Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

From: Vlastimil Babka
Date: Thu Aug 08 2019 - 07:09:28 EST

On 8/8/19 8:21 AM, Michal Hocko wrote:
> On Wed 07-08-19 16:32:08, John Hubbard wrote:
>> On 8/7/19 4:01 AM, Michal Hocko wrote:
>>> On Mon 05-08-19 15:20:17, john.hubbard@xxxxxxxxx wrote:
>>>> From: John Hubbard <jhubbard@xxxxxxxxxx>
>>>> For pages that were retained via get_user_pages*(), release those pages
>>>> via the new put_user_page*() routines, instead of via put_page() or
>>>> release_pages().
>>> Hmm, this is an interesting code path. There seems to be a mix of pages
>>> in the game. We get one page via follow_page_mask but then other pages
>>> in the range are filled by __munlock_pagevec_fill and that does a direct
>>> pte walk. Is using put_user_page correct in this case? Could you explain
>>> why in the changelog?
>> Actually, I think follow_page_mask() gets all the pages, right? And the
>> get_page() in __munlock_pagevec_fill() is there to allow a pagevec_release()
>> later.
> Maybe I am misreading the code (looking at Linus tree) but munlock_vma_pages_range
> calls follow_page for the start address and then if not THP tries to
> fill up the pagevec with few more pages (up to end), do the shortcut
> via manual pte walk as an optimization and use generic get_page there.

That's true. However, I'm not sure munlocking is where the
put_user_page() machinery is intended to be used anyway? These are
short-term pins for struct page manipulation, not e.g. dirtying of page
contents. Reading commit fc1d8e7cca2d I don't think this case falls
within the reasoning there. Perhaps not all GUP users should be
converted to the planned separate GUP tracking, and instead we should
have a GUP/follow_page_mask() variant that keeps using get_page/put_page?