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Re: [PATCHv6 29/36] thp: implement split_huge_pmd()

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  Re: [PATCHv6 29/36] thp: implement split_huge_pmd() Vlastimil Babka <vbabka@suse.cz> - 2015-06-11 12:00 +0200
    Re: [PATCHv6 29/36] thp: implement split_huge_pmd() "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-06-22 13:20 +0200

#1163112 — Re: [PATCHv6 29/36] thp: implement split_huge_pmd()

On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> Original split_huge_page() combined two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> implements split_huge_pmd() which split given PMD without splitting
> other PMDs this page mapped with or underlying compound page.
>
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>

[...]

> +
> +	if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> +		/* Last compound_mapcount is gone. */
> +		__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> +		if (PageDoubleMap(page)) {
> +			/* No need in mapcount reference anymore */
> +			ClearPageDoubleMap(page);
> +			for (i = 0; i < HPAGE_PMD_NR; i++)
> +				atomic_dec(&page[i]._mapcount);
> +		}
> +	} else if (!TestSetPageDoubleMap(page)) {
> +		/*
> +		 * The first PMD split for the compound page and we still
> +		 * have other PMD mapping of the page: bump _mapcount in
> +		 * every small page.
> +		 * This reference will go away with last compound_mapcount.
> +		 */
> +		for (i = 0; i < HPAGE_PMD_NR; i++)
> +			atomic_inc(&page[i]._mapcount);

The order of actions here means that between TestSetPageDoubleMap() and 
the atomic incs, anyone calling page_mapcount() on one of the pages not 
processed by the for loop yet, will see a value lower by 1 from what he 
should see. I wonder if that can cause any trouble somewhere, especially 
if there's only one other compound mapping and page_mapcount() will 
return 0 instead of 1?

Conversely, when clearing PageDoubleMap() above (or in one of those rmap 
functions IIRC), one could see mapcount inflated by one. But I guess 
that's less dangerous.

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#1169841

On Thu, Jun 11, 2015 at 11:49:48AM +0200, Vlastimil Babka wrote:
> On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> >Original split_huge_page() combined two operations: splitting PMDs into
> >tables of PTEs and splitting underlying compound page. This patch
> >implements split_huge_pmd() which split given PMD without splitting
> >other PMDs this page mapped with or underlying compound page.
> >
> >Without tail page refcounting, implementation of split_huge_pmd() is
> >pretty straight-forward.
> >
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >Tested-by: Sasha Levin <sasha.levin@oracle.com>
> 
> [...]
> 
> >+
> >+	if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> >+		/* Last compound_mapcount is gone. */
> >+		__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> >+		if (PageDoubleMap(page)) {
> >+			/* No need in mapcount reference anymore */
> >+			ClearPageDoubleMap(page);
> >+			for (i = 0; i < HPAGE_PMD_NR; i++)
> >+				atomic_dec(&page[i]._mapcount);
> >+		}
> >+	} else if (!TestSetPageDoubleMap(page)) {
> >+		/*
> >+		 * The first PMD split for the compound page and we still
> >+		 * have other PMD mapping of the page: bump _mapcount in
> >+		 * every small page.
> >+		 * This reference will go away with last compound_mapcount.
> >+		 */
> >+		for (i = 0; i < HPAGE_PMD_NR; i++)
> >+			atomic_inc(&page[i]._mapcount);
> 
> The order of actions here means that between TestSetPageDoubleMap() and the
> atomic incs, anyone calling page_mapcount() on one of the pages not
> processed by the for loop yet, will see a value lower by 1 from what he
> should see. I wonder if that can cause any trouble somewhere, especially if
> there's only one other compound mapping and page_mapcount() will return 0
> instead of 1?

Good catch. Thanks.

What about this?

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0f1f5731a893..cd0e6addb662 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2636,15 +2636,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                        for (i = 0; i < HPAGE_PMD_NR; i++)
                                atomic_dec(&page[i]._mapcount);
                }
-       } else if (!TestSetPageDoubleMap(page)) {
+       } else if (!PageDoubleMap(page)) {
                /*
                 * The first PMD split for the compound page and we still
                 * have other PMD mapping of the page: bump _mapcount in
                 * every small page.
+                *
                 * This reference will go away with last compound_mapcount.
+                *
+                * Note, we need to increment mapcounts before setting
+                * PG_double_map to avoid false-negative page_mapped().
                 */
                for (i = 0; i < HPAGE_PMD_NR; i++)
                        atomic_inc(&page[i]._mapcount);
+
+               if (TestSetPageDoubleMap(page)) {
+                       /* Race with another  __split_huge_pmd() for the page */
+                       for (i = 0; i < HPAGE_PMD_NR; i++)
+                               atomic_dec(&page[i]._mapcount);
+               }
        }
 
        smp_wmb(); /* make pte visible before pmd */

> Conversely, when clearing PageDoubleMap() above (or in one of those rmap
> functions IIRC), one could see mapcount inflated by one. But I guess that's
> less dangerous.

I think it's safe.

-- 
 Kirill A. Shutemov
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