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From: Anshuman Khandual <anshuman.khandual@arm.com>
To: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: kirill.shutemov@linux.intel.com, akpm@linux-foundation.org,
	mhocko@suse.com, zi.yan@cs.rutgers.edu, will.deacon@arm.com
Subject: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry
Date: Tue,  9 Oct 2018 09:28:58 +0530	[thread overview]
Message-ID: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com> (raw)

A normal mapped THP page at PMD level should be correctly differentiated
from a PMD migration entry while walking the page table. A mapped THP would
additionally check positive for pmd_present() along with pmd_trans_huge()
as compared to a PMD migration entry. This just adds a new conditional test
differentiating the two while walking the page table.

Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually
exclusive which makes the current conditional block work for both mapped
and migration entries. This is not same with arm64 where pmd_trans_huge()
returns positive for both mapped and migration entries. Could some one
please explain why pmd_trans_huge() has to return false for migration
entries which just install swap bits and its still a PMD ? Nonetheless
pmd_present() seems to be a better check to distinguish between mapped
and (non-mapped non-present) migration entries without any ambiguity.

 mm/page_vma_mapped.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index ae3c2a3..b384396 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -161,7 +161,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 	pmde = READ_ONCE(*pvmw->pmd);
 	if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
 		pvmw->ptl = pmd_lock(mm, pvmw->pmd);
-		if (likely(pmd_trans_huge(*pvmw->pmd))) {
+		if (likely(pmd_trans_huge(*pvmw->pmd) &&
+					pmd_present(*pvmw->pmd))) {
 			if (pvmw->flags & PVMW_MIGRATION)
 				return not_found(pvmw);
 			if (pmd_page(*pvmw->pmd) != page)
-- 
2.7.4


             reply	other threads:[~2018-10-09  3:59 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-09  3:58 Anshuman Khandual [this message]
2018-10-09 13:04 ` [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry Kirill A. Shutemov
2018-10-09 13:18   ` Will Deacon
2018-10-12  8:02     ` Anshuman Khandual
2018-10-15  8:32       ` Aneesh Kumar K.V
2018-10-16 13:16         ` Anshuman Khandual
2018-10-09 13:42   ` Anshuman Khandual
2018-10-09 13:58 ` Zi Yan
2018-10-10  4:05   ` Anshuman Khandual
2018-10-10 12:43     ` Zi Yan
2018-10-12  8:00       ` Anshuman Khandual
2018-10-15  0:53         ` Zi Yan
2018-10-15  4:06           ` Anshuman Khandual
2018-10-16 14:31             ` Zi Yan
2018-10-18  2:17               ` Naoya Horiguchi
2018-11-02  5:22                 ` Anshuman Khandual
2018-10-25  8:10               ` Anshuman Khandual
2018-10-25 18:45                 ` Zi Yan
2018-10-26  1:39                   ` Anshuman Khandual
2018-10-17  2:09           ` Andrea Arcangeli
2018-10-22 14:00             ` Zi Yan
2018-11-02  6:15             ` Anshuman Khandual
2018-11-06  0:35               ` Will Deacon
2018-11-06  9:51                 ` Anshuman Khandual

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