Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2012-08-22 00:06:59
Also in:
lkml
On Tue, Aug 21, 2012 at 05:45:56PM -0300, Rafael Aquini wrote:
quoted hunk ↗ jump to hunk
On Tue, Aug 21, 2012 at 10:30:31PM +0300, Michael S. Tsirkin wrote:quoted
On Tue, Aug 21, 2012 at 04:23:58PM -0300, Rafael Aquini wrote:quoted
On Tue, Aug 21, 2012 at 10:13:30PM +0300, Michael S. Tsirkin wrote:quoted
quoted
I believe rcu_dereference_protected() is what I want/need here, since this code is always called for pages which we hold locked (PG_locked bit).It would only help if we locked the page while updating the mapping, as far as I can see we don't.But we can do it. In fact, by doing it (locking the page) we can easily avoid the nasty race balloon_isolate_page / leak_balloon, in a much simpler way, IMHO.Absolutely. Further, we should look hard at whether most RCU uses in this patchset can be replaced with page lock.Yeah, In fact, by testing/grabbing the page lock at leak_balloon() even the module unload X migration / putback race seems to fade away, since migration code holds the page locked all the way. And that seems a quite easy task to be accomplished: ....@@ -169,21 +197,61 @@ static void leak_balloon(struct virtio_balloon *vb, size_tnum) /* We can only do one array worth at a time. */ num = min(num, ARRAY_SIZE(vb->pfns)); + mutex_lock(&vb->balloon_lock); for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) { + spin_lock(&vb->pages_lock); + /* + * 'virtballoon_isolatepage()' can drain vb->pages list + * making us to stumble across a _temporarily_ empty list.
This still worries me. If this happens we do not lock the page so module can go away? if not need to document why.
+ *
+ * Release the spinlock and resume from here in order to
+ * give page migration a shot to refill vb->pages list.
+ */
+ if (unlikely(list_empty(&vb->pages))) {
+ spin_unlock(&vb->pages_lock);
+ break;
+ }
+
page = list_first_entry(&vb->pages, struct page, lru);
+
+ /*
+ * Grab the page lock to avoid racing against threads isolating
+ * pages from vb->pages list (it's done under page lock).
+ *
+ * Failing to grab the page lock here means this page has been
+ * selected for isolation already.
+ */
+ if (!trylock_page(page)) {
+ spin_unlock(&vb->pages_lock);
+ break;
+ }
+
+ clear_balloon_mapping(page);
list_del(&page->lru);
set_page_pfns(vb->pfns + vb->num_pfns, page);
vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
+ unlock_page(page);
+ spin_unlock(&vb->pages_lock);
}
.....-- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>