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

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Message-ID: <CAJuCfpE-T4Cs_h6LfrgHE+T_iOVywU2oNzLquYETudOaBMauMQ@mail.gmail.com>
Date:   Tue, 9 Feb 2021 09:33:15 -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 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%7Cc4eadb0a9cf6491d99ba08d8ca173457%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637481548325174885%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=FUjZK5NSDMUYfU7vGeE4fDU2HCF%2FYyNBwc30aoLLPQ4%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.

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