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SubjectRe: [PATCH v2 19/20] mm, hugetlb: retry if failed to allocate and there is concurrent user
On Fri, Aug 09, 2013 at 06:26:37PM +0900, Joonsoo Kim wrote:
> If parallel fault occur, we can fail to allocate a hugepage,
> because many threads dequeue a hugepage to handle a fault of same address.
> This makes reserved pool shortage just for a little while and this cause
> faulting thread who can get hugepages to get a SIGBUS signal.
>
> To solve this problem, we already have a nice solution, that is,
> a hugetlb_instantiation_mutex. This blocks other threads to dive into
> a fault handler. This solve the problem clearly, but it introduce
> performance degradation, because it serialize all fault handling.
>
> Now, I try to remove a hugetlb_instantiation_mutex to get rid of
> performance degradation. For achieving it, at first, we should ensure that
> no one get a SIGBUS if there are enough hugepages.
>
> For this purpose, if we fail to allocate a new hugepage when there is
> concurrent user, we return just 0, instead of VM_FAULT_SIGBUS. With this,
> these threads defer to get a SIGBUS signal until there is no
> concurrent user, and so, we can ensure that no one get a SIGBUS if there
> are enough hugepages.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e29e28f..981c539 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -242,6 +242,7 @@ struct hstate {
> int next_nid_to_free;
> unsigned int order;
> unsigned long mask;
> + unsigned long nr_dequeue_users;
> unsigned long max_huge_pages;
> unsigned long nr_huge_pages;
> unsigned long free_huge_pages;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8743e5c..0501fe5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -561,6 +561,7 @@ retry_cpuset:
> if (cpuset_zone_allowed_softwall(zone, htlb_alloc_mask)) {
> page = dequeue_huge_page_node(h, zone_to_nid(zone));
> if (page) {
> + h->nr_dequeue_users++;

So, nr_dequeue_users doesn't seem to be incremented in the
alloc_huge_page_node() path. I'm not sure exactly where that's used,
so I'm not sure if it's a problem.

> if (!use_reserve)
> break;
>
> @@ -577,6 +578,16 @@ retry_cpuset:
> return page;
> }
>
> +static void commit_dequeued_huge_page(struct hstate *h, bool do_dequeue)
> +{
> + if (!do_dequeue)
> + return;

Seems like it would be easier to do this test in the callers, but I
doubt it matters much.

> + spin_lock(&hugetlb_lock);
> + h->nr_dequeue_users--;
> + spin_unlock(&hugetlb_lock);
> +}
> +
> static void update_and_free_page(struct hstate *h, struct page *page)
> {
> int i;
> @@ -1110,7 +1121,9 @@ static void vma_commit_reservation(struct hstate *h,
> }
>
> static struct page *alloc_huge_page(struct vm_area_struct *vma,
> - unsigned long addr, int use_reserve)
> + unsigned long addr, int use_reserve,
> + unsigned long *nr_dequeue_users,
> + bool *do_dequeue)
> {
> struct hugepage_subpool *spool = subpool_vma(vma);
> struct hstate *h = hstate_vma(vma);
> @@ -1138,8 +1151,11 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
> return ERR_PTR(-ENOSPC);
> }
> spin_lock(&hugetlb_lock);
> + *do_dequeue = true;
> page = dequeue_huge_page_vma(h, vma, addr, use_reserve);
> if (!page) {
> + *nr_dequeue_users = h->nr_dequeue_users;

So, the nr_dequeue_users parameter is only initialized if !page here.
It's not obvious to me that the callers only use it in hat case.

> + *do_dequeue = false;
> spin_unlock(&hugetlb_lock);
> page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
> if (!page) {

I think the counter also needs to be incremented in the case where we
call alloc_buddy_huge_page() from alloc_huge_page(). Even though it's
new, it gets added to the hugepage pool at this point and could still
be a contended page for the last allocation, unless I'm missing
something.

> @@ -1894,6 +1910,7 @@ void __init hugetlb_add_hstate(unsigned order)
> h->mask = ~((1ULL << (order + PAGE_SHIFT)) - 1);
> h->nr_huge_pages = 0;
> h->free_huge_pages = 0;
> + h->nr_dequeue_users = 0;
> for (i = 0; i < MAX_NUMNODES; ++i)
> INIT_LIST_HEAD(&h->hugepage_freelists[i]);
> INIT_LIST_HEAD(&h->hugepage_activelist);
> @@ -2500,6 +2517,8 @@ static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
> int outside_reserve = 0;
> long chg;
> bool use_reserve = false;
> + unsigned long nr_dequeue_users = 0;
> + bool do_dequeue = false;
> int ret = 0;
> unsigned long mmun_start; /* For mmu_notifiers */
> unsigned long mmun_end; /* For mmu_notifiers */
> @@ -2551,11 +2570,17 @@ retry_avoidcopy:
> use_reserve = !chg;
> }
>
> - new_page = alloc_huge_page(vma, address, use_reserve);
> + new_page = alloc_huge_page(vma, address, use_reserve,
> + &nr_dequeue_users, &do_dequeue);
>
> if (IS_ERR(new_page)) {
> page_cache_release(old_page);
>
> + if (nr_dequeue_users) {
> + ret = 0;
> + goto out_lock;
> + }
> +
> /*
> * If a process owning a MAP_PRIVATE mapping fails to COW,
> * it is due to references held by a child and an insufficient
> @@ -2580,6 +2605,9 @@ retry_avoidcopy:
> WARN_ON_ONCE(1);
> }
>
> + if (use_reserve)
> + WARN_ON_ONCE(1);
> +
> ret = VM_FAULT_SIGBUS;
> goto out_lock;
> }
> @@ -2614,6 +2642,7 @@ retry_avoidcopy:
> page_cache_release(new_page);
> out_old_page:
> page_cache_release(old_page);
> + commit_dequeued_huge_page(h, do_dequeue);
> out_lock:
> /* Caller expects lock to be held */
> spin_lock(&mm->page_table_lock);
> @@ -2666,6 +2695,8 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
> pte_t new_pte;
> long chg;
> bool use_reserve;
> + unsigned long nr_dequeue_users = 0;
> + bool do_dequeue = false;
>
> /*
> * Currently, we are forced to kill the process in the event the
> @@ -2699,9 +2730,17 @@ retry:
> }
> use_reserve = !chg;
>
> - page = alloc_huge_page(vma, address, use_reserve);
> + page = alloc_huge_page(vma, address, use_reserve,
> + &nr_dequeue_users, &do_dequeue);
> if (IS_ERR(page)) {
> - ret = VM_FAULT_SIGBUS;
> + if (nr_dequeue_users)
> + ret = 0;
> + else {
> + if (use_reserve)
> + WARN_ON_ONCE(1);
> +
> + ret = VM_FAULT_SIGBUS;
> + }
> goto out;
> }
> clear_huge_page(page, address, pages_per_huge_page(h));
> @@ -2714,22 +2753,24 @@ retry:
> err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
> if (err) {
> put_page(page);
> + commit_dequeued_huge_page(h, do_dequeue);
> if (err == -EEXIST)
> goto retry;
> goto out;
> }
> ClearPagePrivate(page);
> + commit_dequeued_huge_page(h, do_dequeue);
>
> spin_lock(&inode->i_lock);
> inode->i_blocks += blocks_per_huge_page(h);
> spin_unlock(&inode->i_lock);
> } else {
> lock_page(page);
> + anon_rmap = 1;
> if (unlikely(anon_vma_prepare(vma))) {
> ret = VM_FAULT_OOM;
> goto backout_unlocked;
> }
> - anon_rmap = 1;
> }
> } else {
> /*
> @@ -2783,6 +2824,8 @@ retry:
> spin_unlock(&mm->page_table_lock);
> unlock_page(page);
> out:
> + if (anon_rmap)
> + commit_dequeued_huge_page(h, do_dequeue);
> return ret;
>
> backout:

Otherwise I think it looks good.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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