Re: [PATCH v3 16/16] mm/gup: sanity-check with CONFIG_DEBUG_VM that anonymous pages are exclusive when (un)pinning

[Vlastimil Babka <vbabka@suse.cz>]

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@redhat.com>
>> 
>> Acked-by: Vlastimil Babka <vbabka@suse.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
> 
> 
>