linux-kernel - [PATCH v2 1/2] mm/migrate: optimize migrate_vma

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Date:   Fri, 10 Jul 2020 12:48:39 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     <linux-mm@...ck.org>, <kvm-ppc@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "Bharata B Rao" <bharata@...ux.ibm.com>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: [PATCH v2 1/2] mm/migrate: optimize migrate_vma_setup() for holes

When migrating system memory to device private memory, if the source
address range is a valid VMA range and there is no memory or a zero page,
the source PFN array is marked as valid but with no PFN. This lets the
device driver allocate private memory and clear it, then insert the new
device private struct page into the CPU's page tables when
migrate_vma_pages() is called. migrate_vma_pages() only inserts the
new page if the VMA is an anonymous range. There is no point in telling
the device driver to allocate device private memory and then not migrate
the page. Instead, mark the source PFN array entries as not migrating to
avoid this overhead.

Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
---
 mm/migrate.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index b0125c082549..ec00b7a6ea2a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2205,6 +2205,16 @@ static int migrate_vma_collect_hole(unsigned long start,
 	struct migrate_vma *migrate = walk->private;
 	unsigned long addr;
 
+	/* Only allow populating anonymous memory. */
+	if (!vma_is_anonymous(walk->vma)) {
+		for (addr = start; addr < end; addr += PAGE_SIZE) {
+			migrate->src[migrate->npages] = 0;
+			migrate->dst[migrate->npages] = 0;
+			migrate->npages++;
+		}
+		return 0;
+	}
+
 	for (addr = start; addr < end; addr += PAGE_SIZE) {
 		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
 		migrate->dst[migrate->npages] = 0;
@@ -2297,8 +2307,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		pte = *ptep;
 
 		if (pte_none(pte)) {
-			mpfn = MIGRATE_PFN_MIGRATE;
-			migrate->cpages++;
+			if (vma_is_anonymous(vma)) {
+				mpfn = MIGRATE_PFN_MIGRATE;
+				migrate->cpages++;
+			}
 			goto next;
 		}
 
-- 
2.20.1