On Wed, Nov 07, 2012 at 04:11:46PM -0800, Andrew Morton wrote: > On Thu, 08 Nov 2012 09:32:18 +1030 > Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > > Rafael Aquini <aquini@xxxxxxxxxx> writes: > > > + * virtballoon_migratepage - perform the balloon page migration on behalf of > > > + * a compation thread. (called under page lock) > > > > > + if (!mutex_trylock(&vb->balloon_lock)) > > > + return -EAGAIN; > > > > Erk, OK... > > Not really. As is almost always the case with a trylock, it needs a > comment explaining why we couldn't use the far superior mutex_lock(). > Data: this reader doesn't know! > That was just to alleviate balloon_lock contention if we're migrating pages concurrently with balloon_fill() or balloon_leak(), as it's easier to retry the page migration later (in the contended case). > > > + /* balloon's page migration 1st step -- inflate "newpage" */ > > > + spin_lock_irqsave(&vb_dev_info->pages_lock, flags); > > > + balloon_page_insert(newpage, mapping, &vb_dev_info->pages); > > > + vb_dev_info->isolated_pages--; > > > + spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags); > > > + vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE; > > > + set_page_pfns(vb->pfns, newpage); > > > + tell_host(vb, vb->inflate_vq); > > > > tell_host does wait_event(), so you can't call it under the page_lock. > > Right? > > Sleeping inside lock_page() is OK. More problematic is that GFP_KERNEL > allocation. iirc it _should_ be OK. Core VM uses trylock_page() and > the filesystems shouldn't be doing a synchronous lock_page() in the > pageout path. But I suspect it isn't a well-tested area. The locked page under migration is not contended by any other FS / core VM path, as it is already isolated by compaction to a private migration page list. OTOH, there's a chance of another parallel compaction thread hitting this page while scanning page blocks for isolation, but that path can be considered safe as it uses trylock_page() > > > You probably get away with it because current qemu will service you > > immediately. You could spin here in this case for the moment. > > > > There's a second call to tell_host(): > > > > > + /* > > > + * balloon's page migration 2nd step -- deflate "page" > > > + * > > > + * It's safe to delete page->lru here because this page is at > > > + * an isolated migration list, and this step is expected to happen here > > > + */ > > > + balloon_page_delete(page); > > > + vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE; > > > + set_page_pfns(vb->pfns, page); > > > + tell_host(vb, vb->deflate_vq); > > > > The first one can be delayed, the second one can be delayed if the host > > didn't ask for VIRTIO_BALLOON_F_MUST_TELL_HOST (qemu doesn't). > > > > We could implement a proper request queue for these, and return -EAGAIN > > if the queue fills. Though in practice, it's not important (it might > > help performance). > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>