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SubjectRe: [PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism of raw error pages
On Fri, Jun 24, 2022 at 08:51:48AM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>
> When handling memory error on a hugetlb page, the error handler tries to
> dissolve and turn it into 4kB pages. If it's successfully dissolved,
> PageHWPoison flag is moved to the raw error page, so that's all right.
> However, dissolve sometimes fails, then the error page is left as
> hwpoisoned hugepage. It's useful if we can retry to dissolve it to save
> healthy pages, but that's not possible now because the information about
> where the raw error pages is lost.
>
> Use the private field of a few tail pages to keep that information. The
> code path of shrinking hugepage pool uses this info to try delayed dissolve.
> In order to remember multiple errors in a hugepage, a singly-linked list
> originated from SUBPAGE_INDEX_HWPOISON-th tail page is constructed. Only
> simple operations (adding an entry or clearing all) are required and the
> list is assumed not to be very long, so this simple data structure should
> be enough.
>
> If we failed to save raw error info, the hwpoison hugepage has errors on
> unknown subpage, then this new saving mechanism does not work any more,
> so disable saving new raw error info and freeing hwpoison hugepages.
>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

Thanks for your work on this. I have several quesions below.

> ---
> v1 -> v2:
> - support hwpoison hugepage with multiple errors,
> - moved the new interface functions to mm/memory-failure.c,
> - define additional subpage index SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
> - stop freeing/dissolving hwpoison hugepages with unreliable raw error info,
> - drop hugetlb_clear_page_hwpoison() in dissolve_free_huge_page() because
> that's done in update_and_free_page(),
> - move setting/clearing PG_hwpoison flag to the new interfaces,
> - checking already hwpoisoned or not on a subpage basis.
>
> ChangeLog since previous post on 4/27:
> - fixed typo in patch description (by Miaohe)
> - fixed config value in #ifdef statement (by Miaohe)
> - added sentences about "multiple hwpoison pages" scenario in patch
> description
>
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---
> include/linux/hugetlb.h | 13 ++++++
> mm/hugetlb.c | 39 +++++++++--------
> mm/memory-failure.c | 95 ++++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 125 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e4cff27d1198..ac13c2022517 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -42,6 +42,10 @@ enum {
> SUBPAGE_INDEX_CGROUP, /* reuse page->private */
> SUBPAGE_INDEX_CGROUP_RSVD, /* reuse page->private */
> __MAX_CGROUP_SUBPAGE_INDEX = SUBPAGE_INDEX_CGROUP_RSVD,
> +#endif
> +#ifdef CONFIG_MEMORY_FAILURE
> + SUBPAGE_INDEX_HWPOISON,
> + SUBPAGE_INDEX_HWPOISON_UNRELIABLE,
> #endif
> __NR_USED_SUBPAGE,
> };
> @@ -798,6 +802,15 @@ extern int dissolve_free_huge_page(struct page *page);
> extern int dissolve_free_huge_pages(unsigned long start_pfn,
> unsigned long end_pfn);
>
> +#ifdef CONFIG_MEMORY_FAILURE
> +extern int hugetlb_clear_page_hwpoison(struct page *hpage);
> +#else
> +static inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> +{
> + return 0;
> +}
> +#endif
> +
> #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
> #ifndef arch_hugetlb_migration_supported
> static inline bool arch_hugetlb_migration_supported(struct hstate *h)
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b7ae5f73f3b2..19ef427aa1e8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1541,17 +1541,15 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> return;
>
> - if (hugetlb_vmemmap_alloc(h, page)) {
> - spin_lock_irq(&hugetlb_lock);
> - /*
> - * If we cannot allocate vmemmap pages, just refuse to free the
> - * page and put the page back on the hugetlb free list and treat
> - * as a surplus page.
> - */
> - add_hugetlb_page(h, page, true);
> - spin_unlock_irq(&hugetlb_lock);
> - return;
> - }
> + if (hugetlb_vmemmap_alloc(h, page))
> + goto fail;
> +
> + /*
> + * Move PageHWPoison flag from head page to the raw error pages,
> + * which makes any healthy subpages reusable.
> + */
> + if (unlikely(PageHWPoison(page) && hugetlb_clear_page_hwpoison(page)))
> + goto fail;
>
> for (i = 0; i < pages_per_huge_page(h);
> i++, subpage = mem_map_next(subpage, page, i)) {
> @@ -1572,6 +1570,16 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
> } else {
> __free_pages(page, huge_page_order(h));
> }
> + return;
> +fail:
> + spin_lock_irq(&hugetlb_lock);
> + /*
> + * If we cannot allocate vmemmap pages or cannot identify raw hwpoison
> + * subpages reliably, just refuse to free the page and put the page
> + * back on the hugetlb free list and treat as a surplus page.
> + */
> + add_hugetlb_page(h, page, true);
> + spin_unlock_irq(&hugetlb_lock);
> }
>
> /*
> @@ -2115,15 +2123,6 @@ int dissolve_free_huge_page(struct page *page)
> */
> rc = hugetlb_vmemmap_alloc(h, head);
> if (!rc) {
> - /*
> - * Move PageHWPoison flag from head page to the raw
> - * error page, which makes any subpages rather than
> - * the error page reusable.
> - */
> - if (PageHWPoison(head) && page != head) {
> - SetPageHWPoison(page);
> - ClearPageHWPoison(head);
> - }
> update_and_free_page(h, head, false);
> } else {
> spin_lock_irq(&hugetlb_lock);
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index fb3feb1f363e..af571fa6f2af 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1499,6 +1499,97 @@ static int try_to_split_thp_page(struct page *page, const char *msg)
> }
>
> #ifdef CONFIG_HUGETLB_PAGE
> +/*
> + * Struct raw_hwp_page represents information about "raw error page",
> + * constructing singly linked list originated from ->private field of
> + * SUBPAGE_INDEX_HWPOISON-th tail page.
> + */
> +struct raw_hwp_page {
> + struct llist_node node;
> + struct page *page;
> +};
> +
> +static inline struct llist_head *raw_hwp_list_head(struct page *hpage)
> +{
> + return (struct llist_head *)&page_private(hpage + SUBPAGE_INDEX_HWPOISON);
> +}
> +
> +static inline int raw_hwp_unreliable(struct page *hpage)
> +{
> + return page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE);
> +}
> +
> +static inline void set_raw_hwp_unreliable(struct page *hpage)
> +{
> + set_page_private(hpage + SUBPAGE_INDEX_HWPOISON_UNRELIABLE, 1);
> +}

Why not use HPAGEFLAG(HwpoisonUnreliable, hwpoison_unreliable) directly?

> +
> +/*
> + * about race consideration
> + */
> +static inline int hugetlb_set_page_hwpoison(struct page *hpage,
> + struct page *page)
> +{
> + struct llist_head *head;
> + struct raw_hwp_page *raw_hwp;
> + struct llist_node *t, *tnode;
> + int ret;
> +
> + /*
> + * Once the hwpoison hugepage has lost reliable raw error info,
> + * there is little mean to keep additional error info precisely,
> + * so skip to add additional raw error info.
> + */
> + if (raw_hwp_unreliable(hpage))
> + return -EHWPOISON;
> + head = raw_hwp_list_head(hpage);
> + llist_for_each_safe(tnode, t, head->first) {
> + struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> +
> + if (p->page == page)
> + return -EHWPOISON;
> + }
> +
> + ret = TestSetPageHWPoison(hpage) ? -EHWPOISON : 0;
> + /* the first error event will be counted in action_result(). */
> + if (ret)
> + num_poisoned_pages_inc();
> +
> + raw_hwp = kmalloc(sizeof(struct raw_hwp_page), GFP_KERNEL);

This function can be called in atomic context, GFP_ATOMIC should be used
here.

> + if (raw_hwp) {
> + raw_hwp->page = page;
> + llist_add(&raw_hwp->node, head);

The maximum amount of items in the list is one, right?

> + } else {
> + /*
> + * Failed to save raw error info. We no longer trace all
> + * hwpoisoned subpages, and we need refuse to free/dissolve
> + * this hwpoisoned hugepage.
> + */
> + set_raw_hwp_unreliable(hpage);
> + return ret;
> + }
> + return ret;
> +}
> +
> +inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> +{
> + struct llist_head *head;
> + struct llist_node *t, *tnode;
> +
> + if (raw_hwp_unreliable(hpage))
> + return -EBUSY;

IIUC, we use head page's PageHWPoison to synchronize hugetlb_clear_page_hwpoison()
and hugetlb_set_page_hwpoison(), right? If so, who can set hwp_unreliable here?

> + ClearPageHWPoison(hpage);
> + head = raw_hwp_list_head(hpage);
> + llist_for_each_safe(tnode, t, head->first) {

Is it possible that a new item is added hugetlb_set_page_hwpoison() and we do not
traverse it (we have cleared page's PageHWPoison)? Then we ignored a real hwpoison
page, right?

> + struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);
> +
> + SetPageHWPoison(p->page);
> + kfree(p);
> + }
> + llist_del_all(head);

If the above issue exists, moving ClearPageHWPoison(hpage) to here could
fix it. We should clear PageHWPoison carefully since the head page is
also can be poisoned.

Thanks.

> + return 0;
> +}
> +
> /*
> * Called from hugetlb code with hugetlb_lock held.
> *
> @@ -1533,7 +1624,7 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
> goto out;
> }
>
> - if (TestSetPageHWPoison(head)) {
> + if (hugetlb_set_page_hwpoison(head, page)) {
> ret = -EHWPOISON;
> goto out;
> }
> @@ -1585,7 +1676,7 @@ static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb
> lock_page(head);
>
> if (hwpoison_filter(p)) {
> - ClearPageHWPoison(head);
> + hugetlb_clear_page_hwpoison(head);
> res = -EOPNOTSUPP;
> goto out;
> }
> --
> 2.25.1
>
>

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