Re: [PATCH v2 1/3] mm/memory-failure.c: fix race with changing page compound again

Mike Kravetz
2022-03-14T18:20:54Z

On 3/11/22 23:46, Miaohe Lin wrote:
> There is a race window where we got the compound_head, the hugetlb page
> could be freed to buddy, or even changed to another compound page just
> before we try to get hwpoison page. Think about the below race window:
> CPU 1 CPU 2
> memory_failure_hugetlb
> struct page *head = compound_head(p);
> hugetlb page might be freed to
> buddy, or even changed to another
> compound page.
>
> get_hwpoison_page -- page is not what we want now...
>
> If this race happens, just bail out. Also MF_MSG_DIFFERENT_PAGE_SIZE is
> introduced to record this event.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
> include/linux/mm.h | 1 +
> include/ras/ras_event.h | 1 +
> mm/memory-failure.c | 12 ++++++++++++
> 3 files changed, 14 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c9bada4096ac..ef98cff2b253 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3253,6 +3253,7 @@ enum mf_action_page_type {
> MF_MSG_BUDDY,
> MF_MSG_DAX,
> MF_MSG_UNSPLIT_THP,
> + MF_MSG_DIFFERENT_PAGE_SIZE,
> MF_MSG_UNKNOWN,
> };
>
> diff --git a/include/ras/ras_event.h b/include/ras/ras_event.h
> index d0337a41141c..1e694fd239b9 100644
> --- a/include/ras/ras_event.h
> +++ b/include/ras/ras_event.h
> @@ -374,6 +374,7 @@ TRACE_EVENT(aer_event,
> EM ( MF_MSG_BUDDY, "free buddy page" ) \
> EM ( MF_MSG_DAX, "dax page" ) \
> EM ( MF_MSG_UNSPLIT_THP, "unsplit thp" ) \
> + EM ( MF_MSG_DIFFERENT_PAGE_SIZE, "different page size" ) \
> EMe ( MF_MSG_UNKNOWN, "unknown page" )
>
> /*
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5444a8ef4867..dabecd87ad3f 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -733,6 +733,7 @@ static const char * const action_page_types[] = {
> [MF_MSG_BUDDY] = "free buddy page",
> [MF_MSG_DAX] = "dax page",
> [MF_MSG_UNSPLIT_THP] = "unsplit thp",
> + [MF_MSG_DIFFERENT_PAGE_SIZE] = "different page size",
> [MF_MSG_UNKNOWN] = "unknown page",
> };
>
> @@ -1534,6 +1535,17 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
> }
>
> lock_page(head);
> +
> + /**
> + * The page could have changed compound pages due to race window.
> + * If this happens just bail out.
> + */
> + if (!PageHuge(p) || compound_head(p) != head) {
> + action_result(pfn, MF_MSG_DIFFERENT_PAGE_SIZE, MF_IGNORED);
> + res = -EBUSY;

We have discussed this race in other versions of the patch. When we encounter
the race, we have likely marked poison on the wrong page. Correct?

Instead of printing a "different page size", would it be better to perhaps:
- Print a message that wrong page may be marked for poison?
- Clear the poison flag in the "head page" previously set?

--
Mike Kravetz

> + goto out;
> + }
> +
> page_flags = head->flags;
>
> if (hwpoison_filter(p)) {