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    Subject[PATCH RFC 4/8] fsnotify: stop walking child dentries if remaining tail is negative
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    When notification starts/stops listening events from inode's children it
    have to update dentry->d_flags of all positive child dentries. Scanning
    may took a long time if directory has a lot of negative child dentries.

    This is main beneficiary of sweeping cached negative dentries to the end.

    Before patch:

    nr_dentry = 24172597 24.2M
    nr_buckets = 8388608 2.9 avg
    nr_unused = 24158110 99.9%
    nr_negative = 24142810 99.9%

    inotify time: 0.507182 seconds

    After patch:

    nr_dentry = 24562747 24.6M
    nr_buckets = 8388608 2.9 avg
    nr_unused = 24548714 99.9%
    nr_negative = 24543867 99.9%

    inotify time: 0.000010 seconds

    Negative dentries no longer slow down inotify op at parent directory.

    Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    ---
    fs/notify/fsnotify.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
    index 72d332ce8e12..072974302950 100644
    --- a/fs/notify/fsnotify.c
    +++ b/fs/notify/fsnotify.c
    @@ -127,8 +127,12 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
    * original inode) */
    spin_lock(&alias->d_lock);
    list_for_each_entry(child, &alias->d_subdirs, d_child) {
    - if (!child->d_inode)
    + if (!child->d_inode) {
    + /* all remaining children are negative */
    + if (d_is_tail_negative(child))
    + break;
    continue;
    + }

    spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
    if (watched)
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