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    SubjectRe: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list
    Date
    On Monday 30 October 2006 15:24, Vasily Averin wrote:
    > Andrew Morton wrote:
    > > On Fri, 27 Oct 2006 18:05:50 +0400
    > >
    > > Vasily Averin <vvs@sw.ru> wrote:
    > >> Virtuozzo/OpenVZ linux kernel team has discovered that umount/remount
    > >> can last for hours looping in shrink_dcache_sb() without much successes.
    > >> Since during shrinking s_umount semaphore is taken lots of other
    > >> unrelated operations like sync can stop working until shrink finished.
    > >
    > > Did you consider altering shrink_dcache_sb() so that it holds onto
    > > dcache_lock and moves all the to-be-pruned dentries onto a private list
    > > in a single pass, then prunes them all outside the lock?
    >
    > At the first glance it is wrong because of 2 reasons:
    > 1) it continues to check the whole global LRU list (we propose to use
    > per-sb LRU, it will provide very quick search)

    Quick search maybe, but your patch adds 2 pointers to each dentry in the
    system... That's pretty expensive, as dentries are already using a *lot* of
    ram.

    Maybe an alternative would be to not have anymore a global dentry_unused, but
    only per-sb unused dentries lists ?


    > 2) we have not any guarantee that someone will add new unused dentries to
    > the list when we prune it outside the lock. And to the contrary, some of
    > unused dentries can be used again. As far as I understand we should hold
    > dcache_lock beginning at the removing dentry from unused_list until
    > dentry_iput() call.
    >
    > David did it inside shrink_dcache_for_umount() just because it have
    > guarantee that all the filesystem operations are finished and new ones
    > cannot be started.

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