On 2022/2/15 16:48, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Feb 15, 2022 at 11:14:07AM +0800, Miaohe Lin wrote:
>> On 2022/2/14 22:50, Naoya Horiguchi wrote:
>>> On Thu, Feb 10, 2022 at 10:17:29PM +0800, Miaohe Lin wrote:
>>>> orig_head is used to check whether the page have changed compound pages
>>>> during the locking. But it's always equal to hpage. So we can use hpage
>>>> directly and remove this redundant one.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>> ---
>>>> mm/memory-failure.c | 5 ++---
>>>> 1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index 2dd7f35ee65a..4370c2f407c5 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -1691,7 +1691,6 @@ int memory_failure(unsigned long pfn, int flags)
>>>> {
>>>> struct page *p;
>>>> struct page *hpage;
>>>> - struct page *orig_head;
>>>> struct dev_pagemap *pgmap;
>>>> int res = 0;
>>>> unsigned long page_flags;
>>>> @@ -1737,7 +1736,7 @@ int memory_failure(unsigned long pfn, int flags)
>>>> goto unlock_mutex;
>>>> }
>>>>
>>>> - orig_head = hpage = compound_head(p);
>>>> + hpage = compound_head(p);
>>>> num_poisoned_pages_inc();
>>>>
>>>> /*
>>>> @@ -1821,7 +1820,7 @@ int memory_failure(unsigned long pfn, int flags)
>>>> * The page could have changed compound pages during the locking.
>>>> * If this happens just bail out.
>>>> */
>>>> - if (PageCompound(p) && compound_head(p) != orig_head) {
>>>> + if (PageCompound(p) && compound_head(p) != hpage) {
>>>
>>> I think that this if-check was intended to detect the case that page p
>>> belongs to a thp when memory_failure() is called and belongs to a compound
>>> page in different size (like slab or some driver page) after the thp is
>>> split. But your suggestion makes me aware that the page p could be embedded
>>> on a thp again after thp split. I think this might be rare, but if it
>>
>> IIUC, this page can't be embedded on a thp again after thp split because memory_failure hold
>> an __extra__ page refcnt. I think there exist below race windows:
>>
>> memory_failure
>> orig_head = hpage = compound_head(p); -- page is Non-compound yet
>> < -- Page becomes compound page, like thp, slab, some driver page and even hugetlb page -- >
>> get_hwpoison_page
>> failed to split thp page, as hpage is Non-compound ...
>> lock_page
>>
>>> happens the current if-check (or suggested one) cannot detect it.
>>> So I feel that simply dropping compound_head() check might be better?
>>>
>>> - if (PageCompound(p) && compound_head(p) != orig_head) {
>>> + if (PageCompound(p)) {
>>
>> However this change could also catch the above race correctly. In fact, we can't handle
>> compound page here. But is it enough to just return -EBUSY here as it's really rare or
>> we should do more things (like split thp, retry if in PageHuge case)?
>
> Hmm, both could make sense and hard to judge to me, so it's upto you.
> We already have goto label "try_again" so retrying might not be so surprising.
>
try_again sounds a good idea. Will send a V2. Many thanks.
> Thanks,
> Naoya Horiguchi
>
