[PATCH] generic_file_buffered_write(): deadlock on vectored write

generic_file_buffered_write() prefaults in user pages in order to avoid
deadlock on copying from the same page as write goes to.

However, it looks like there is a problem when write is vectored:
fault_in_pages_readable brings in current segment or its part (maxlen).
OTOH, filemap_copy_from_user_iovec is called to copy number of bytes
(bytes) which may exceed current segment, so filemap_copy_from_user_iovec
switches to the next segment which is not brought in yet.  Pagefault is
generated.  That causes the deadlock if pagefault is for the same page
write goes to: page being written is locked and not uptodate, pagefault
will deadlock trying to lock locked page.

[akpm@osdl.org: somewhat rewritten]
Cc: Neil Brown <neilb@suse.de>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 7 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 9c7334bafda8..d504d6e98886 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2095,14 +2095,21 @@ 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;
-                if (bytes > count)
-                        bytes = count;
+
+                /* 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);
 
                 /*
                  * Bring in the user page that we will copy from _first_.
@@ -2110,10 +2117,7 @@ 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.
                  */
-                maxlen = cur_iov->iov_len - iov_base;
-                if (maxlen > bytes)
-                        maxlen = bytes;
-                fault_in_pages_readable(buf, maxlen);
+                fault_in_pages_readable(buf, bytes);
 
                 page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
                 if (!page) {