linux-kernel - Re: [v2 PATCH] mm: pvmw: check possible huge PMD map by transhuge_vma

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Date:   Wed, 18 May 2022 13:31:29 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     willy@...radead.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: pvmw: check possible huge PMD map by
 transhuge_vma_suitable()

On Fri, May 13, 2022 at 12:17:05PM -0700, Yang Shi wrote:
> IIUC PVMW checks if the vma is possibly huge PMD mapped by
> transparent_hugepage_active() and "pvmw->nr_pages >= HPAGE_PMD_NR".
> 
> Actually pvmw->nr_pages is returned by compound_nr() or
> folio_nr_pages(), so the page should be THP as long as "pvmw->nr_pages
> >= HPAGE_PMD_NR".  And it is guaranteed THP is allocated for valid VMA
> in the first place.  But it may be not PMD mapped if the VMA is file
> VMA and it is not properly aligned.  The transhuge_vma_suitable()
> is used to do such check, so replace transparent_hugepage_active() to
> it, which is too heavy and overkilling.
> 
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Muchun Song <songmuchun@...edance.com>
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
> v2: * Fixed build error for !CONFIG_TRANSPARENT_HUGEPAGE
>     * Removed fixes tag per Willy
> 
>  include/linux/huge_mm.h | 8 ++++++--
>  mm/page_vma_mapped.c    | 2 +-
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index fbf36bb1be22..c2826b1f4069 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -117,8 +117,10 @@ extern struct kobj_attribute shmem_enabled_attr;
>  extern unsigned long transparent_hugepage_flags;
>  
>  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> -		unsigned long haddr)
> +		unsigned long addr)
>  {
> +	unsigned long haddr;
> +
>  	/* Don't have to check pgoff for anonymous vma */
>  	if (!vma_is_anonymous(vma)) {
>  		if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> @@ -126,6 +128,8 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>  			return false;
>  	}
>  
> +	haddr = addr & HPAGE_PMD_MASK;
> +
>  	if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
>  		return false;
>  	return true;
> @@ -328,7 +332,7 @@ static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
>  }
>  
>  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> -		unsigned long haddr)
> +		unsigned long addr)
>  {
>  	return false;
>  }
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index c10f839fc410..e971a467fcdf 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -243,7 +243,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			 * cleared *pmd but not decremented compound_mapcount().
>  			 */
>  			if ((pvmw->flags & PVMW_SYNC) &&
> -			    transparent_hugepage_active(vma) &&
> +			    transhuge_vma_suitable(vma, pvmw->address) &&

How about the following diff? Then we do not need to change
transhuge_vma_suitable().  All the users of transhuge_vma_suitable()
are already do the alignment by themselves.

Thanks.

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index c10f839fc410..0aed5ca60c67 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -243,7 +243,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
                         * cleared *pmd but not decremented compound_mapcount().
                         */
                        if ((pvmw->flags & PVMW_SYNC) &&
-                           transparent_hugepage_active(vma) &&
+                           IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+                           transhuge_vma_suitable(vma, pvmw->address & HPAGE_PMD_MASK) &&
                            (pvmw->nr_pages >= HPAGE_PMD_NR)) {
                                spinlock_t *ptl = pmd_lock(mm, pvmw->pmd);

>  			    (pvmw->nr_pages >= HPAGE_PMD_NR)) {
>  				spinlock_t *ptl = pmd_lock(mm, pvmw->pmd);
>  
> -- 
> 2.26.3
> 
>