Re: [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch

From: Andrew Morton (akpm_at_osdl.org)
Date: 12/17/03

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    Date:	Tue, 16 Dec 2003 18:03:19 -0800
To: Daniel McNeil <daniel@osdl.org>

    Daniel McNeil <daniel@osdl.org> wrote:
    >
    > I have found something else that might be causing the problem.
    > filemap_fdatawait() skips pages that are not marked PG_writeback.
    > However, when a page is going to be written, PG_dirty is cleared
    > before PG_writeback is set (while the PG_locked is set). So it
    > looks like filemap_fdatawait() can see a page just before it is
    > going to be written and not wait for it. Here is a patch that
    > makes filemap_fdatawait() wait for locked pages as well to make
    > sure it does not missed any pages.

    This filemap_fdatawait() behaviour is as-designed. That function is only
    responsible for waiting on I/O which was initiated prior to it being
    invoked. Because it is designed for fsync(), fdatasync(), O_SYNC, msync(),
    sync(), etc.

    Now, it could be that this behaviour is not appropriate for the O_DIRECT
    sync function - the result of your testing will be interesting.

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