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    SubjectRe: [patch 03/13] io-accounting: write accounting
    Andrew Morton wrote:
    > 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.

    Ah yes indeed. I'm unable to keep up with all the new developments. :-(

    However, if my understanding of this code is correct, it seems that the
    page fault is only done for shared writable VMAs, so you still can't
    rely on set_page_dirty() always being called by the process that
    dirtied the page in the first place.

    Am I wrong?

    -- Suleiman
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