On 2/27/20 3:30 AM, Dave Chinner wrote:

On Wed, Feb 26, 2020 at 11:13:53AM -0500, Waiman Long wrote:

quoted

As there is no limit for negative dentries, it is possible that a sizeable
portion of system memory can be tied up in dentry cache slabs. Dentry slabs
are generally recalimable if the dentries are in the LRUs. Still having
too much memory used up by dentries can be problematic:

I don't get it.

Why isn't the solution simply "constrain the application generating
unbound numbers of dentries to a memcg"?

Then when the memcg runs out of memory, it will start reclaiming the
dentries that were allocated inside the memcg that are using all
it's resources, thereby preventing unbound growth of the dentry
cache.

I mean, this sort of resource control is exactly what memcgs are
supposed to be used for and are already used for. I don't see why we
need all this complexity for global dentry resource management when
memcgs should already provide an effective means of managing and
placing bounds on the amount of memory any specific application can
use...

Using memcg is one way to limit the damage. The argument that excessive
negative dentries can push out existing memory objects that can be more
useful if left alone still applies. Daemons that run in the root memcg
has no limitation on how much memory that they can use.

There can also be memcgs with high memory limits and long running
applications. memcg is certainly a useful tool in this regards, but it
doesn't solve all the problem.

Cheers,
Longman