On Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <[email protected]> wrote: > On 07/19/2017 04:24 PM, Miklos Szeredi wrote: >> On Mon, Jul 17, 2017 at 3:39 PM, Waiman Long <[email protected]> wrote: >>> The number of positive dentries is limited by the number of files >>> in the filesystems. The number of negative dentries, however, >>> has no limit other than the total amount of memory available in >>> the system. So a rogue application that generates a lot of negative >>> dentries can potentially exhaust most of the memory available in the >>> system impacting performance on other running applications. >>> >>> To prevent this from happening, the dcache code is now updated to limit >>> the amount of the negative dentries in the LRU lists that can be kept >>> as a percentage of total available system memory. The default is 5% >>> and can be changed by specifying the "neg_dentry_pc=" kernel command >>> line option. >>> >>> Signed-off-by: Waiman Long <[email protected]> >>> --- >> [...] >> >>> @@ -603,7 +698,13 @@ static struct dentry *dentry_kill(struct dentry >>> *dentry) >>> >>> if (!IS_ROOT(dentry)) { >>> parent = dentry->d_parent; >>> - if (unlikely(!spin_trylock(&parent->d_lock))) { >>> + /* >>> + * Force the killing of this negative dentry when >>> + * DCACHE_KILL_NEGATIVE flag is set. >>> + */ >>> + if (unlikely(dentry->d_flags & DCACHE_KILL_NEGATIVE)) { >>> + spin_lock(&parent->d_lock); >> This looks like d_lock ordering problem (should be parent first, child >> second). Why is this needed, anyway? >> > > Yes, that is a bug. I should have used lock_parent() instead.
lock_parent() can release dentry->d_lock, which means it's perfectly useless for this. I still feel forcing free is wrong here. Why not just block until the number of negatives goes below the limit (start reclaim if not already doing so, etc...)? Thanks, Miklos
