Re: [PATCH v4 02/31] vmscan: take at least one pass with shrinkers

[Glauber Costa <glommer@parallels.com>]

On 04/30/2013 05:22 PM, Mel Gorman wrote:
> On Sat, Apr 27, 2013 at 03:18:58AM +0400, Glauber Costa wrote:
>> In very low free kernel memory situations, it may be the case that we
>> have less objects to free than our initial batch size. If this is the
>> case, it is better to shrink those, and open space for the new workload
>> then to keep them and fail the new allocations.
>>
>> More specifically, this happens because we encode this in a loop with
>> the condition: "while (total_scan >= batch_size)". So if we are in such
>> a case, we'll not even enter the loop.
>>
>> This patch modifies turns it into a do () while {} loop, that will
>> guarantee that we scan it at least once, while keeping the behaviour
>> exactly the same for the cases in which total_scan > batch_size.
>>
>> Signed-off-by: Glauber Costa <glommer@openvz.org>
>> Reviewed-by: Dave Chinner <david@fromorbit.com>
>> Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
>> CC: "Theodore Ts'o" <tytso@mit.edu>
>> CC: Al Viro <viro@zeniv.linux.org.uk>
> 
> There are two cases where this *might* cause a problem and worth keeping
> an eye out for.
> 

Any test case that you envision that could help bringing those issues
forward should they exist ? (aside from getting it into upstream trees
early?)

> 
> Neither potential issue are the fault of your patch so I'm not going
> to nack. However, bear in mind that we should watch out for bugs where
> processes stall making forward progress because they are constantly
> reallocating inodes or bugs like graphics stalls because their objects
> keep getting fully reclaimed as only a small number exist.
> 
> I suspect that you need this patch because it's very reasonable for a memcg
> to have slabs with tiny numbers of objects you want to reclaim.

Precisely.

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