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Date: Thu, 21 Apr 2022 11:15:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Rientjes <rientjes@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
John Hubbard <jhubbard@...dia.com>,
Jason Gunthorpe <jgg@...dia.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Mike Rapoport <rppt@...ux.ibm.com>,
Yang Shi <shy828301@...il.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
Roman Gushchin <guro@...com>,
Andrea Arcangeli <aarcange@...hat.com>,
Peter Xu <peterx@...hat.com>,
Donald Dutile <ddutile@...hat.com>,
Christoph Hellwig <hch@....de>,
Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
Liang Zhang <zhangliang5@...wei.com>,
Pedro Gomes <pedrodemargomes@...il.com>,
Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 16/16] mm/gup: sanity-check with CONFIG_DEBUG_VM that
anonymous pages are exclusive when (un)pinning
On 19.04.22 19:40, Vlastimil Babka wrote:
> On 3/29/22 18:04, David Hildenbrand wrote:
>> Let's verify when (un)pinning anonymous pages that we always deal with
>> exclusive anonymous pages, which guarantees that we'll have a reliable
>> PIN, meaning that we cannot end up with the GUP pin being inconsistent
>> with he pages mapped into the page tables due to a COW triggered
>> by a write fault.
>>
>> When pinning pages, after conditionally triggering GUP unsharing of
>> possibly shared anonymous pages, we should always only see exclusive
>> anonymous pages. Note that anonymous pages that are mapped writable
>> must be marked exclusive, otherwise we'd have a BUG.
>>
>> When pinning during ordinary GUP, simply add a check after our
>> conditional GUP-triggered unsharing checks. As we know exactly how the
>> page is mapped, we know exactly in which page we have to check for
>> PageAnonExclusive().
>>
>> When pinning via GUP-fast we have to be careful, because we can race with
>> fork(): verify only after we made sure via the seqcount that we didn't
>> race with concurrent fork() that we didn't end up pinning a possibly
>> shared anonymous page.
>>
>> Similarly, when unpinning, verify that the pages are still marked as
>> exclusive: otherwise something turned the pages possibly shared, which
>> can result in random memory corruptions, which we really want to catch.
>>
>> With only the pinned pages at hand and not the actual page table entries
>> we have to be a bit careful: hugetlb pages are always mapped via a
>> single logical page table entry referencing the head page and
>> PG_anon_exclusive of the head page applies. Anon THP are a bit more
>> complicated, because we might have obtained the page reference either via
>> a PMD or a PTE -- depending on the mapping type we either have to check
>> PageAnonExclusive of the head page (PMD-mapped THP) or the tail page
>> (PTE-mapped THP) applies: as we don't know and to make our life easier,
>> check that either is set.
>>
>> Take care to not verify in case we're unpinning during GUP-fast because
>> we detected concurrent fork(): we might stumble over an anonymous page
>> that is now shared.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>
> Nits:
>
>> @@ -510,6 +563,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>> page = ERR_PTR(-EMLINK);
>> goto out;
>> }
>> +
>> + VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) &&
>> + !PageAnonExclusive(page));
>
> Do we rather want VM_BUG_ON_PAGE? Also for the same tests in mm/huge*.c below.
Make sense, thanks:
diff --git a/mm/gup.c b/mm/gup.c
index 5c17d4816441..46ffd8c51c6e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -564,8 +564,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
goto out;
}
- VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) &&
- !PageAnonExclusive(page));
+ VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
+ !PageAnonExclusive(page), page);
/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
if (unlikely(!try_grab_page(page, flags))) {
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 390f22334ee9..a2f44d8d3d47 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1392,8 +1392,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
return ERR_PTR(-EMLINK);
- VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) &&
- !PageAnonExclusive(page));
+ VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
+ !PageAnonExclusive(page), page);
if (!try_grab_page(page, flags))
return ERR_PTR(-ENOMEM);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8a635b5b5270..0ba2b1930b21 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6100,8 +6100,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
page = pte_page(huge_ptep_get(pte));
- VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) &&
- !PageAnonExclusive(page));
+ VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
+ !PageAnonExclusive(page), page);
/*
* If subpage information not requested, update counters
--
Thanks,
David / dhildenb
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