Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped_walk() safe to pmd unshare

David Hildenbrand
2022-11-30T16:41:08Z

On 30.11.22 17:32, Peter Xu wrote:
> On Wed, Nov 30, 2022 at 05:18:45PM +0100, David Hildenbrand wrote:
>> On 29.11.22 20:35, Peter Xu wrote:
>>> Since page_vma_mapped_walk() walks the pgtable, it needs the vma lock
>>> to make sure the pgtable page will not be freed concurrently.
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>> include/linux/rmap.h | 4 ++++
>>> mm/page_vma_mapped.c | 5 ++++-
>>> 2 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>> index bd3504d11b15..a50d18bb86aa 100644
>>> --- a/include/linux/rmap.h
>>> +++ b/include/linux/rmap.h
>>> @@ -13,6 +13,7 @@
>>> #include <linux/highmem.h>
>>> #include <linux/pagemap.h>
>>> #include <linux/memremap.h>
>>> +#include <linux/hugetlb.h>
>>> /*
>>> * The anon_vma heads a list of private "related" vmas, to scan if
>>> @@ -408,6 +409,9 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
>>> pte_unmap(pvmw->pte);
>>> if (pvmw->ptl)
>>> spin_unlock(pvmw->ptl);
>>> + /* This needs to be after unlock of the spinlock */
>>> + if (is_vm_hugetlb_page(pvmw->vma))
>>> + hugetlb_vma_unlock_read(pvmw->vma);
>>> }
>>> bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index 93e13fc17d3c..f94ec78b54ff 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -169,10 +169,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>> if (pvmw->pte)
>>> return not_found(pvmw);
>>> + hugetlb_vma_lock_read(vma);
>>> /* when pud is not present, pte will be NULL */
>>> pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
>>> - if (!pvmw->pte)
>>> + if (!pvmw->pte) {
>>> + hugetlb_vma_unlock_read(vma);
>>> return false;
>>> + }
>>> pvmw->ptl = huge_pte_lock(hstate, mm, pvmw->pte);
>>> if (!check_pte(pvmw))
>>
>> Looking at code like mm/damon/paddr.c:__damon_pa_mkold() and reading the
>> doc of page_vma_mapped_walk(), this might be broken.
>>
>> Can't we get page_vma_mapped_walk() called multiple times?
>
> Yes it normally can, but not for hugetlbfs? Feel free to check:
>
> if (unlikely(is_vm_hugetlb_page(vma))) {
> ...
> /* The only possible mapping was handled on last iteration */
> if (pvmw->pte)
> return not_found(pvmw);
> }

Ah, I see, thanks.

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb