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Subject: Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()
To:     Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>
Cc:     john.hubbard@gmail.com, Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jerome Glisse <jglisse@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>,
        Daniel Black <daniel@linux.ibm.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
References: <20190805222019.28592-1-jhubbard@nvidia.com>
 <20190805222019.28592-2-jhubbard@nvidia.com>
 <20190807110147.GT11812@dhcp22.suse.cz>
 <01b5ed91-a8f7-6b36-a068-31870c05aad6@nvidia.com>
 <20190808062155.GF11812@dhcp22.suse.cz>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <875dca95-b037-d0c7-38bc-4b4c4deea2c7@suse.cz>
Date:   Thu, 8 Aug 2019 13:09:20 +0200
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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@gmail.com wrote:
>>>> From: John Hubbard <jhubbard@nvidia.com>
>>>>
>>>> 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?