On 10/22/2017 01:20 PM, Tetsuo Handa wrote:
Wei Wang wrote:
The balloon_lock was used to synchronize the access demand to elements
of struct virtio_balloon and its queue operations (please see commit
e22504296d). This prevents the concurrent run of the leak_balloon and
fill_balloon functions, thereby resulting in a deadlock issue on OOM:
fill_balloon: take balloon_lock and wait for OOM to get some memory;
oom_notify: release some inflated memory via leak_balloon();
leak_balloon: wait for balloon_lock to be released by fill_balloon.
This patch breaks the lock into two fine-grained inflate_lock and
deflate_lock, and eliminates the unnecessary use of the shared data
(i.e. vb->pnfs, vb->num_pfns). This enables leak_balloon and
fill_balloon to run concurrently and solves the deadlock issue.
@@ -162,20 +160,20 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
msleep(200);
break;
}
- set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
- vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
+ set_page_pfns(vb, pfns + num_pfns, page);
if (!virtio_has_feature(vb->vdev,
VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
adjust_managed_page_count(page, -1);
}
- num_allocated_pages = vb->num_pfns;
+ mutex_lock(&vb->inflate_lock);
/* Did we get any? */
- if (vb->num_pfns != 0)
- tell_host(vb, vb->inflate_vq);
- mutex_unlock(&vb->balloon_lock);
+ if (num_pfns != 0)
+ tell_host(vb, vb->inflate_vq, pfns, num_pfns);
+ mutex_unlock(&vb->inflate_lock);
+ atomic64_add(num_pfns, &vb->num_pages);
Isn't this addition too late? If leak_balloon() is called due to
out_of_memory(), it will fail to find up to dated vb->num_pages value.
Not really. I think the old way of implementation above:
"vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE"
isn't quite accurate, because "vb->num_page" should reflect the number of
pages that have already been inflated, which means those pages have
already been given to the host via "tell_host()".
If we update "vb->num_page" earlier before tell_host(), then it will
include the pages
that haven't been given to the host, which I think shouldn't be counted
as inflated pages.
On the other hand, OOM will use leak_balloon() to release the pages that
should
have already been inflated.
In addition, I think we would also need to move balloon_page_insert(),
which puts the
page onto the inflated page list, after tell_host().
- return num_allocated_pages;
+ return num_pfns;
}
static void release_pages_balloon(struct virtio_balloon *vb,
@@ -194,38 +192,39 @@ static void release_pages_balloon(struct virtio_balloon *vb,
static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
{
- unsigned num_freed_pages;
struct page *page;
struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
LIST_HEAD(pages);
+ unsigned int num_pfns;
+ __virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
This array consumes 1024 bytes of kernel stack, doesn't it?
leak_balloon() might be called from out_of_memory() where kernel stack
is already largely consumed before entering __alloc_pages_nodemask().
For reducing possibility of stack overflow, since out_of_memory() is
serialized by oom_lock, I suggest using static (maybe kmalloc()ed as
vb->oom_pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX]) buffer when called from
out_of_memory().
In that case, we might as well to use
vb->inflate_pfns = kmalloc(VIRTIO_BALLOON_ARRAY_PFNS_MAX..);
vb->deflate_pfns = kmalloc(VIRTIO_BALLOON_ARRAY_PFNS_MAX..);
which are allocated in probe().
/* We can only do one array worth at a time. */
- num = min(num, ARRAY_SIZE(vb->pfns));
+ num = min_t(size_t, num, VIRTIO_BALLOON_ARRAY_PFNS_MAX);
- mutex_lock(&vb->balloon_lock);
/* We can't release more pages than taken */
- num = min(num, (size_t)vb->num_pages);
- for (vb->num_pfns = 0; vb->num_pfns < num;
- vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+ num = min_t(size_t, num, atomic64_read(&vb->num_pages));
+ for (num_pfns = 0; num_pfns < num;
+ num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
page = balloon_page_dequeue(vb_dev_info);
If balloon_page_dequeue() can be concurrently called by both host's request
and guest's OOM event, is (!dequeued_page) test in balloon_page_dequeue() safe?
I'm not sure about the question. The "dequeue_page" is a local variable
in the function, why would it be unsafe for two invocations (the shared
b_dev_info->pages are operated under a lock)?
Is such concurrency needed?
Thanks for this question, it triggers another optimization, which I want to
introduce if this direction could be accepted:
I think it is not quite necessary to deflate pages in OOM-->leak_balloon()
when the host request leak_ballon() is running. In that case, I think OOM
can just count the pages that are deflated by the host request.
The implementation logic will be simple, here is the major part:
1) Introduce a "vb->deflating" flag, to tell whether deflating is in
progress
2) At the beginning of leak_balloon():
if (READ_ONCE(vb->deflating)) {
npages = atomic64_read(&vb->num_pages);
/* Wait till the other run of leak_balloon() returns */
while (READ_ONCE(vb->deflating));
npages = npages - atomic64_read(&vb->num_pages)
} else {
WRITE_ONCE(vb->deflating, true);
}
...
3) At the end of leak_balloon():
WRITE_ONCE(vb->deflating, false);
(The above vb->deflating doesn't have to be in vb though, it can be a
static variable inside leak_balloon(). we can
discuss more about the implementation when reaching that step)
Best,
Wei
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