linux-kernel - Re: [PATCH v1 5/5] mm, hwpoison: enable memory error handling on 1GB hugepage

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Message-ID: <20220608061635.GA1413099@hori.linux.bs1.fc.nec.co.jp>
Date:   Wed, 8 Jun 2022 06:16:36 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Miaohe Lin <linmiaohe@...wei.com>
CC:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v1 5/5] mm, hwpoison: enable memory error handling on 1GB
 hugepage

On Tue, Jun 07, 2022 at 10:11:24PM +0800, Miaohe Lin wrote:
> On 2022/6/2 13:06, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@....com>
> > 
> > Now error handling code is prepared, so remove the blocking code and
> > enable memory error handling on 1GB hugepage.
> > 
> 
> I'm nervous about this change. It seems there are many code paths not awared of pud swap entry.
> I browsed some of them:
> apply_to_pud_range called from apply_to_page_range:
> 
> apply_to_pud_range:
> 	next = pud_addr_end(addr, end);
> 	if (pud_none(*pud) && !create)
> 		continue;
> 	if (WARN_ON_ONCE(pud_leaf(*pud)))
> 		return -EINVAL;
> 	if (!pud_none(*pud) && WARN_ON_ONCE(pud_bad(*pud))) {
> 		if (!create)
> 			continue;
> 		pud_clear_bad(pud);
> 	}
> 	err = apply_to_pmd_range(mm, pud, addr, next,
> 				 fn, data, create, mask);
> 
> For !pud_present case, it will mostly reach apply_to_pmd_range and call pmd_offset on it. And invalid
> pointer will be de-referenced.

apply_to_pmd_range() has BUG_ON(pud_huge(*pud)) and apply_to_pte_range() has
BUG_ON(pmd_huge(*pmd)), so this page table walking code seems to not expect
to handle pmd/pud level mapping.

> 
> Another example might be copy_pud_range and so on. So I think it might not be prepared to enable the
> 1GB hugepage or all of these places should be fixed?

I think that most of page table walker for user address space should first
check is_vm_hugetlb_page() and call hugetlb specific walking code for vma
with VM_HUGETLB.
copy_page_range() is a good example.  It calls copy_hugetlb_page_range()
for vma with VM_HUGETLB and the function should support hwpoison entry.
But I feel that I need testing for confirmation.

And I'm not sure that all other are prepared for non-present pud-mapping,
so I'll need somehow code inspection and testing for each.

Thanks,
Naoya Horiguchi