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Subject[PATCH v2 19/20] mm, hugetlb: retry if failed to allocate and there is concurrent user
Date
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++;
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;
+
+ 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;
+ *do_dequeue = false;
spin_unlock(&hugetlb_lock);
page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
if (!page) {
@@ -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:
--
1.7.9.5


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