Re: [Qemu-devel] [PATCH v19 3/7] xbitmap: add more operations

[Tetsuo Handa]

Wang, Wei W wrote:
> > Wei Wang wrote:
> > > > But passing GFP_NOWAIT means that we can handle allocation failure.
> > > > There is no need to use preload approach when we can handle allocation 
> > > > failure.
> > >
> > > I think the reason we need xb_preload is because radix tree insertion
> > > needs the memory being preallocated already (it couldn't suffer from
> > > memory failure during the process of inserting, probably because
> > > handling the failure there isn't easy, Matthew may know the backstory
> > > of
> > > this)
> > 
> > According to https://lwn.net/Articles/175432/ , I think that preloading is
> > needed only when failure to insert an item into a radix tree is a 
> > significant
> > problem.
> > That is, when failure to insert an item into a radix tree is not a problem, 
> > I
> > think that we don't need to use preloading.
> 
> It also mentions that the preload attempts to allocate sufficient memory to 
> *guarantee* that the next radix tree insertion cannot fail.
> 
> If we check radix_tree_node_alloc(), the comments there says "this assumes 
> that the caller has performed appropriate preallocation".

If you read what radix_tree_node_alloc() is doing, you will find that
radix_tree_node_alloc() returns NULL when memory allocation failed.

I think that "this assumes that the caller has performed appropriate 
preallocation"
means "The caller has to perform appropriate preallocation if the caller does 
not
want radix_tree_node_alloc() to return NULL".

> 
> So, I think we would get a risk of triggering some issue without preload().
> 
> > >
> > > So, I think we can handle the memory failure with xb_preload, which
> > > stops going into the radix tree APIs, but shouldn't call radix tree
> > > APIs without the related memory preallocated.
> > 
> > It seems to me that virtio-ballon case has no problem without using
> > preloading.
> 
> Why is that?
> 

Because you are saying in PATCH 4/7 that it is OK to fail xb_set_page()
due to -ENOMEM (apart from lack of ability to fallback to !use_sg path
when all xb_set_page() calls failed (i.e. no page will be handled because
there is no "1" bit in the xbitmap)).


+static inline int xb_set_page(struct virtio_balloon *vb,
+                              struct page *page,
+                              unsigned long *pfn_min,
+                              unsigned long *pfn_max)
+{
+       unsigned long pfn = page_to_pfn(page);
+       int ret;
+
+       *pfn_min = min(pfn, *pfn_min);
+       *pfn_max = max(pfn, *pfn_max);
+
+       do {
+               ret = xb_preload_and_set_bit(&vb->page_xb, pfn,
+                                            GFP_NOWAIT | __GFP_NOWARN);
+       } while (unlikely(ret == -EAGAIN));
+
+       return ret;
+}

@@ -173,8 +290,15 @@ static unsigned fill_balloon(struct virtio_balloon *vb, 
size_t num)
 
        while ((page = balloon_page_pop(&pages))) {
                balloon_page_enqueue(&vb->vb_dev_info, page);
+               if (use_sg) {
+                       if (xb_set_page(vb, page, &pfn_min, &pfn_max) < 0) {
+                               __free_page(page);
+                               continue;
+                       }
+               } else {
+                       set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+               }
 

@@ -223,7 +354,14 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
size_t num)
                page = balloon_page_dequeue(vb_dev_info);
                if (!page)
                        break;
-               set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+               if (use_sg) {
+                       if (xb_set_page(vb, page, &pfn_min, &pfn_max) < 0) {
+                               balloon_page_enqueue(&vb->vb_dev_info, page);
+                               break;
+                       }
+               } else {
+                       set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+               }
                list_add(&page->lru, &pages);
                vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
        }