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SubjectRe: [PATCH V3 1/2] mm: Add get_user_pages_cma_migrate
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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@linux.ibm.com>
> ---
> 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?


> + 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?

> + *
> + * 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() :(


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

> +
> + 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:"?


> + goto get_user_again;
> + }
> + return ret;
> +}
>

--
Alexey

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