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)?
>
> This should ensure the assumption (mentioned in 8/8 patch) more that
> the error page never belongs to compound page after taking page lock.
Agree, this really helps.
>> Thanks,
> Naoya Horiguchi
> .
>
Many thanks.
