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Subject[patch 3/9] mm: revert "generic_file_buffered_write(): deadlock on vectored write"
Date
From: Andrew Morton <akpm@osdl.org>

Revert 6527c2bdf1f833cc18e8f42bd97973d583e4aa83

This patch 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.

Signed-off-by: Andrew Morton <akpm@osdl.org>

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: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -2090,21 +2090,14 @@ generic_file_buffered_write(struct kiocb
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_.
@@ -2112,7 +2105,10 @@ generic_file_buffered_write(struct kiocb
* 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) {
-
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