Re: [PATCH 6/8] mm/memory-failure: Convert memory_failure() to use a folio

[Miaohe Lin <linmiaohe@huawei.com>]

On 2024/3/15 2:15, Jane Chu wrote:
> On 3/13/2024 7:34 PM, Miaohe Lin wrote:
> 
>> On 2024/3/13 9:23, Jane Chu wrote:
>>> On 3/12/2024 7:14 AM, Matthew Wilcox wrote:
>>>
>>>> On Tue, Mar 12, 2024 at 03:07:39PM +0800, Miaohe Lin wrote:
>>>>> On 2024/3/11 20:31, Matthew Wilcox wrote:
>>>>>> Assuming we have a refcount on this page so it can't be simultaneously
>>>>>> split/freed/whatever, these three sequences are equivalent:
>>>>> If page is stable after page refcnt is held, I agree below three sequences are equivalent.
>>>>>
>>>>>> 1    if (PageCompound(p))
>>>>>>
>>>>>> 2    struct page *head = compound_head(p);
>>>>>> 2    if (PageHead(head))
>>>>>>
>>>>>> 3    struct folio *folio = page_folio(p);
>>>>>> 3    if (folio_test_large(folio))
>>>>>>
>>>>>> .
>>>>>>
>>>>> But please see below commit:
>>>>>
>>>>> """
>>>>> commit f37d4298aa7f8b74395aa13c728677e2ed86fdaf
>>>>> Author: Andi Kleen <ak@linux.intel.com>
>>>>> Date:   Wed Aug 6 16:06:49 2014 -0700
>>>>>
>>>>>       hwpoison: fix race with changing page during offlining
>>>>>
>>>>>       When a hwpoison page is locked it could change state due to parallel
>>>>>       modifications.  The original compound page can be torn down and then
>>>>>       this 4k page becomes part of a differently-size compound page is is a
>>>>>       standalone regular page.
>>>>>
>>>>>       Check after the lock if the page is still the same compound page.
>>>> I can't speak to what the rules were ten years ago, but this is not
>>>> true now.  Compound pages cannot be split if you hold a refcount.
>>>> Since we don't track a per-page refcount, we wouldn't know which of
>>>> the split pages to give the excess refcount to.
>>> I noticed this recently
>>>
>>>   * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
>>>   * they are not mapped.
>>>   *
>>>   * Returns 0 if the hugepage is split successfully.
>>>   * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
>>>   * us.
>>>   */
>>> int split_huge_page_to_list(struct page *page, struct list_head *list)
>>> {
>>>
>>> I have a test case with poisoned shmem THP page that was mlocked and
>>>
>>> GUP pinned (FOLL_LONGTERM|FOLL_WRITE), but the split succeeded.
>> Thanks for points this out. Compound pages can be split even if extra refcnt is held. So folio_test_large
>> check is not stable if we hold a refcnt now? Will it introduce some obscure races?
>>
>> Except from that, I think a page cannot become a subpage of a THP when extra refcnt is held now. So below code can be removed.
>> Any thought?
>>
>>     /*
>>      * We're only intended to deal with the non-Compound page here.
>>      * However, the page could have changed compound pages due to
>>      * race window. If this happens, we could try again to hopefully
>>      * handle the page next round.
>>      */
>>     if (PageCompound(p)) {
>>         if (retry) {
>>             ClearPageHWPoison(p);
>>             unlock_page(p);
>>             put_page(p);
>>             flags &= ~MF_COUNT_INCREASED;
>>             retry = false;
>>             goto try_again;
>>         }
>>         res = action_result(pfn, MF_MSG_DIFFERENT_COMPOUND, MF_IGNORED);
>>         goto unlock_page;
>>     }
> Not sure of what scenario it was meant to deal with.  How about adding a warning instead of removal? It'll be interesting to see how the warning got triggered. But if after a while nothing happens, then remove it.

This sounds like a good alternative. Thanks.