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; } -- 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>
