Re: [PATCH v2 2/2] mm: hugetlb: soft-offline: dissolve_free_huge_page() return zero on !PageHuge

[Naoya Horiguchi]

On Tue, Jun 11, 2019 at 03:20:26PM +0530, Anshuman Khandual wrote:
> 
> On 06/10/2019 01:48 PM, Naoya Horiguchi wrote:
> > madvise(MADV_SOFT_OFFLINE) often returns -EBUSY when calling soft offline
> > for hugepages with overcommitting enabled. That was caused by the suboptimal
> > code in current soft-offline code. See the following part:
> > 
> >     ret = migrate_pages(&pagelist, new_page, NULL, MPOL_MF_MOVE_ALL,
> >                             MIGRATE_SYNC, MR_MEMORY_FAILURE);
> >     if (ret) {
> >             ...
> >     } else {
> >             /*
> >              * We set PG_hwpoison only when the migration source hugepage
> >              * was successfully dissolved, because otherwise hwpoisoned
> >              * hugepage remains on free hugepage list, then userspace will
> >              * find it as SIGBUS by allocation failure. That's not expected
> >              * in soft-offlining.
> >              */
> >             ret = dissolve_free_huge_page(page);
> >             if (!ret) {
> >                     if (set_hwpoison_free_buddy_page(page))
> >                             num_poisoned_pages_inc();
> >             }
> >     }
> >     return ret;
> > 
> > Here dissolve_free_huge_page() returns -EBUSY if the migration source page
> > was freed into buddy in migrate_pages(), but even in that case we actually
> 
> Over committed source pages will be released into buddy and the normal ones
> will not be ? dissolve_free_huge_page() returns -EBUSY because PageHuge()
> return negative on already released pages ? 

The answers for both questions here are yes.

> How dissolve_free_huge_page()
> will behave differently with over committed pages. I might be missing some
> recent developments here.

This dissolve_free_huge_page() should see a (free or reused) 4kB page when
overcommitting, and should see a (free or reused) huge page for non
overcommitting case.

> 
> > has a chance that set_hwpoison_free_buddy_page() succeeds. So that means
> > current code gives up offlining too early now.
> 
> Hmm. It gives up early as the return value from dissolve_free_huge_page(EBUSY)
> gets back as the return code for soft_offline_huge_page() without attempting
> set_hwpoison_free_buddy_page() which still has a chance to succeed for freed
> normal buddy pages.

Exactly.

> 
> > 
> > dissolve_free_huge_page() checks that a given hugepage is suitable for
> > dissolving, where we should return success for !PageHuge() case because
> > the given hugepage is considered as already dissolved.
> 
> Right. It should return 0 (as a success) for freed normal buddy pages. Should
> not it then check explicitly for PageBuddy() as well ?

in new semantics, dissolve_free_huge_page() returns:

  0: successfully dissolved free hugepages or the page is already dissolved
  EBUSY: failed to dissolved free hugepages or the hugepage is in-use.

so for any types of non hugepages, the return value is 0.

Thanks,
- Naoya 

> > 
> > This change also affects other callers of dissolve_free_huge_page(),
> > which are cleaned up together.
> > 
> > Reported-by: Chen, Jerry T <jerry.t.chen@xxxxxxxxx>
> > Tested-by: Chen, Jerry T <jerry.t.chen@xxxxxxxxx>
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > Fixes: 6bc9b56433b76 ("mm: fix race on soft-offlining")
> > Cc: <stable@xxxxxxxxxxxxxxx> # v4.19+
> > ---
> >  mm/hugetlb.c        | 15 +++++++++------
> >  mm/memory-failure.c |  5 +----
> >  2 files changed, 10 insertions(+), 10 deletions(-)
> > 
> > diff --git v5.2-rc3/mm/hugetlb.c v5.2-rc3_patched/mm/hugetlb.c
> > index ac843d3..048d071 100644
> > --- v5.2-rc3/mm/hugetlb.c
> > +++ v5.2-rc3_patched/mm/hugetlb.c
> > @@ -1519,7 +1519,12 @@ int dissolve_free_huge_page(struct page *page)
> >  	int rc = -EBUSY;
> >  
> >  	spin_lock(&hugetlb_lock);
> > -	if (PageHuge(page) && !page_count(page)) {
> > +	if (!PageHuge(page)) {
> > +		rc = 0;
> > +		goto out;
> > +	}
> 
> With this early bail out it maintains the functionality when called from
> soft_offline_free_page() for normal pages. For huge page, it continues
> on the previous path.
> 
> > +
> > +	if (!page_count(page)) {
> >  		struct page *head = compound_head(page);
> >  		struct hstate *h = page_hstate(head);
> >  		int nid = page_to_nid(head);
> > @@ -1564,11 +1569,9 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
> >  
> >  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
> >  		page = pfn_to_page(pfn);
> > -		if (PageHuge(page) && !page_count(page)) {
> 
> Right. These checks are now redundant.
>