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

[David Hildenbrand <david@redhat.com>]

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:

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