Re: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries

[Waiman Long]

On 08/16/2017 06:33 AM, Wangkai (Kevin,C) wrote:
>> -----Original Message-----
>> From: linux-fsdevel-owner@xxxxxxxxxxxxxxx
>> [mailto:linux-fsdevel-owner@xxxxxxxxxxxxxxx] On Behalf Of Waiman Long
>> Sent: Wednesday, August 16, 2017 1:15 AM
>> To: Alexander Viro; Jonathan Corbet
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx;
>> linux-fsdevel@xxxxxxxxxxxxxxx; Paul E. McKenney; Andrew Morton; Ingo Molnar;
>> Miklos Szeredi; Matthew Wilcox; Larry Woodman; James Bottomley
>> Subject: Re: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries
>>
>> On 07/28/2017 02:34 PM, Waiman Long wrote:
>>>  v2->v3:
>>>   - Add a faster pruning rate when the free pool is closed to depletion.
>>>   - As suggested by James Bottomley, add an artificial delay waiting
>>>     loop before killing a negative dentry and properly clear the
>>>     DCACHE_KILL_NEGATIVE flag if killing doesn't happen.
>>>   - Add a new patch to track number of negative dentries that are
>>>     forcifully killed.
>>>
>>>  v1->v2:
>>>   - Move the new nr_negative field to the end of dentry_stat_t structure
>>>     as suggested by Matthew Wilcox.
>>>   - With the help of Miklos Szeredi, fix incorrect locking order in
>>>     dentry_kill() by using lock_parent() instead of locking the parent's
>>>     d_lock directly.
>>>   - Correctly account for positive to negative dentry transitions.
>>>   - Automatic pruning of negative dentries will now ignore the reference
>>>     bit in negative dentries but not the regular shrinking.
>>>
>>> A rogue application can potentially create a large number of negative
>>> dentries in the system consuming most of the memory available. This
>>> can impact performance of other applications running on the system.
>>>
>>> This patchset introduces changes to the dcache subsystem to limit the
>>> number of negative dentries allowed to be created thus limiting the
>>> amount of memory that can be consumed by negative dentries.
>>>
>>> Patch 1 tracks the number of negative dentries used and disallow the
>>> creation of more when the limit is reached.
>>>
>>> Patch 2 enables /proc/sys/fs/dentry-state to report the number of
>>> negative dentries in the system.
>>>
>>> Patch 3 enables automatic pruning of negative dentries when it is
>>> close to the limit so that we won't end up killing recently used
>>> negative dentries.
>>>
>>> Patch 4 prevents racing between negative dentry pruning and umount
>>> operation.
>>>
>>> Patch 5 shows the number of forced negative dentry killings in
>>> /proc/sys/fs/dentry-state. End users can then tune the neg_dentry_pc=
>>> kernel boot parameter if they want to reduce forced negative dentry
>>> killings.
>>>
>>> Waiman Long (5):
>>>   fs/dcache: Limit numbers of negative dentries
>>>   fs/dcache: Report negative dentry number in dentry-state
>>>   fs/dcache: Enable automatic pruning of negative dentries
>>>   fs/dcache: Protect negative dentry pruning from racing with umount
>>>   fs/dcache: Track count of negative dentries forcibly killed
>>>
>>>  Documentation/admin-guide/kernel-parameters.txt |   7 +
>>>  fs/dcache.c                                     | 451
>> ++++++++++++++++++++++--
>>>  include/linux/dcache.h                          |   8 +-
>>>  include/linux/list_lru.h                        |   1 +
>>>  mm/list_lru.c                                   |   4 +-
>>>  5 files changed, 435 insertions(+), 36 deletions(-)
>>>
>> I haven't received any comment on this v3 patch for over 2 weeks. Is there
>> anything I can do to make it more ready to be merged?
>>
>> Thanks,
>> Longman
> Hi Longman,
> I am a fresher of fsdevel, about 2 weeks before, I have joined this mail list, recently I have met the same problem of negative dentries, 
> in my opinion, the dentries should be remove together with the files or directories, I don't know you have submit this patch, I have
> another patch about this:
>
> http://marc.info/?l=linux-fsdevel&m=150209902215266&w=2
>
> maybe this is a foo idea...
>
> regards
> Kevin

If you look at the code, the front dentries of the LRU list are removed
when there are too many negative dentries. That includes positive
dentries as well as it is not practical to just remove the negative
dentries.

I have looked at your patch. The dentry of a removed file becomes a
negative dentry. The kernel can keep track of those negative entries and
there is no need to add an additional flag for that.

Cheers,
Longman