Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[John Hubbard]

On 12/4/18 3:03 PM, Dan Williams wrote:
> On Tue, Dec 4, 2018 at 1:56 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>>
>> On 12/4/18 12:28 PM, Dan Williams wrote:
>>> On Mon, Dec 3, 2018 at 4:17 PM <john.hubbard@xxxxxxxxx> wrote:
>>>>
>>>> From: John Hubbard <jhubbard@xxxxxxxxxx>
>>>>
>>>> Introduces put_user_page(), which simply calls put_page().
>>>> This provides a way to update all get_user_pages*() callers,
>>>> so that they call put_user_page(), instead of put_page().
>>>>
>>>> Also introduces put_user_pages(), and a few dirty/locked variations,
>>>> as a replacement for release_pages(), and also as a replacement
>>>> for open-coded loops that release multiple pages.
>>>> These may be used for subsequent performance improvements,
>>>> via batching of pages to be released.
>>>>
>>>> This is the first step of fixing the problem described in [1]. The steps
>>>> are:
>>>>
>>>> 1) (This patch): provide put_user_page*() routines, intended to be used
>>>>    for releasing pages that were pinned via get_user_pages*().
>>>>
>>>> 2) Convert all of the call sites for get_user_pages*(), to
>>>>    invoke put_user_page*(), instead of put_page(). This involves dozens of
>>>>    call sites, and will take some time.
>>>>
>>>> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
>>>>    implement tracking of these pages. This tracking will be separate from
>>>>    the existing struct page refcounting.
>>>>
>>>> 4) Use the tracking and identification of these pages, to implement
>>>>    special handling (especially in writeback paths) when the pages are
>>>>    backed by a filesystem. Again, [1] provides details as to why that is
>>>>    desirable.
>>>
>>> I thought at Plumbers we talked about using a page bit to tag pages
>>> that have had their reference count elevated by get_user_pages()? That
>>> way there is no need to distinguish put_page() from put_user_page() it
>>> just happens internally to put_page(). At the conference Matthew was
>>> offering to free up a page bit for this purpose.
>>>
>>
>> ...but then, upon further discussion in that same session, we realized that
>> that doesn't help. You need a reference count. Otherwise a random put_page
>> could affect your dma-pinned pages, etc, etc.
> 
> Ok, sorry, I mis-remembered. So, you're effectively trying to capture
> the end of the page pin event separate from the final 'put' of the
> page? Makes sense.
> 

Yes, that's it exactly.

>> I was not able to actually find any place where a single additional page
>> bit would help our situation, which is why this still uses LRU fields for
>> both the two bits required (the RFC [1] still applies), and the dma_pinned_count.
> 
> Except the LRU fields are already in use for ZONE_DEVICE pages... how
> does this proposal interact with those?

Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?

If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole 
LRU field approach is unusable.


thanks,
-- 
John Hubbard
NVIDIA