*

		 * Limit the size of the copy to that of the current segment,

		 * because fault_in_pages_readable() doesn't know how to walk

		 * segments.

		 */

		bytes = min(bytes, cur_iov->iov_len - iov_base);

		if (bytes > count)

			bytes = count;



		/*

		 * Bring in the user page that we will copy from _first_.
@@ -1887,7 +1880,10 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, 
		 * same page as we're writing to, without it being marked

		 * up-to-date.

		 */

		fault_in_pages_readable(buf, bytes);

		maxlen = cur_iov->iov_len - iov_base;

		if (maxlen > bytes)

			maxlen = bytes;

		fault_in_pages_readable(buf, maxlen);



		page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);

		if (!page) {