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    SubjectRe: [PATCH 13/14] FS-Cache: Release page->private in failed readahead [try #8]
    Date
    Andrew Morton <akpm@osdl.org> wrote:

    > > SetPageLocked(page);
    >
    > if (TestSetPagLocked(page))
    > BUG();
    >
    > would make me more comfortable..

    That shouldn't be necessary if add_to_page_cache() also doesn't do that, but
    if you wish, I can do that - it's the error handling path, so it doesn't
    matter too much performancewise.

    > > > For the second hunk, is it not possible to do this cleanup in the callback
    > > > function?
    > >
    > > Which callback function?
    >
    > I was referring to the filler_t thingy. Is it not possible to get control
    > of that?

    Well, the filler_t thing is generally a_ops->readpage from the caller fs, but
    we don't want to call that if add_to_page_cache() failed, and we don't want to
    call it if we're just discarding a bunch of pages we've now no intention of
    actually reading.

    I suppose we could add another callback for ditching pages we don't want to
    keep. This has the potential to be called quite a lot because of the way
    readahead works on Linux.

    > So please, can we have some comments in there which describe the new
    > behaviour in a manner sufficient for a maintainer to follow so people don't
    > break your stuff?

    Okay... I'll add more comments. I should probably also extend the
    documentation on releasepage(). It won't be till Monday though.

    > > Out of interest, why do we need PG_private to say there's something in
    > > page->private? Can't it just be assumed either that if page->private is
    > > non-zero or that if a_ops->releasepage() is non-NULL, then we need to
    > > "release" the page?
    >
    > page->private is an unsigned long, not a pointer. The core kernel hence
    > cannot determine from its value whether or not it is live. For example, the
    > fs might choose to treat it as a bitmap of which-blocks-are-uptodate and
    > which-blocks-are-dirty.

    Then the second option is still possible (calling releasepage()
    unconditionally).

    David
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