On Mon, Mar 14, 2022 at 03:10:25PM +0800, Miaohe Lin wrote:
> On 2022/3/14 10:13, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> >
> > There is a race condition between memory_failure_hugetlb() and hugetlb
> > free/demotion, which causes setting PageHWPoison flag on the wrong page
> > (which was a hugetlb when memory_failure() was called, but was removed
> > or demoted when memory_failure_hugetlb() is called). This results in
> > killing wrong processes. So set PageHWPoison flag with holding page lock,
>
> It seems hold page lock could not help solve this race condition as hugetlb
> page demotion is not required to hold the page lock. Could you please explain
> this a bit more?
Sorry, the last line in the paragraph need change. What prevents the current
race is hugetlb_lock, not page lock. The page lock is here to prevent the
race with hugepage allocation (not directly related to the current issue,
but it's still necessary).
>
> BTW:Is there some words missing or here should be 'page lock.' instead of 'page lock,' ?
I should use a period here, I'll fix it.
[...]
> > @@ -1503,24 +1502,11 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
> > int res;
> > unsigned long page_flags;
> >
> > - if (TestSetPageHWPoison(head)) {
> > - pr_err("Memory failure: %#lx: already hardware poisoned\n",
> > - pfn);
> > - res = -EHWPOISON;
> > - if (flags & MF_ACTION_REQUIRED)
> > - res = kill_accessing_process(current, page_to_pfn(head), flags);
> > - return res;
> > - }
> > -
> > - num_poisoned_pages_inc();
> > -
> > if (!(flags & MF_COUNT_INCREASED)) {
> > res = get_hwpoison_page(p, flags);
> > if (!res) {
>
> In this (res == 0) case, hugetlb page could be dissolved via __page_handle_poison.
> But since PageHWPoison is not set yet, we can't set the PageHWPoison to the correct
> page. Think about the below code in dissolve_free_huge_page:
> /*
> * 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);
> }
>
> SetPageHWPoison won't be called for the error page. Or am I miss something?
No, you're right. We need call page_handle_poison() instead of
__page_handle_poison().
@@ -1512,7 +1512,7 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
}
unlock_page(head);
res = MF_FAILED;
- if (__page_handle_poison(p)) {
+ if (page_handle_poison(p, true, false)) {
page_ref_inc(p);
res = MF_RECOVERED;
}
>
> > lock_page(head);
> > if (hwpoison_filter(p)) {
> > - if (TestClearPageHWPoison(head))
> > - num_poisoned_pages_dec();
> > unlock_page(head);
> > return -EOPNOTSUPP;
> > }
> > @@ -1553,13 +1539,16 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
> > page_flags = head->flags;
> >
> > if (hwpoison_filter(p)) {
> > - if (TestClearPageHWPoison(head))
> > - num_poisoned_pages_dec();
> > put_page(p);
> > res = -EOPNOTSUPP;
> > goto out;
> > }
> >
> > + if (TestSetPageHWPoison(head))
> > + goto already_hwpoisoned;
> > +
> > + num_poisoned_pages_inc();
> > +
> > /*
> > * TODO: hwpoison for pud-sized hugetlb doesn't work right now, so
> > * simply disable it. In order to make it work properly, we need
> > @@ -1585,6 +1574,14 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
> > out:
> > unlock_page(head);
> > return res;
> > +already_hwpoisoned:
> > + put_page(p);
> > + unlock_page(head);
>
> Generally speaking, we should do unlock_page before put_page or page might be disappeared
> before we unlock the page. This should be ok when memory_failure succeeds to handle the
> page previously as it holds one extra page refcnt. But it might be problematic when
> memory_failure failed to handle the page last time. We might be the last user here.
OK, so another code path in "if (hwpoison_filter)@ block seems to need
the same change in the order.
Thanks,
Naoya Horiguchi
