Re: [PATCH V3 1/2] mm: Add get_user_pages_cma_migrate

[Alexey Kardashevskiy]

On 16/10/2018 18:16, Aneesh Kumar K.V wrote:
> Alexey Kardashevskiy <aik@xxxxxxxxx> writes:
> 
>> On 18/09/2018 21:58, Aneesh Kumar K.V wrote:
>>> This helper does a get_user_pages_fast and if it find pages in the CMA area
>>> it will try to migrate them before taking page reference. This makes sure that
>>> we don't keep non-movable pages (due to page reference count) in the CMA area.
>>> Not able to move pages out of CMA area result in CMA allocation failures.
>>>
>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>>> ---
>>>  include/linux/hugetlb.h |   2 +
>>>  include/linux/migrate.h |   3 +
>>>  mm/hugetlb.c            |   4 +-
>>>  mm/migrate.c            | 132 ++++++++++++++++++++++++++++++++++++++++
>>>  4 files changed, 139 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>>> index 6b68e345f0ca..1abccb1a1ecc 100644
>>> --- a/include/linux/hugetlb.h
>>> +++ b/include/linux/hugetlb.h
>>> @@ -357,6 +357,8 @@ struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
>>>  				nodemask_t *nmask);
>>>  struct page *alloc_huge_page_vma(struct hstate *h, struct vm_area_struct *vma,
>>>  				unsigned long address);
>>> +struct page *alloc_migrate_huge_page(struct hstate *h, gfp_t gfp_mask,
>>> +				     int nid, nodemask_t *nmask);
>>>  int huge_add_to_page_cache(struct page *page, struct address_space *mapping,
>>>  			pgoff_t idx);
>>>  
>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>>> index f2b4abbca55e..d82b35afd2eb 100644
>>> --- a/include/linux/migrate.h
>>> +++ b/include/linux/migrate.h
>>> @@ -286,6 +286,9 @@ static inline int migrate_vma(const struct migrate_vma_ops *ops,
>>>  }
>>>  #endif /* IS_ENABLED(CONFIG_MIGRATE_VMA_HELPER) */
>>>  
>>> +extern int get_user_pages_cma_migrate(unsigned long start, int nr_pages, int write,
>>> +				      struct page **pages);
>>> +
>>>  #endif /* CONFIG_MIGRATION */
>>>  
>>>  #endif /* _LINUX_MIGRATE_H */
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 3c21775f196b..1abbfcb84f66 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -1585,8 +1585,8 @@ static struct page *alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
>>>  	return page;
>>>  }
>>>  
>>> -static struct page *alloc_migrate_huge_page(struct hstate *h, gfp_t gfp_mask,
>>> -		int nid, nodemask_t *nmask)
>>> +struct page *alloc_migrate_huge_page(struct hstate *h, gfp_t gfp_mask,
>>> +				     int nid, nodemask_t *nmask)
>>>  {
>>>  	struct page *page;
>>>  
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index d6a2e89b086a..2f92534ea7a1 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -3006,3 +3006,135 @@ int migrate_vma(const struct migrate_vma_ops *ops,
>>>  }
>>>  EXPORT_SYMBOL(migrate_vma);
>>>  #endif /* defined(MIGRATE_VMA_HELPER) */
>>> +
>>> +static struct page *new_non_cma_page(struct page *page, unsigned long private)
>>> +{
>>> +	/*
>>> +	 * We want to make sure we allocate the new page from the same node
>>> +	 * as the source page.
>>> +	 */
>>> +	int nid = page_to_nid(page);
>>> +	gfp_t gfp_mask = GFP_USER | __GFP_THISNODE;
>>> +
>>> +	if (PageHighMem(page))
>>> +		gfp_mask |= __GFP_HIGHMEM;
>>> +
>>> +	if (PageTransHuge(page)) {
>>> +		struct page *thp;
>>> +		gfp_t thp_gfpmask = GFP_TRANSHUGE | __GFP_THISNODE;
>>> +
>>> +		/*
>>> +		 * Remove the movable mask so that we don't allocate from
>>> +		 * CMA area again.
>>> +		 */
>>> +		thp_gfpmask &= ~__GFP_MOVABLE;
>>> +		thp = __alloc_pages_node(nid, thp_gfpmask, HPAGE_PMD_ORDER);
>>
>>
>> HPAGE_PMD_ORDER is 2MB or 1GB? THP are always that PMD order?
> 
> 2M or 16M. THP is at PMD level.
> 
>>
>>
>>> +		if (!thp)
>>> +			return NULL;
>>> +		prep_transhuge_page(thp);
>>> +		return thp;
>>> +
>>> +#ifdef  CONFIG_HUGETLB_PAGE
>>> +	} else if (PageHuge(page)) {
>>> +
>>> +		struct hstate *h = page_hstate(page);
>>> +		/*
>>> +		 * We don't want to dequeue from the pool because pool pages will
>>> +		 * mostly be from the CMA region.
>>> +		 */
>>> +		return alloc_migrate_huge_page(h, gfp_mask, nid, NULL);
>>> +#endif
>>> +	}
>>> +
>>> +	return __alloc_pages_node(nid, gfp_mask, 0);
>>> +}
>>> +
>>> +/**
>>> + * get_user_pages_cma_migrate() - pin user pages in memory by migrating pages in CMA region
>>> + * @start:	starting user address
>>> + * @nr_pages:	number of pages from start to pin
>>> + * @write:	whether pages will be written to
>>> + * @pages:	array that receives pointers to the pages pinned.
>>> + *		Should be at least nr_pages long.
>>> + *
>>> + * Attempt to pin user pages in memory without taking mm->mmap_sem.
>>> + * If not successful, it will fall back to taking the lock and
>>> + * calling get_user_pages().
>>
>>
>> I do not see any locking or get_user_pages(), hidden somewhere?
>>
> 
> The rules are same as get_user_pages_fast, which does that pin attempt
> without taking mm->mmap_sem. If it fail get_user_pages_fast will take
> the mmap_sem and try to pin the pages. The details are in
> get_user_pages_fast. You can look at get_user_pages_unlocked

Ah, right.


>>> + *
>>> + * If the pinned pages are backed by CMA region, we migrate those pages out,
>>> + * allocating new pages from non-CMA region. This helps in avoiding keeping
>>> + * pages pinned in the CMA region for a long time thereby resulting in
>>> + * CMA allocation failures.
>>> + *
>>> + * Returns number of pages pinned. This may be fewer than the number
>>> + * requested. If nr_pages is 0 or negative, returns 0. If no pages
>>> + * were pinned, returns -errno.
>>> + */
>>> +
>>> +int get_user_pages_cma_migrate(unsigned long start, int nr_pages, int write,
>>> +			       struct page **pages)
>>> +{
>>> +	int i, ret;
>>> +	bool drain_allow = true;
>>> +	bool migrate_allow = true;
>>> +	LIST_HEAD(cma_page_list);
>>> +
>>> +get_user_again:
>>> +	ret = get_user_pages_fast(start, nr_pages, write, pages);
>>> +	if (ret <= 0)
>>> +		return ret;
>>> +
>>> +	for (i = 0; i < ret; ++i) {
>>> +		/*
>>> +		 * If we get a page from the CMA zone, since we are going to
>>> +		 * be pinning these entries, we might as well move them out
>>> +		 * of the CMA zone if possible.
>>> +		 */
>>> +		if (is_migrate_cma_page(pages[i]) && migrate_allow) {
>>> +			if (PageHuge(pages[i]))
>>> +				isolate_huge_page(pages[i], &cma_page_list);
>>> +			else {
>>> +				struct page *head = compound_head(pages[i]);
>>> +
>>> +				if (!PageLRU(head) && drain_allow) {
>>> +					lru_add_drain_all();
>>> +					drain_allow = false;
>>> +				}
>>> +
>>> +				if (!isolate_lru_page(head)) {
>>> +					list_add_tail(&head->lru, &cma_page_list);
>>> +					mod_node_page_state(page_pgdat(head),
>>> +							    NR_ISOLATED_ANON +
>>> +							    page_is_file_cache(head),
>>> +							    hpage_nr_pages(head));
>>
>>
>> Above 10 lines I cannot really comment due to my massive ignorance in
>> this area, especially about what lru_add_drain_all() and
>> mod_node_page_state() :(
> 
> That makes sure we move the pages from per cpu lru vec and add them to
> the right lru list so that we can isolate the pages correctly.

I understand the idea but cannot confirm the correctness :-/


> 
>>
>>
>>> +				}
>>> +			}
>>> +		}
>>> +	}
>>> +	if (!list_empty(&cma_page_list)) {
>>> +		/*
>>> +		 * drop the above get_user_pages reference.
>>> +		 */


btw, can these pages be used by somebody else in this short window
before we migrated and pinned them?


>>> +		for (i = 0; i < ret; ++i)
>>> +			put_page(pages[i]);
>>> +
>>> +		if (migrate_pages(&cma_page_list, new_non_cma_page,
>>> +				  NULL, 0, MIGRATE_SYNC, MR_CONTIG_RANGE)) {
>>> +			/*
>>> +			 * some of the pages failed migration. Do get_user_pages
>>> +			 * without migration.
>>> +			 */
>>> +			migrate_allow = false;
>>
>>
>> migrate_allow seems useless, simply calling get_user_pages_fast() should
>> make the code easier to read imho. And the comment says
>> get_user_pages(), where does this guy hide?
> 
> I didn't get that suggestion. What we want to do here is try to migrate pages as
> long as we find CMA pages in the result of get_user_pages_fast. If we
> failed any migration attempt, don't try to migrate again.


Setting migrate_allow to false here means you jump up, call
get_user_pages_fast() and then run the loop which will do nothing just
because if(...migrate_allow) is false. Instead of jumping up you could
just call get_user_pages_fast().

btw what is migrate_pages() leaves something in cma_page_list (I cannot
see it removing pages)? Won't it loop indefinitely?



> 
>>
>>> +
>>> +			if (!list_empty(&cma_page_list))
>>> +				putback_movable_pages(&cma_page_list);
>>> +		}
>>> +		/*
>>> +		 * We did migrate all the pages, Try to get the page references again
>>> +		 * migrating any new CMA pages which we failed to isolate earlier.
>>> +		 */
>>> +		drain_allow = true;
>>
>> Move this "drain_allow = true" right after "get_user_again:"? 1
>  
>>
>>
>>> +		goto get_user_again;
>>> +	}
>>> +	return ret;
>>> +}
>>>
>>
>> -- 
>> Alexey
> 
> -aneesh
> 

-- 
Alexey