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Date: Fri, 22 Apr 2022 08:54:06 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: David Hildenbrand <david@...hat.com>, 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 4/21/22 11:15, David Hildenbrand wrote:
> 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:
LGTM
> 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
>
>
>
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