Re: [PATCH v2 1/4] mm/balloon_compaction: list interfaces

Michael S. Tsirkin
2019-04-19T22:07:32Z

On Thu, Mar 28, 2019 at 01:07:15AM +0000, Nadav Amit wrote:
> Introduce interfaces for ballooning enqueueing and dequeueing of a list
> of pages. These interfaces reduce the overhead of storing and restoring
> IRQs by batching the operations. In addition they do not panic if the
> list of pages is empty.
>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: virtualization@lists.linux-foundation.org
> Reviewed-by: Xavier Deguillard <xdeguillard@vmware.com>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> include/linux/balloon_compaction.h | 4 +
> mm/balloon_compaction.c | 145 +++++++++++++++++++++--------
> 2 files changed, 111 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index f111c780ef1d..1da79edadb69 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -64,6 +64,10 @@ extern struct page *balloon_page_alloc(void);
> extern void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
> struct page *page);
> extern struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info);
> +extern size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
> + struct list_head *pages);
> +extern size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
> + struct list_head *pages, int n_req_pages);
>

Why size_t I wonder? It can never be > n_req_pages which is int.
Callers also seem to assume int.

> static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
> {

> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index ef858d547e2d..88d5d9a01072 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -10,6 +10,106 @@
> #include <linux/export.h>
> #include <linux/balloon_compaction.h>
>
> +static int balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
> + struct page *page)
> +{
> + /*
> + * Block others from accessing the 'page' when we get around to
> + * establishing additional references. We should be the only one
> + * holding a reference to the 'page' at this point.
> + */
> + if (!trylock_page(page)) {
> + WARN_ONCE(1, "balloon inflation failed to enqueue page\n");
> + return -EFAULT;

Looks like all callers bug on a failure. So let's just do it here,
and then make this void?

> + }
> + list_del(&page->lru);
> + balloon_page_insert(b_dev_info, page);
> + unlock_page(page);
> + __count_vm_event(BALLOON_INFLATE);
> + return 0;
> +}
> +
> +/**
> + * balloon_page_list_enqueue() - inserts a list of pages into the balloon page
> + * list.
> + * @b_dev_info: balloon device descriptor where we will insert a new page to
> + * @pages: pages to enqueue - allocated using balloon_page_alloc.
> + *
> + * Driver must call it to properly enqueue a balloon pages before definitively
> + * removing it from the guest system.

A bunch of grammar error here. Pls fix for clarify.
Also - document that nothing must lock the pages? More assumptions?
What is "it" in this context? All pages? And what does removing from
guest mean? Really adding to the balloon?

> + *
> + * Return: number of pages that were enqueued.
> + */
> +size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
> + struct list_head *pages)
> +{
> + struct page *page, *tmp;
> + unsigned long flags;
> + size_t n_pages = 0;
> +
> + spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> + list_for_each_entry_safe(page, tmp, pages, lru) {
> + balloon_page_enqueue_one(b_dev_info, page);

Do we want to do something about an error here?

> + n_pages++;
> + }
> + spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> + return n_pages;
> +}
> +EXPORT_SYMBOL_GPL(balloon_page_list_enqueue);
> +
> +/**
> + * balloon_page_list_dequeue() - removes pages from balloon's page list and
> + * returns a list of the pages.
> + * @b_dev_info: balloon device decriptor where we will grab a page from.
> + * @pages: pointer to the list of pages that would be returned to the caller.
> + * @n_req_pages: number of requested pages.
> + *
> + * Driver must call it to properly de-allocate a previous enlisted balloon pages
> + * before definetively releasing it back to the guest system. This function
> + * tries to remove @n_req_pages from the ballooned pages and return it to the
> + * caller in the @pages list.
> + *
> + * Note that this function may fail to dequeue some pages temporarily empty due
> + * to compaction isolated pages.
> + *
> + * Return: number of pages that were added to the @pages list.
> + */
> +size_t balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
> + struct list_head *pages, int n_req_pages)
> +{
> + struct page *page, *tmp;
> + unsigned long flags;
> + size_t n_pages = 0;
> +
> + spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> + list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
> + /*
> + * Block others from accessing the 'page' while we get around
> + * establishing additional references and preparing the 'page'
> + * to be released by the balloon driver.
> + */
> + if (!trylock_page(page))
> + continue;
> +
> + if (IS_ENABLED(CONFIG_BALLOON_COMPACTION) &&
> + PageIsolated(page)) {
> + /* raced with isolation */
> + unlock_page(page);
> + continue;
> + }
> + balloon_page_delete(page);
> + __count_vm_event(BALLOON_DEFLATE);
> + unlock_page(page);
> + list_add(&page->lru, pages);
> + if (++n_pages >= n_req_pages)
> + break;
> + }
> + spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> +
> + return n_pages;
> +}
> +EXPORT_SYMBOL_GPL(balloon_page_list_dequeue);
> +
> /*
> * balloon_page_alloc - allocates a new page for insertion into the balloon
> * page list.
> @@ -43,17 +143,9 @@ void balloon_page_enqueue(struct balloon_dev_info *b_dev_info,
> {
> unsigned long flags;
>
> - /*
> - * Block others from accessing the 'page' when we get around to
> - * establishing additional references. We should be the only one
> - * holding a reference to the 'page' at this point.
> - */
> - BUG_ON(!trylock_page(page));
> spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> - balloon_page_insert(b_dev_info, page);
> - __count_vm_event(BALLOON_INFLATE);
> + balloon_page_enqueue_one(b_dev_info, page);

We used to bug on failure to lock page, now we
silently ignore this error. Why?

> spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> - unlock_page(page);
> }
> EXPORT_SYMBOL_GPL(balloon_page_enqueue);
>
> @@ -70,36 +162,13 @@ EXPORT_SYMBOL_GPL(balloon_page_enqueue);
> */
> struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
> {
> - struct page *page, *tmp;
> unsigned long flags;
> - bool dequeued_page;
> + LIST_HEAD(pages);
> + int n_pages;
>
> - dequeued_page = false;
> - spin_lock_irqsave(&b_dev_info->pages_lock, flags);
> - list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
> - /*
> - * Block others from accessing the 'page' while we get around
> - * establishing additional references and preparing the 'page'
> - * to be released by the balloon driver.
> - */
> - if (trylock_page(page)) {
> -#ifdef CONFIG_BALLOON_COMPACTION
> - if (PageIsolated(page)) {
> - /* raced with isolation */
> - unlock_page(page);
> - continue;
> - }
> -#endif
> - balloon_page_delete(page);
> - __count_vm_event(BALLOON_DEFLATE);
> - unlock_page(page);
> - dequeued_page = true;
> - break;
> - }
> - }
> - spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> + n_pages = balloon_page_list_dequeue(b_dev_info, &pages, 1);
>
> - if (!dequeued_page) {
> + if (n_pages != 1) {
> /*
> * If we are unable to dequeue a balloon page because the page
> * list is empty and there is no isolated pages, then something
> @@ -112,9 +181,9 @@ struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
> !b_dev_info->isolated_pages))
> BUG();
> spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> - page = NULL;
> + return NULL;
> }
> - return page;
> + return list_first_entry(&pages, struct page, lru);
> }
> EXPORT_SYMBOL_GPL(balloon_page_dequeue);
>
> --
> 2.19.1