On Thu, Apr 12, 2018 at 8:27 PM, Roman Gushchin [off-list ref] wrote:

On Thu, Apr 12, 2018 at 08:52:52AM +0200, Vlastimil Babka wrote:

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On 04/11/2018 03:56 PM, Roman Gushchin wrote:

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On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:

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[+CC linux-api]

On 03/05/2018 02:37 PM, Roman Gushchin wrote:

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This patch introduces a concept of indirectly reclaimable memory
and adds the corresponding memory counter and /proc/vmstat item.

Indirectly reclaimable memory is any sort of memory, used by
the kernel (except of reclaimable slabs), which is actually
reclaimable, i.e. will be released under memory pressure.

The counter is in bytes, as it's not always possible to
count such objects in pages. The name contains BYTES
by analogy to NR_KERNEL_STACK_KB.

Signed-off-by: Roman Gushchin <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com

Hmm, looks like I'm late and this user-visible API change was just
merged. But it's for rc1, so we can still change it, hopefully?

One problem I see with the counter is that it's in bytes, but among
counters that use pages, and the name doesn't indicate it.

Here I just followed "nr_kernel_stack" path, which is measured in kB,
but this is not mentioned in the field name.

Oh, didn't know. Bad example to follow :P

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Then, I don't
see why users should care about the "indirectly" part, as that's just an
implementation detail. It is reclaimable and that's what matters, right?
(I also wanted to complain about lack of Documentation/... update, but
looks like there's no general file about vmstat, ugh)

I agree, that it's a bit weird, and it's probably better to not expose
it at all; but this is how all vm counters work. We do expose them all
in /proc/vmstat. A good number of them is useless until you are not a
mm developer, so it's arguable more "debug info" rather than "api".

Yeah the problem is that once tools start rely on them, they fall under
the "do not break userspace" rule, however we call them. So being
cautious and conservative can't hurt.

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It's definitely not a reason to make them messy.
Does "nr_indirectly_reclaimable_bytes" look better to you?

It still has has the "indirecly" part and feels arbitrary :/

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I also kind of liked the idea from v1 rfc posting that there would be a
separate set of reclaimable kmalloc-X caches for these kind of
allocations. Besides accounting, it should also help reduce memory
fragmentation. The right variant of cache would be detected via
__GFP_RECLAIMABLE.

Well, the downside is that we have to introduce X new caches
just for this particular problem. I'm not strictly against the idea,
but not convinced that it's much better.

Maybe we can find more cases that would benefit from it. Heck, even slab
itself allocates some management structures from the generic kmalloc
caches, and if they are used for reclaimable caches, they could be
tracked as reclaimable as well.

This is a good catch!

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With that in mind, can we at least for now put the (manually maintained)
byte counter in a variable that's not directly exposed via /proc/vmstat,
and then when printing nr_slab_reclaimable, simply add the value
(divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
subtract the same value. This way we would be simply making the existing
counters more precise, in line with their semantics.

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.

Thanks,
Vinayak