Messages in this thread Patch in this message | | | From | Nick Piggin <> | Subject | [patch 3/9] mm: revert "generic_file_buffered_write(): deadlock on vectored write" | Date | Mon, 29 Jan 2007 11:32:05 +0100 (CET) |
| |
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 @@ -1881,21 +1881,14 @@ 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_. @@ -1903,7 +1896,10 @@ 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) { - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |