Re: [PATCH 3/6] mm/balloon_compaction: list interfaces

Michael S. Tsirkin
2019-02-07T01:26:48Z

On Thu, Feb 07, 2019 at 12:43:51AM +0000, Nadav Amit wrote:
> > On Feb 6, 2019, at 4:32 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Feb 06, 2019 at 03:57:03PM -0800, 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.
> >>
>
> [Snip]
>
> First, thanks for the quick feedback.
>
> >> +
> >> +/**
> >> + * 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.
> >> + */
> >> +void balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
> >> + struct list_head *pages)
> >> +{
> >> + struct page *page, *tmp;
> >> + unsigned long flags;
> >> +
> >> + 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);
> >> + spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
> >
> > As this is scanning pages one by one anyway, it will be useful
> > to have this return the # of pages enqueued.
>
> Sure.
>
> >
> >> +}
> >> +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.
> >> + */
> >> +int balloon_page_list_dequeue(struct balloon_dev_info *b_dev_info,
> >> + struct list_head *pages, int n_req_pages)
> >
> > Are we sure this int never overflows? Why not just use u64
> > or size_t straight away?
>
> size_t it is.
>
> >
> >> +{
> >> + struct page *page, *tmp;
> >> + unsigned long flags;
> >> + int 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);
> >> +
> >
> > This looks quite reasonable. In fact virtio can be reworked to use
> > this too and then the original one can be dropped.
> >
> > Have the time?
>
> Obviously not, but I am willing to make the time. What I cannot “make" is an
> approval to send patches for other hypervisors. Let me run a quick check
> with our FOSS people here.
>
> Anyhow, I hope it would not prevent the patches from getting to the next
> release.
>

No, that's not a blocker.

--
MST