linux-kernel - Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE

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Date:   Wed, 25 Apr 2018 18:23:04 +0100
From:   Roman Gushchin <guro@...com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Vijayanand Jitta <vjitta@...eaurora.org>,
        vinayak menon <vinayakm.list@...il.com>, <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

On Wed, Apr 25, 2018 at 07:02:42PM +0200, Vlastimil Babka wrote:
> 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.

Sure, but even just hiding a value isn't that easy now.
So we have to touch the vmstat printing code anyway.

Thanks!