linux-kernel - Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped

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Message-ID: <8290e578-9eaa-5859-6309-e62634b66fb5@redhat.com>
Date:   Wed, 30 Nov 2022 17:39:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        James Houghton <jthoughton@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped_walk() safe to pmd
 unshare

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@...hat.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@...hat.com>


-- 
Thanks,

David / dhildenb