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From: John Hubbard <jhubbard@nvidia.com>
To: Jerome Glisse <jglisse@redhat.com>, Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Dan Williams <dan.j.williams@intel.com>,
	John Hubbard <john.hubbard@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>, <tom@talpey.com>,
	Al Viro <viro@zeniv.linux.org.uk>, <benve@cisco.com>,
	Christoph Hellwig <hch@infradead.org>,
	Christopher Lameter <cl@linux.com>,
	"Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>,
	<mike.marciniszyn@intel.com>, <rcampbell@nvidia.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Date: Mon, 14 Jan 2019 11:09:20 -0800
Message-ID: <fdece7f8-7e4f-f679-821f-1d05ed748c15@nvidia.com> (raw)
In-Reply-To: <20190114172124.GA3702@redhat.com>

On 1/14/19 9:21 AM, Jerome Glisse wrote:
> On Mon, Jan 14, 2019 at 03:54:47PM +0100, Jan Kara wrote:
>> On Fri 11-01-19 19:06:08, John Hubbard wrote:
>>> On 1/11/19 6:46 PM, Jerome Glisse wrote:
>>>> On Fri, Jan 11, 2019 at 06:38:44PM -0800, John Hubbard wrote:
>>>> [...]
>>>>
>>>>>>> The other idea that you and Dan (and maybe others) pointed out was a debug
>>>>>>> option, which we'll certainly need in order to safely convert all the call
>>>>>>> sites. (Mirror the mappings at a different kernel offset, so that put_page()
>>>>>>> and put_user_page() can verify that the right call was made.)  That will be
>>>>>>> a separate patchset, as you recommended.
>>>>>>>
>>>>>>> I'll even go as far as recommending the page lock itself. I realize that this 
>>>>>>> adds overhead to gup(), but we *must* hold off page_mkclean(), and I believe
>>>>>>> that this (below) has similar overhead to the notes above--but is *much* easier
>>>>>>> to verify correct. (If the page lock is unacceptable due to being so widely used,
>>>>>>> then I'd recommend using another page bit to do the same thing.)
>>>>>>
>>>>>> Please page lock is pointless and it will not work for GUP fast. The above
>>>>>> scheme do work and is fine. I spend the day again thinking about all memory
>>>>>> ordering and i do not see any issues.
>>>>>>
>>>>>
>>>>> Why is it that page lock cannot be used for gup fast, btw?
>>>>
>>>> Well it can not happen within the preempt disable section. But after
>>>> as a post pass before GUP_fast return and after reenabling preempt then
>>>> it is fine like it would be for regular GUP. But locking page for GUP
>>>> is also likely to slow down some workload (with direct-IO).
>>>>
>>>
>>> Right, and so to crux of the matter: taking an uncontended page lock
>>> involves pretty much the same set of operations that your approach does.
>>> (If gup ends up contended with the page lock for other reasons than these
>>> paths, that seems surprising.) I'd expect very similar performance.
>>>
>>> But the page lock approach leads to really dramatically simpler code (and
>>> code reviews, let's not forget). Any objection to my going that
>>> direction, and keeping this idea as a Plan B? I think the next step will
>>> be, once again, to gather some performance metrics, so maybe that will
>>> help us decide.
>>
>> FWIW I agree that using page lock for protecting page pinning (and thus
>> avoid races with page_mkclean()) looks simpler to me as well and I'm not
>> convinced there will be measurable difference to the more complex scheme
>> with barriers Jerome suggests unless that page lock contended. Jerome is
>> right that you cannot just do lock_page() in gup_fast() path. There you
>> have to do trylock_page() and if that fails just bail out to the slow gup
>> path.
>>

Yes, understood about gup fast.

>> Regarding places other than page_mkclean() that need to check pinned state:
>> Definitely page migration will want to check whether the page is pinned or
>> not so that it can deal differently with short-term page references vs
>> longer-term pins.

OK.

>>
>> Also there is one more idea I had how to record number of pins in the page:
>>
>> #define PAGE_PIN_BIAS	1024
>>
>> get_page_pin()
>> 	atomic_add(&page->_refcount, PAGE_PIN_BIAS);
>>
>> put_page_pin();
>> 	atomic_add(&page->_refcount, -PAGE_PIN_BIAS);
>>
>> page_pinned(page)
>> 	(atomic_read(&page->_refcount) - page_mapcount(page)) > PAGE_PIN_BIAS
>>
>> This is pretty trivial scheme. It still gives us 22-bits for page pins
>> which should be plenty (but we should check for that and bail with error if
>> it would overflow). Also there will be no false negatives and false
>> positives only if there are more than 1024 non-page-table references to the
>> page which I expect to be rare (we might want to also subtract
>> hpage_nr_pages() for radix tree references to avoid excessive false
>> positives for huge pages although at this point I don't think they would
>> matter). Thoughts?
> 
> Racing PUP are as likely to cause issues:
> 
> CPU0                        | CPU1       | CPU2
>                             |            |
>                             | PUP()      |
>     page_pinned(page)       |            |
>       (page_count(page) -   |            |
>        page_mapcount(page)) |            |
>                             |            | GUP()
> 
> So here the refcount snap-shot does not include the second GUP and
> we can have a false negative ie the page_pinned() will return false
> because of the PUP happening just before on CPU1 despite the racing
> GUP on CPU2 just after.
> 
> I believe only either lock or memory ordering with barrier can
> guarantee that we do not miss GUP ie no false negative. Still the
> bias idea might be usefull as with it we should not need a flag.
> 
> So to make the above safe it would still need the page write back
> double check that i described so that GUP back-off if it raced with
> page_mkclean,clear_page_dirty_for_io and the fs write page call back
> which call test_set_page_writeback() (yes it is very unlikely but
> might still happen).
> 
> 
> I still need to ponder some more on all the races.
> 

Tentatively, so far I prefer the _mapcount scheme, because it seems more
accurate to add mapcounts than to overload the _refcount field. And the 
implementation is going to be cleaner. And we've already figured out the
races.

For example, the following already survives a basic boot to graphics mode.
It requires a bunch of callsite conversions, and a page flag (neither of which
is shown here), and may also have "a few" gross conceptual errors, but take a 
peek:

From 1b6e611238a45badda7e63d3ffc089cefb621cb2 Mon Sep 17 00:00:00 2001
From: John Hubbard <jhubbard@nvidia.com>
Date: Sun, 13 Jan 2019 15:10:31 -0800
Subject: [PATCH 2/2] mm: track gup-pinned pages
X-NVConfidentiality: public
Cc: John Hubbard <jhubbard@nvidia.com>

Track GUP-pinned pages.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  8 ++++---
 mm/gup.c           | 59 +++++++++++++++++++++++++++++++++++++++++++---
 mm/rmap.c          | 23 ++++++++++++++----
 3 files changed, 79 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 809b7397d41e..3221a13b4891 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1004,12 +1004,14 @@ static inline void put_page(struct page *page)
  * particular, interactions with RDMA and filesystems need special
  * handling.
  *
- * put_user_page() and put_page() are not interchangeable, despite this early
- * implementation that makes them look the same. put_user_page() calls must
- * be perfectly matched up with get_user_page() calls.
+ * put_user_page() and put_page() are not interchangeable. put_user_page()
+ * calls must be perfectly matched up with get_user_page() calls.
  */
 static inline void put_user_page(struct page *page)
 {
+	page = compound_head(page);
+
+	atomic_dec(&page->_mapcount);
 	put_page(page);
 }
 
diff --git a/mm/gup.c b/mm/gup.c
index 05acd7e2eb22..af3909814be7 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -615,6 +615,48 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 	return 0;
 }
 
+/*
+ * Manages the PG_gup_pinned flag.
+ *
+ * Note that page->_mapcount counting part of managing that flag, because the
+ * _mapcount is used to determine if PG_gup_pinned can be cleared, in
+ * page_mkclean().
+ */
+static void track_gup_page(struct page *page)
+{
+	page = compound_head(page);
+
+	lock_page(page);
+
+	wait_on_page_writeback(page);
+
+	atomic_inc(&page->_mapcount);
+	SetPageGupPinned(page);
+
+	unlock_page(page);
+}
+
+/*
+ * A variant of track_gup_page() that returns -EBUSY, instead of waiting.
+ */
+static int track_gup_page_atomic(struct page *page)
+{
+	page = compound_head(page);
+
+	if (PageWriteback(page) || !trylock_page(page))
+		return -EBUSY;
+
+	if (PageWriteback(page)) {
+		unlock_page(page);
+		return -EBUSY;
+	}
+	atomic_inc(&page->_mapcount);
+	SetPageGupPinned(page);
+
+	unlock_page(page);
+	return 0;
+}
+
 /**
  * __get_user_pages() - pin user pages in memory
  * @tsk:	task_struct of target task
@@ -761,6 +803,9 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			ret = PTR_ERR(page);
 			goto out;
 		}
+
+		track_gup_page(page);
+
 		if (pages) {
 			pages[i] = page;
 			flush_anon_page(vma, page, start);
@@ -1439,6 +1484,11 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 
+		if (track_gup_page_atomic(page)) {
+			put_page(head);
+			goto pte_unmap;
+		}
+
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		(*nr)++;
@@ -1574,7 +1624,8 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 		return 0;
 	}
 
-	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
+	if (unlikely(pmd_val(orig) != pmd_val(*pmdp)) ||
+	    track_gup_page_atomic(head)) {
 		*nr -= refs;
 		while (refs--)
 			put_page(head);
@@ -1612,7 +1663,8 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 		return 0;
 	}
 
-	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
+	if (unlikely(pud_val(orig) != pud_val(*pudp)) ||
+	    track_gup_page_atomic(head)) {
 		*nr -= refs;
 		while (refs--)
 			put_page(head);
@@ -1649,7 +1701,8 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 		return 0;
 	}
 
-	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
+	if (unlikely(pgd_val(orig) != pgd_val(*pgdp)) ||
+	    track_gup_page_atomic(head)) {
 		*nr -= refs;
 		while (refs--)
 			put_page(head);
diff --git a/mm/rmap.c b/mm/rmap.c
index 0454ecc29537..434283898bb0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -880,6 +880,11 @@ int page_referenced(struct page *page,
 	return pra.referenced;
 }
 
+struct page_mkclean_args {
+	int cleaned;
+	int mapcount;
+};
+
 static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
 			    unsigned long address, void *arg)
 {
@@ -890,7 +895,7 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
 		.flags = PVMW_SYNC,
 	};
 	struct mmu_notifier_range range;
-	int *cleaned = arg;
+	struct page_mkclean_args *pma = arg;
 
 	/*
 	 * We have to assume the worse case ie pmd for invalidation. Note that
@@ -940,6 +945,8 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
 #endif
 		}
 
+		pma->mapcount++;
+
 		/*
 		 * No need to call mmu_notifier_invalidate_range() as we are
 		 * downgrading page table protection not changing it to point
@@ -948,7 +955,7 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
 		 * See Documentation/vm/mmu_notifier.rst
 		 */
 		if (ret)
-			(*cleaned)++;
+			pma->cleaned++;
 	}
 
 	mmu_notifier_invalidate_range_end(&range);
@@ -966,10 +973,13 @@ static bool invalid_mkclean_vma(struct vm_area_struct *vma, void *arg)
 
 int page_mkclean(struct page *page)
 {
-	int cleaned = 0;
+	struct page_mkclean_args pma = {
+		.cleaned = 0,
+		.mapcount = 0
+	};
 	struct address_space *mapping;
 	struct rmap_walk_control rwc = {
-		.arg = (void *)&cleaned,
+		.arg = (void *)&pma,
 		.rmap_one = page_mkclean_one,
 		.invalid_vma = invalid_mkclean_vma,
 	};
@@ -985,7 +995,10 @@ int page_mkclean(struct page *page)
 
 	rmap_walk(page, &rwc);
 
-	return cleaned;
+	if (pma.mapcount == page_mapcount(page))
+		ClearPageGupPinned(page);
+
+	return pma.cleaned;
 }
 EXPORT_SYMBOL_GPL(page_mkclean);
 
-- 
2.20.1



thanks,
-- 
John Hubbard
NVIDIA

  reply index

Thread overview: 179+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-04  0:17 [PATCH 0/2] put_user_page*(): start converting the call sites john.hubbard
2018-12-04  0:17 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2018-12-04  7:53   ` Mike Rapoport
2018-12-05  1:40     ` John Hubbard
2018-12-04 20:28   ` Dan Williams
2018-12-04 21:56     ` John Hubbard
2018-12-04 23:03       ` Dan Williams
2018-12-05  0:36         ` Jerome Glisse
2018-12-05  0:40           ` Dan Williams
2018-12-05  0:59             ` John Hubbard
2018-12-05  0:58         ` John Hubbard
2018-12-05  1:00           ` Dan Williams
2018-12-05  1:15           ` Matthew Wilcox
2018-12-05  1:44             ` Jerome Glisse
2018-12-05  1:57               ` John Hubbard
2018-12-07  2:45                 ` John Hubbard
2018-12-07 19:16                   ` Jerome Glisse
2018-12-07 19:26                     ` Dan Williams
2018-12-07 19:40                       ` Jerome Glisse
2018-12-08  0:52                     ` John Hubbard
2018-12-08  2:24                       ` Jerome Glisse
2018-12-10 10:28                         ` Jan Kara
2018-12-12 15:03                           ` Jerome Glisse
2018-12-12 16:27                             ` Dan Williams
2018-12-12 17:02                               ` Jerome Glisse
2018-12-12 17:49                                 ` Dan Williams
2018-12-12 19:07                                   ` John Hubbard
2018-12-12 21:30                               ` Jerome Glisse
2018-12-12 21:40                                 ` Dan Williams
2018-12-12 21:53                                   ` Jerome Glisse
2018-12-12 22:11                                     ` Matthew Wilcox
2018-12-12 22:16                                       ` Jerome Glisse
2018-12-12 23:37                                     ` Jason Gunthorpe
2018-12-12 23:46                                       ` John Hubbard
2018-12-12 23:54                                       ` Dan Williams
2018-12-13  0:01                                       ` Jerome Glisse
2018-12-13  0:18                                         ` Dan Williams
2018-12-13  0:44                                           ` Jerome Glisse
2018-12-13  3:26                                             ` Jason Gunthorpe
2018-12-13  3:20                                         ` Jason Gunthorpe
2018-12-13 12:43                                           ` Jerome Glisse
2018-12-13 13:40                                             ` Tom Talpey
2018-12-13 14:18                                               ` Jerome Glisse
2018-12-13 14:51                                                 ` Tom Talpey
2018-12-13 15:18                                                   ` Jerome Glisse
2018-12-13 18:12                                                     ` Tom Talpey
2018-12-13 19:18                                                       ` Jerome Glisse
2018-12-14 10:41                                             ` Jan Kara
2018-12-14 15:25                                               ` Jerome Glisse
2018-12-12 21:56                                 ` John Hubbard
2018-12-12 22:04                                   ` Jerome Glisse
2018-12-12 22:11                                     ` John Hubbard
2018-12-12 22:14                                       ` Jerome Glisse
2018-12-12 22:17                                         ` John Hubbard
2018-12-12 21:46                             ` Dave Chinner
2018-12-12 21:59                               ` Jerome Glisse
2018-12-13  0:51                                 ` Dave Chinner
2018-12-13  2:02                                   ` Jerome Glisse
2018-12-13 15:56                                     ` Christopher Lameter
2018-12-13 16:02                                       ` Jerome Glisse
2018-12-14  6:00                                     ` Dave Chinner
2018-12-14 15:13                                       ` Jerome Glisse
2018-12-14  3:52                                   ` John Hubbard
2018-12-14  5:21                                     ` Dan Williams
2018-12-14  6:11                                       ` John Hubbard
2018-12-14 15:20                                         ` Jerome Glisse
2018-12-14 19:38                                         ` Dan Williams
2018-12-14 19:48                                           ` Matthew Wilcox
2018-12-14 19:53                                             ` Dave Hansen
2018-12-14 20:03                                               ` Matthew Wilcox
2018-12-14 20:17                                                 ` Dan Williams
2018-12-14 20:29                                                   ` Matthew Wilcox
2018-12-15  0:41                                                 ` John Hubbard
2018-12-17  8:56                                           ` Jan Kara
2018-12-17 18:28                                             ` Dan Williams
2018-12-14 15:43                               ` Jan Kara
2018-12-16 21:58                                 ` Dave Chinner
2018-12-17 18:11                                   ` Jerome Glisse
2018-12-17 18:34                                     ` Matthew Wilcox
2018-12-17 19:48                                       ` Jerome Glisse
2018-12-17 19:51                                         ` Matthew Wilcox
2018-12-17 19:54                                           ` Jerome Glisse
2018-12-17 19:59                                             ` Matthew Wilcox
2018-12-17 20:55                                               ` Jerome Glisse
2018-12-17 21:03                                                 ` Matthew Wilcox
2018-12-17 21:15                                                   ` Jerome Glisse
2018-12-18  1:09                                       ` Dave Chinner
2018-12-18  6:12                                       ` Darrick J. Wong
2018-12-18  9:30                                       ` Jan Kara
2018-12-18 23:29                                         ` John Hubbard
2018-12-19  2:07                                           ` Jerome Glisse
2018-12-19 11:08                                             ` Jan Kara
2018-12-20 10:54                                               ` John Hubbard
2018-12-20 16:50                                                 ` Jerome Glisse
2018-12-20 16:57                                                   ` Dan Williams
2018-12-20 16:49                                               ` Jerome Glisse
2019-01-03  1:55                                               ` Jerome Glisse
2019-01-03  3:27                                                 ` John Hubbard
2019-01-03 14:57                                                   ` Jerome Glisse
2019-01-03  9:26                                                 ` Jan Kara
2019-01-03 14:44                                                   ` Jerome Glisse
2019-01-11  2:59                                                     ` John Hubbard
2019-01-11 16:51                                                       ` Jerome Glisse
2019-01-12  1:04                                                         ` John Hubbard
2019-01-12  2:02                                                           ` Jerome Glisse
2019-01-12  2:38                                                             ` John Hubbard
2019-01-12  2:46                                                               ` Jerome Glisse
2019-01-12  3:06                                                                 ` John Hubbard
2019-01-12  3:25                                                                   ` Jerome Glisse
2019-01-12 20:46                                                                     ` John Hubbard
2019-01-14 14:54                                                                   ` Jan Kara
2019-01-14 17:21                                                                     ` Jerome Glisse
2019-01-14 19:09                                                                       ` John Hubbard [this message]
2019-01-15  8:34                                                                         ` Jan Kara
2019-01-15 21:39                                                                           ` John Hubbard
2019-01-15  8:07                                                                       ` Jan Kara
2019-01-15 17:15                                                                         ` Jerome Glisse
2019-01-15 21:56                                                                           ` John Hubbard
2019-01-15 22:12                                                                             ` Jerome Glisse
2019-01-16  0:44                                                                               ` John Hubbard
2019-01-16  1:56                                                                                 ` Jerome Glisse
2019-01-16  2:01                                                                                   ` Dan Williams
2019-01-16  2:23                                                                                     ` Jerome Glisse
2019-01-16  4:34                                                                                       ` Dave Chinner
2019-01-16 14:50                                                                                         ` Jerome Glisse
2019-01-16 22:51                                                                                           ` Dave Chinner
2019-01-16 11:38                                                                         ` Jan Kara
2019-01-16 13:08                                                                           ` Jerome Glisse
2019-01-17  5:42                                                                             ` John Hubbard
2019-01-17 15:21                                                                               ` Jerome Glisse
2019-01-18  0:16                                                                                 ` Dave Chinner
2019-01-18  1:59                                                                                   ` Jerome Glisse
2019-01-17  9:30                                                                             ` Jan Kara
2019-01-17 15:17                                                                               ` Jerome Glisse
2019-01-22 15:24                                                                                 ` Jan Kara
2019-01-22 16:46                                                                                   ` Jerome Glisse
2019-01-23 18:02                                                                                     ` Jan Kara
2019-01-23 19:04                                                                                       ` Jerome Glisse
2019-01-29  0:22                                                                                         ` John Hubbard
2019-01-29  1:23                                                                                           ` Jerome Glisse
2019-01-29  6:41                                                                                             ` John Hubbard
2019-01-29 10:12                                                                                               ` Jan Kara
2019-01-30  2:21                                                                                                 ` John Hubbard
2019-01-17  5:25                                                                         ` John Hubbard
2019-01-17  9:04                                                                           ` Jan Kara
2019-01-12  3:14                                                               ` Jerome Glisse
2018-12-18 10:33                                   ` Jan Kara
2018-12-18 23:42                                     ` Dave Chinner
2018-12-19  3:03                                       ` Jason Gunthorpe
2018-12-19  5:26                                         ` Dan Williams
2018-12-19 11:19                                           ` Jan Kara
2018-12-19 10:28                                         ` Dave Chinner
2018-12-19 11:35                                           ` Jan Kara
2018-12-19 16:56                                             ` Jason Gunthorpe
2018-12-19 22:33                                             ` Dave Chinner
2018-12-20  9:07                                               ` Jan Kara
2018-12-20 16:54                                               ` Jerome Glisse
2018-12-19 13:24                                       ` Jan Kara
2018-12-08  5:18                       ` Matthew Wilcox
2018-12-12 19:13                         ` John Hubbard
2018-12-08  7:16                       ` Dan Williams
2018-12-08 16:33                         ` Jerome Glisse
2018-12-08 16:48                           ` Christoph Hellwig
2018-12-08 17:47                             ` Jerome Glisse
2018-12-08 18:26                               ` Christoph Hellwig
2018-12-08 18:45                                 ` Jerome Glisse
2018-12-08 18:09                             ` Dan Williams
2018-12-08 18:12                               ` Christoph Hellwig
2018-12-11  6:18                               ` Dave Chinner
2018-12-05  5:52             ` Dan Williams
2018-12-05 11:16       ` Jan Kara
2018-12-04  0:17 ` [PATCH 2/2] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2018-12-04 17:10 ` [PATCH 0/2] put_user_page*(): start converting the call sites David Laight
2018-12-05  1:05   ` John Hubbard
2018-12-05 14:08     ` David Laight
2018-12-28  8:37       ` Pavel Machek
2019-02-08  7:56 [PATCH 0/2] mm: put_user_page() call site conversion first john.hubbard
2019-02-08  7:56 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2019-02-08 10:32   ` Mike Rapoport
2019-02-08 20:44     ` John Hubbard

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