linux-kernel - [patch] mm, thp: always direct reclaim for MADV

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Date:   Wed, 21 Dec 2016 16:21:54 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Jonathan Corbet <corbet@....net>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [patch] mm, thp: always direct reclaim for MADV_HUGEPAGE even when
 deferred

Currently, when defrag is set to "madvise", thp allocations will direct
reclaim.  However, when defrag is set to "defer", all thp allocations do
not attempt reclaim regardless of MADV_HUGEPAGE.

This patch always directly reclaims for MADV_HUGEPAGE regions when defrag
is not set to "never."  The idea is that MADV_HUGEPAGE regions really
want to be backed by hugepages and are willing to endure the latency at
fault as it was the default behavior prior to commit 444eb2a449ef ("mm:
thp: set THP defrag by default to madvise and add a stall-free defrag
option").

In this form, "defer" is a stronger, more heavyweight version of
"madvise".

Signed-off-by: David Rientjes <rientjes@...gle.com>
---
 Documentation/vm/transhuge.txt |  7 +++++--
 mm/huge_memory.c               | 10 ++++++----
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
--- a/Documentation/vm/transhuge.txt
+++ b/Documentation/vm/transhuge.txt
@@ -121,8 +121,11 @@ to utilise them.
 
 "defer" means that an application will wake kswapd in the background
 to reclaim pages and wake kcompact to compact memory so that THP is
-available in the near future. It's the responsibility of khugepaged
-to then install the THP pages later.
+available in the near future, unless it is for a region where
+madvise(MADV_HUGEPAGE) has been used, in which case direct reclaim will be
+used. Kcompactd will attempt to make hugepages available for allocation in
+the near future and khugepaged will try to collapse existing memory into
+hugepages later.
 
 "madvise" will enter direct reclaim like "always" but only for regions
 that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -619,15 +619,17 @@ static int __do_huge_pmd_anonymous_page(struct vm_fault *vmf, struct page *page,
  */
 static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
 {
-	bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
+	const bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
 
 	if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG,
 				&transparent_hugepage_flags) && vma_madvised)
 		return GFP_TRANSHUGE;
 	else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
-						&transparent_hugepage_flags))
-		return GFP_TRANSHUGE_LIGHT | __GFP_KSWAPD_RECLAIM;
-	else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
+						&transparent_hugepage_flags)) {
+		return GFP_TRANSHUGE_LIGHT |
+		       (vma_madvised ? __GFP_DIRECT_RECLAIM :
+				       __GFP_KSWAPD_RECLAIM);
+	} else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
 						&transparent_hugepage_flags))
 		return GFP_TRANSHUGE | (vma_madvised ? 0 : __GFP_NORETRY);