Commit:     6814d7a91289ceb143285975e244a8f00fd3a830
Parent:     4b49643fbb3fa8bf4910f82be02d45e94e8972a4
Author:     Andrew Morton <[EMAIL PROTECTED]>
AuthorDate: Tue Oct 16 01:24:54 2007 -0700
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Tue Oct 16 09:42:54 2007 -0700

    Revert "[PATCH] generic_file_buffered_write(): deadlock on vectored write"
    This reverts commit 6527c2bdf1f833cc18e8f42bd97973d583e4aa83, 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 
      the already-locked page (to bring it up to date) and deadlocks.
      An exploit for this bug is in writev-deadlock-demo.c, in
      (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 
    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 
    <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: Nick Piggin <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
 mm/filemap.c |   18 +++++++-----------
 1 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index caaaa7a..4bf7d1a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -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) {
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