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    SubjectRe: [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch
    On Wed, Dec 31, 2003 at 03:42:39PM -0800, Andrew Morton wrote:
    > Daniel McNeil <daniel@osdl.org> wrote:
    > >
    > > The other potential race in filemap_fdatawait() was that it
    > > removed the page from the locked list while waiting for a writeback
    > > and if there was a 2nd filemap_fdatawait() running on another cpu,
    > > it would not wait for the page being written since it would never see
    > > it on the list.
    >
    > That would only happen if one thread or the other was not running under
    > i_sem. The only path I see doing that is in generic_file_direct_IO()?

    Yes, and we should simply fix generic_file_direct_IO to avoid doing so.
    We anyway issue filemap_fdatawait later with i_sem held.

    The race that we need to worry about is between background writeouts
    (which don't take i_sem) and filemap_fdatawrite/filemap_fdatawait - i.e
    the first one discussed.

    >
    > > + /*
    > > + * If the page is locked, it might be in process of being
    > > + * setup for writeback but without PG_writeback set
    > > + * and with PG_dirty cleared.
    > > + * (PG_dirty is cleared BEFORE PG_writeback is set)
    > > + * So, wait for the PG_locked to clear, then start over.
    > > + */
    > > + if (PageLocked(page)) {
    > > + page_cache_get(page);
    > > + spin_unlock(&mapping->page_lock);
    > > + wait_on_page_locked(page);
    > > + page_cache_release(page);
    > > + goto restart;
    > > + }
    >
    > Why is this a problem which needs addressing here? If some other thread is
    > in the process of starting I/O against this page then the page must have
    > been clean when this thread ran filemap_fdatawrite()?

    This is the same race that we have been discussing (background writer
    pulled this page off io_pages, put it on locked pages but hasn't set
    PG_writeback as yet). To me it seemed that Daniel's solution was just an
    alternative to what you proposed - i.e. adding lock_page() to filemap_fdatawait.
    I have to think a little about the fix -- AFAICS but we are all talking
    about the same (real) problem here.

    Regards
    Suparna

    --
    Suparna Bhattacharya (suparna@in.ibm.com)
    Linux Technology Center
    IBM Software Lab, India

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