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    SubjectRe: [patch resend v4] update ctime and mtime for mmaped write
    On Mon, 26 Mar 2007 23:43:08 +0200
    Miklos Szeredi <miklos@szeredi.hu> wrote:

    > > > This patch makes writing to shared memory mappings update st_ctime and
    > > > st_mtime as defined by SUSv3:
    > >
    > > Boy this is complicated.
    >
    > You tell me?
    >
    > > Is there a simpler way of doing all this? Say, we define a new page flag
    > > PG_dirtiedbywrite and we do SetPageDirtiedByWrite() inside write() and
    > > ClearPageDirtiedByWrite() whenever we propagate pte-dirtiness into
    > > page-dirtiness. Then, when performing writeback we look to see if any of
    > > the dirty pages are !PageDirtiedByWrite() and, if so, we update [mc]time to
    > > current-time.
    >
    > I don't think a page flag gains anything over the address_space flag
    > that this patch already has.
    >
    > The complexity is not about keeping track of the "data modified
    > through mmap" state, but about msync() guarantees, that POSIX wants.
    >
    > And these requirements do in fact make some sense: msync() basically
    > means:
    >
    > "I want the data written through mmaps to be visible to the world"
    >
    > And that obviously includes updating the timestamps.
    >
    > So how do we know if the data was modified between two msync()
    > invocations? The only sane way I can think of is to walk the page
    > tables in msync() and test/clear the pte dirty bit.

    clear_page_dirty_for_io() already does that.

    So we should be able to test PageDirtiedByWrite() after running
    clear_page_dirty_for_io() to discover whether this page was dirtied via
    MAP_SHARED, and then update the inode times if so.

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