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    SubjectRe: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.)
    On Thu, Mar 10, 2011 at 02:24:07PM -0700, Andreas Dilger wrote:
    > On 2011-03-10, at 2:15 PM, Chris Mason wrote:
    > > Excerpts from Vivek Goyal's message of 2011-03-10 14:41:06 -0500:
    > >> On Thu, Mar 10, 2011 at 02:11:15PM -0500, Vivek Goyal wrote:
    > >>>>> I think the person who dirtied the page can store the information in
    > >>>>> page->private (assuming buffer heads were not generated) and if flusher
    > >>>>> thread later ends up generating buffer heads and ends up modifying
    > >>>>> page->private, this can be copied in buffer heads?
    > >>>>
    > >>>> This scares me a bit.
    > >>>>
    > >>>> As I understand it, fs/ code expects total ownership of page->private.
    > >>>> This adds a responsibility for every user to copy the data through and
    > >>>> store it in the buffer head (or anything else). btrfs seems to do
    > >>>> something entirely different in some cases and store a different kind
    > >>>> of value.
    > >>>
    > >>> If filesystems are using page->private for some other purpose also, then
    > >>> I guess we have issues.
    > >>>
    > >>> I am ccing linux-fsdevel to have some feedback on the idea of trying
    > >>> to store cgroup id of page dirtying thread in page->private and/or buffer
    > >>> head for tracking which group originally dirtied the page in IO controller
    > >>> during writeback.
    > >>
    > >> A quick "grep" showed that btrfs, ceph and logfs are using page->private
    > >> for other purposes also.
    > >>
    > >> I was under the impression that either page->private is null or it
    > >> points to buffer heads for the writeback case. So storing the info
    > >> directly in either buffer head directly or first in page->private and
    > >> then transferring it to buffer heads would have helped.
    > >
    > > Right, btrfs has its own uses for page->private, and we expect to own
    > > it. With a proper callback, the FS could store the extra information you
    > > need in out own structs.
    >
    > There is no requirement that page->private ever points to a buffer_head, and Lustre clients use it for its own tracking structure (never touching buffer_heads at all). Any assumption about what a filesystem is storing in page->private in other parts of the code is just broken.

    Andreas,

    As Chris mentioned, will providing callbacks so that filesystem can
    save/restore page->private be reasonable?

    Thanks
    Vivek


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