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

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