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    SubjectRe: [patch 03/13] io-accounting: write accounting
    On Wed, 13 Dec 2006 00:45:50 -0800
    Suleiman Souhlal <ssouhlal@FreeBSD.org> wrote:

    > akpm@osdl.org wrote:
    > > From: Andrew Morton <akpm@osdl.org>
    > >
    > > Accounting writes is fairly simple: whenever a process flips a page from clean
    > > to dirty, we accuse it of having caused a write to underlying storage of
    > > PAGE_CACHE_SIZE bytes.
    >
    > On architectures where dirtying a page doesn't cause a page fault (like i386), couldn't you end up billing the wrong process (in fact, I think that even on other archituctures set_page_dirty() doesn't get called immediately in the page fault handler)?

    Yes, that would be a problem in 2.6.18 and earlier.

    In 2.6.19 and later, we do take a fault when transitioning a page from
    pte-clean to pte-dirty. That was done to get the dirty-page accounting
    right - to avoid the all-of-memory-is-dirty-but-the-kernel-doesn't-know-it
    problem.


    > AFAICS, set_page_dirty() is mostly called when trying to unmap a page when trying to shrink LRU lists, and there is no guarantee that this happens under the process that dirtied it (in fact, the set_page_dirty() is often done by kswapd).

    hm, that code is still there in zap_pte_range(). If all is well, that
    set_page_dirty() call should never return true. Peter did, you ever test
    for that?

    (Well, it might return true in rare races, because zap_pte_range() doesn't
    lock the pages)
    -
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