linux-kernel - Re: [RFC][PATCH v6 1/7] drm: Add a sharable drm page-pool implementation

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Date:   Tue, 9 Feb 2021 09:52:35 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Christian König <christian.koenig@....com>
Cc:     John Stultz <john.stultz@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Laura Abbott <labbott@...nel.org>,
        Brian Starkey <Brian.Starkey@....com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Daniel Mentz <danielmentz@...gle.com>,
        Ørjan Eide <orjan.eide@....com>,
        Robin Murphy <robin.murphy@....com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Simon Ser <contact@...rsion.fr>,
        James Jones <jajones@...dia.com>,
        linux-media <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [RFC][PATCH v6 1/7] drm: Add a sharable drm page-pool implementation

On Tue, Feb 9, 2021 at 9:46 AM Christian König <christian.koenig@....com> wrote:
>
>
>
> Am 09.02.21 um 18:33 schrieb Suren Baghdasaryan:
> > On Tue, Feb 9, 2021 at 4:57 AM Christian König <christian.koenig@....com> wrote:
> >> Am 09.02.21 um 13:11 schrieb Christian König:
> >>> [SNIP]
> >>>>>> +void drm_page_pool_add(struct drm_page_pool *pool, struct page *page)
> >>>>>> +{
> >>>>>> +     spin_lock(&pool->lock);
> >>>>>> +     list_add_tail(&page->lru, &pool->items);
> >>>>>> +     pool->count++;
> >>>>>> +     atomic_long_add(1 << pool->order, &total_pages);
> >>>>>> +     spin_unlock(&pool->lock);
> >>>>>> +
> >>>>>> +     mod_node_page_state(page_pgdat(page),
> >>>>>> NR_KERNEL_MISC_RECLAIMABLE,
> >>>>>> +                         1 << pool->order);
> >>>>> Hui what? What should that be good for?
> >>>> This is a carryover from the ION page pool implementation:
> >>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Ftree%2Fdrivers%2Fstaging%2Fandroid%2Fion%2Fion_page_pool.c%3Fh%3Dv5.10%23n28&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7Cdccccff8edcd4d147a5b08d8cd20cff2%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637484888114923580%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=9%2BIBC0tezSV6Ci4S3kWfW%2BQvJm4mdunn3dF6C0kyfCw%3D&amp;reserved=0
> >>>>
> >>>>
> >>>> My sense is it helps with the vmstat/meminfo accounting so folks can
> >>>> see the cached pages are shrinkable/freeable. This maybe falls under
> >>>> other dmabuf accounting/stats discussions, so I'm happy to remove it
> >>>> for now, or let the drivers using the shared page pool logic handle
> >>>> the accounting themselves?
> >> Intentionally separated the discussion for that here.
> >>
> >> As far as I can see this is just bluntly incorrect.
> >>
> >> Either the page is reclaimable or it is part of our pool and freeable
> >> through the shrinker, but never ever both.
> > IIRC the original motivation for counting ION pooled pages as
> > reclaimable was to include them into /proc/meminfo's MemAvailable
> > calculations. NR_KERNEL_MISC_RECLAIMABLE defined as "reclaimable
> > non-slab kernel pages" seems like a good place to account for them but
> > I might be wrong.
>
> Yeah, that's what I see here as well. But exactly that is utterly nonsense.
>
> Those pages are not "free" in the sense that get_free_page could return
> them directly.

Any ideas where these pages would fit better? We do want to know that
under memory pressure these pages can be made available (which is I
think what MemAvailable means).

>
> Regards,
> Christian.
>
> >
> >> In the best case this just messes up the accounting, in the worst case
> >> it can cause memory corruption.
> >>
> >> Christian.
>