Re: [PATCH 4/8] mm/memory-failure.c: remove unneeded orig_head

[HORIGUCHI NAOYA(堀口 直也)]

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.

Thanks,
Naoya Horiguchi