Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

From: Vlastimil Babka <vbabka@suse.cz>
To: Roman Gushchin <guro@fb.com>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>,
	vinayak menon <vinayakm.list@gmail.com>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES
Date: Wed, 25 Apr 2018 19:02:42 +0200	[thread overview]
Message-ID: <7fc2986e-b867-eb32-9124-d10ef6c1a3a3@suse.cz> (raw)
In-Reply-To: <20180425164845.GA7223@castle>

On 04/25/2018 06:48 PM, Roman Gushchin wrote:
> On Wed, Apr 25, 2018 at 05:47:26PM +0200, Vlastimil Babka wrote:
>> On 04/25/2018 02:52 PM, Roman Gushchin wrote:
>>> On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
>>>>>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
>>>>>>>> infrastructure, and I definitely wouldn't touch the exposed
>>>>>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
>>>>>>>
>>>>>>> We would be just making the reported values more precise wrt reality.
>>>>>>
>>>>>> It depends on if we believe that only slab memory can be reclaimable
>>>>>> or not. If yes, this is true, otherwise not.
>>>>>>
>>>>>> My guess is that some drivers (e.g. networking) might have buffers,
>>>>>> which are reclaimable under mempressure, and are allocated using
>>>>>> the page allocator. But I have to look closer...
>>>>>>
>>>>>
>>>>> One such case I have encountered is that of the ION page pool. The page pool
>>>>> registers a shrinker. When not in any memory pressure page pool can go high
>>>>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
>>>>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
>>
>> FYI, we have discussed this at LSF/MM and agreed to try the kmalloc
>> reclaimable caches idea. The existing counter could then remain for page
>> allocator users such as ION. It's a bit weird to have it in bytes and
>> not pages then, IMHO. What if we hid it from /proc/vmstat now so it
>> doesn't become ABI, and later convert it to page granularity and expose
>> it under a name such as "nr_other_reclaimable" ?
> 
> I've nothing against hiding it from /proc/vmstat, as long as we keep
> the counter in place and the main issue resolved.

Sure.

> Maybe it's better to add nr_reclaimable = nr_slab_reclaimable + nr_other_reclaimable,
> which will have a simpler meaning that nr_other_reclaimable (what is other?).

"other" can be changed, sure. nr_reclaimable is possible if we change
slab to adjust that counter as well - vmstat code doesn't support
arbitrary calculations when printing.

> Thanks!
> 