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    SubjectRe: [patch 6/6] mm: fix pagecache write deadlocks
    On Fri, Oct 13, 2006 at 06:44:52PM +0200, Nick Piggin wrote:
    > From: Andrew Morton <akpm@osdl.org> and Nick Piggin <npiggin@suse.de>
    >
    > The idea is to modify the core write() code so that it won't take a pagefault
    > while holding a lock on the pagecache page. There are a number of different
    > deadlocks possible if we try to do such a thing:

    Here is a patch to improve the comment a little. This is a pretty tricky
    situation so we must be clear as to why it works.
    --

    Comment was not entirely clear about why we must eliminate all other
    possibilities.

    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
    @@ -1946,12 +1946,19 @@ retry_noprogress:
    if (!PageUptodate(page)) {
    /*
    * If the page is not uptodate, we cannot allow a
    - * partial commit_write, because that might expose
    - * uninitialised data.
    + * partial commit_write because when we unlock the
    + * page below, someone else might bring it uptodate
    + * and we lose our write. We cannot allow a full
    + * commit_write, because that exposes uninitialised
    + * data. We cannot zero the rest of the file and do
    + * a full commit_write because that exposes transient
    + * zeroes.
    *
    - * We will enter the single-segment path below, which
    - * should get the filesystem to bring the page
    - * uputodate for us next time.
    + * Abort the operation entirely with a zero length
    + * commit_write. Retry. We will enter the
    + * single-segment path below, which should get the
    + * filesystem to bring the page uputodate for us next
    + * time.
    */
    if (unlikely(copied != bytes))
    copied = 0;
    -
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