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

["Christian König" <christian.koenig@amd.com>]

Am 09.02.21 um 18:33 schrieb Suren Baghdasaryan:
> On Tue, Feb 9, 2021 at 4:57 AM Christian König <christian.koenig@amd.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.

Regards,
Christian.

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