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SubjectRe: [PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism of raw error pages
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On Mon, Jun 27, 2022 at 05:26:01PM +0800, Muchun Song wrote:
> 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.
>
...

> > @@ -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?
>

OK, that sounds better, I'll do it.

> > +
> > +/*
> > + * 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.

OK, I'll use GFP_ATOMIC.

>
> > + if (raw_hwp) {
> > + raw_hwp->page = page;
> > + llist_add(&raw_hwp->node, head);
>
> The maximum amount of items in the list is one, right?

The maximum is the number of subpages in the hugepage (512 for 2MB hugepage,
262144 for 1GB hugepage).

>
> > + } 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?

Sorry if I might miss your point, but raw_hwp_unreliable is set when
allocating raw_hwp_page failed. hugetlb_set_page_hwpoison() can be called
multiple times on a hugepage and if one of the calls fails, the hwpoisoned
hugepage becomes unreliable.

BTW, as you pointed out above, if we switch to passing GFP_ATOMIC to kmalloc(),
the kmalloc() never fails, so we no longer have to implement this unreliable
flag, so things get simpler.

>
> > + 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?

Maybe you are mentioning the race like below. Yes, that's possible.

CPU 0 CPU 1

free_huge_page
lock hugetlb_lock
ClearHPageMigratable
unlock hugetlb_lock
get_huge_page_for_hwpoison
lock hugetlb_lock
__get_huge_page_for_hwpoison
hugetlb_set_page_hwpoison
allocate raw_hwp_page
TestSetPageHWPoison
update_and_free_page
__update_and_free_page
if (PageHWPoison)
hugetlb_clear_page_hwpoison
TestClearPageHWPoison
// remove all list items
llist_add
unlock hugetlb_lock


The end result seems not critical (leaking raced raw_hwp_page?), but
we need fix.

>
> > + 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.

The reason why I put ClearPageHWPoison(hpage) before llist_for_each_safe()
was that raw error page can be the head page. But this can be solved
with some additional code to remember whether raw_hwp_page list has an item
associated with the head page.

Or another approach in my mind now is to call hugetlb_clear_page_hwpoison()
with taking mf_mutex.

>
> Thanks.

Thank you for valuable feedbacks.

- Naoya Horiguchi

>
> > + 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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