Message-ID: <20210629080835.GA427744@u2004>
Date:   Tue, 29 Jun 2021 17:08:35 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     Bin Wang <wangbin224@...wei.com>, mike.kravetz@...cle.com
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, naoya.horiguchi@....com, wuxu.wu@...wei.com
Subject: Re: Re: [PATCH v2] mm: hugetlb: add hwcrp_hugepages to record memory
 failure on hugetlbfs

Hi Bin, Mike,

On Mon, Jun 28, 2021 at 05:27:52PM +0800, Bin Wang wrote:
> > > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> > > index 926eeb9bf4eb..ffb6e7b6756b 100644
> > > --- a/fs/hugetlbfs/inode.c
> > > +++ b/fs/hugetlbfs/inode.c
> > > @@ -986,8 +986,7 @@ static int hugetlbfs_error_remove_page(struct address_space *mapping,
> > >  	pgoff_t index = page->index;
> > >  
> > >  	remove_huge_page(page);
> > > -	if (unlikely(hugetlb_unreserve_pages(inode, index, index + 1, 1)))
> > > -		hugetlb_fix_reserve_counts(inode);
> > 
> > As mentioned, huge page reserve counts are not used to record number of
> > poisioned pages.  The calls to hugetlb_unreserve_pages and possibly
> > hugetlb_fix_reserve_counts are necessary for reserve accounting.  They
> > can not be removed.
> 
> Thanks for your explanation very much. I didn't get the point of the
> comments from the first patch. hugetlb_fix_reserve_counts() shouldn't
> be removed and I will fix this.
> 
> > > +	hugetlb_fix_hwcrp_counts(page);
> > 
> > This new routine just counts memory errors on 'in use' huge pages.
> > I do not see a call anywhere to count memory errors on huge pages
> > not in use.
> 
> It's my oversight. I should have considered this situation. I tested
> it with hwcrp_hugepages count, and this is the result:
> # cat /proc/meminfo |grep -E 'HugePages_|Hard'
> HardwareCorrupted:     0 kB
> HugePages_Total:      64
> HugePages_Free:       64
> HugePages_Rsvd:        0
> HugePages_Surp:        0
> HugePages_Hwcrp:       0
> No count changes, even the HardwareCorrupted. I'm not sure if this is
> normal. This is what happens in kernel(stable master branch):
> static int memory_failure_hugetlb(unsigned long pfn, int flags)
> {
> 	...
> 	/* TestSetPageHWPoison return 0 */
> 	if (TestSetPageHWPoison(head)) {
> 		...
> 	}
> 
> 	num_poisoned_pages_inc(); /* HardwareCorrupted += PAGE_SIZE */
> 
> 	/* get_hwpoison_page() return 0 */
> 	if (!(flags & MF_COUNT_INCREASED) && !get_hwpoison_page(p, flags, 0)) {
> 		/*
> 		 * Check "filter hit" and "race with other subpage."
> 		 */
> 		lock_page(head);
> 		/* PageHWPoison() return 1 */
> 		if (PageHWPoison(head)) {
> 			/* (p != head && TestSetPageHWPoison(head)) is hit */
> 			if ((hwpoison_filter(p) && TestClearPageHWPoison(p))
> 			    || (p != head && TestSetPageHWPoison(head))) {
> 				/* HardwareCorrupted -= PAGE_SIZE */
> 				num_poisoned_pages_dec();
> 				unlock_page(head);
> 				return 0;
> 			}
> 		}
> 		...
> 	}
> 	...
> }
> It seems like that memory errors on huge pages not in use hit the
> "race with other subpage". I think I shouldn't add hwcrp_hugepages in
> this routine. Maybe we need more conditions to distinguish this.

BTW, I think that the race handled by this if-block will be no longer
necessary if mf_mutex is extended to cover unpoison_memory() by patch
"mm/hwpoison: mf_mutex for soft offline and unpoison" [1].
I'm thinking of updating patch 2/6 [2] to remove this if-block.

  [1]: https://lore.kernel.org/linux-mm/20210614021212.223326-2-nao.horiguchi@gmail.com/
  [2]: https://lore.kernel.org/linux-mm/20210614021212.223326-3-nao.horiguchi@gmail.com/

# .. so you don't have to care for the detail of this part.

> 
> > >  
> > >  	return 0;
> > >  }
> > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > index f7ca1a3870ea..1d5bada80aa5 100644
> > > --- a/include/linux/hugetlb.h
> > > +++ b/include/linux/hugetlb.h
> > > @@ -171,6 +171,7 @@ void putback_active_hugepage(struct page *page);
> > >  void move_hugetlb_state(struct page *oldpage, struct page *newpage, int reason);
> > >  void free_huge_page(struct page *page);
> > >  void hugetlb_fix_reserve_counts(struct inode *inode);
> > > +void hugetlb_fix_hwcrp_counts(struct page *page);
> > >  extern struct mutex *hugetlb_fault_mutex_table;
> > >  u32 hugetlb_fault_mutex_hash(struct address_space *mapping, pgoff_t idx);
> > >  
> > > @@ -602,12 +603,14 @@ struct hstate {
> > >  	unsigned long free_huge_pages;
> > >  	unsigned long resv_huge_pages;
> > >  	unsigned long surplus_huge_pages;
> > > +	unsigned long hwcrp_huge_pages;
> > >  	unsigned long nr_overcommit_huge_pages;
> > >  	struct list_head hugepage_activelist;
> > >  	struct list_head hugepage_freelists[MAX_NUMNODES];
> > >  	unsigned int nr_huge_pages_node[MAX_NUMNODES];
> > >  	unsigned int free_huge_pages_node[MAX_NUMNODES];
> > >  	unsigned int surplus_huge_pages_node[MAX_NUMNODES];
> > > +	unsigned int hwcrp_huge_pages_node[MAX_NUMNODES];
> > 
> > I understand your requirement to count the number of memory errors on
> > hugetlb pages.  However, we need to think carefully about we represent
> > that count.
> > 
> > Noaya, do you have opinions on where would be the best place to store
> > this information?  The hugetlb memory error code has the comment 'needs
> > work'.  Ideally, we could isolate memory errors to a single base (4K for
> > x86) page and free the remaining base pages to buddy.  We could also
> > potentially allocate a 'replacement' hugetlb page doing something like
> > alloc_and_dissolve_huge_page.

Yes, these two are the important goals of hugetlb support.

> > 
> > If we get an error on a hugetlb page and can isolate it to a base page
> > and replace the huge page, is it still a huge page memory error?

That's not counted as a huge page error, but just a base page error.

> > 
> > IMO, we should work on isolating memory errors to a base page and
> > replacing the huge page.  Then, the existing count of base pages with
> > memory errors would be sufficient?

Yes, I agree with Mike.  Adding new fields dedicated to the (maybe temporary)
counters in struct hstate seems to me an overkill.

> > 
> > This is something I would like to work, but I have higher priorities
> > right now.
> 
> Yes, splitting the huge pages and isolating a base page is ideal. And
> we do this with dissolve_free_huge_page() when page_mapping() return
> NULL. I think there is a reason(but I do not get it) why we don't split
> huge pags in hugetlbfs_error_remove_page() or after. So I choose to 
> add a new count.

Maybe the resource is the main reason of this incompleteness, I noticed this
for years and continued to say "this is in my todo list", but still don't
make it (really sorry about that...).  Anyway, if you can (I hope) solve
your problem with "/proc/kpageflag" approach, which is a recommended solution.

Thanks,
Naoya Horiguchi

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