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

[Anshuman Khandual]

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

> 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.

> 
> 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 ?

> 
> 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.