On 07/19/2017 04:24 PM, Miklos Szeredi wrote:
> On Mon, Jul 17, 2017 at 3:39 PM, Waiman Long <longman@xxxxxxxxxx> 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 <longman@xxxxxxxxxx>
>> ---
> [...]
>
>> @@ -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.
I have a test program that generate a lot of negative dentries
continuously. Using spin_trylock(), it failed most of the time when that
test program was running. So I need to actually acquire the parent's
d_lock to make sure that the offending negative dentry was really
killed. It was there to protect against the worst case situation. I will
update the patch to correct that.
Thanks for spotting this.
Cheers,
Longman
