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

[David Hildenbrand]

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@xxxxxxxxxx>
> ---
>   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? Wouldn't we 
have to remember that we already took the lock to not lock twice, and to 
see if we really have to unlock in page_vma_mapped_walk_done() ?

--
Thanks,

David / dhildenb