Re: [PATCH 1/2] mm: hugetlb: proc: check for hugetlb shared PMD in /proc/PID/smaps

[Andrew Morton]

On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 26.01.23 23:27, Mike Kravetz wrote:
> > A hugetlb page will have a mapcount of 1 if mapped by multiple processes
> > via a shared PMD.  This is because only the first process increases the
> > map count, and subsequent processes just add the shared PMD page to
> > their page table.
> > 
> > page_mapcount is being used to decide if a hugetlb page is shared or
> > private in /proc/PID/smaps.  Pages referenced via a shared PMD were
> > incorrectly being counted as private.
> > 
> > To fix, check for a shared PMD if mapcount is 1.  If a shared PMD is
> > found count the hugetlb page as shared.  A new helper to check for a
> > shared PMD is added.
> > 
> ...
>
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
> >   
> >   		if (mapcount >= 2)
> >   			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> > -		else
> > -			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
> > +		else {
> 
> Better:
> 
> if (mapcount >= 2 || hugetlb_pmd_shared(pte))
> 	mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
> else
> 	mss->private_hugetlb += huge_page_size(hstate_vma(vma));

Yup.  And that local doesn't add any value?

--- a/fs/proc/task_mmu.c~mm-hugetlb-proc-check-for-hugetlb-shared-pmd-in-proc-pid-smaps-fix
+++ a/fs/proc/task_mmu.c
@@ -745,18 +745,10 @@ static int smaps_hugetlb_range(pte_t *pt
 			page = pfn_swap_entry_to_page(swpent);
 	}
 	if (page) {
-		int mapcount = page_mapcount(page);
-
-		if (mapcount >= 2)
+		if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte))
 			mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
-		else {
-			if (hugetlb_pmd_shared(pte))
-				mss->shared_hugetlb +=
-						huge_page_size(hstate_vma(vma));
-			else
-				mss->private_hugetlb +=
-						huge_page_size(hstate_vma(vma));
-		}
+		else
+			mss->private_hugetlb += huge_page_size(hstate_vma(vma));
 	}
 	return 0;
 }
_