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Subject[PATCH] Lock mmap_sem when calling migrate_pages() in do_move_pages_to_node()
Date
From: Zi Yan <zi.yan@cs.rutgers.edu>

migrate_pages() requires at least down_read(mmap_sem) to protect
related page tables and VMAs from changing. Let's do it in
do_page_moves() for both do_move_pages_to_node() and
add_page_for_migration().

Also add this lock requirement in the comment of migrate_pages().

Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
---
mm/migrate.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 5d0dc7b85f90..52d029953c32 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1354,6 +1354,9 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
* or free list only if ret != 0.
*
* Returns the number of pages that were not migrated, or an error code.
+ *
+ * The caller must hold at least down_read(mmap_sem) for to-be-migrated pages
+ * to protect related page tables and VMAs from changing.
*/
int migrate_pages(struct list_head *from, new_page_t get_new_page,
free_page_t put_new_page, unsigned long private,
@@ -1457,6 +1460,12 @@ static int store_status(int __user *status, int start, int value, int nr)
return 0;
}

+/*
+ * Migrates the pages from pagelist and put back those not migrated.
+ *
+ * The caller must at least hold down_read(mmap_sem), which is required
+ * for migrate_pages()
+ */
static int do_move_pages_to_node(struct mm_struct *mm,
struct list_head *pagelist, int node)
{
@@ -1487,7 +1496,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
unsigned int follflags;
int err;

- down_read(&mm->mmap_sem);
err = -EFAULT;
vma = find_vma(mm, addr);
if (!vma || addr < vma->vm_start || !vma_migratable(vma))
@@ -1540,7 +1548,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
*/
put_page(page);
out:
- up_read(&mm->mmap_sem);
return err;
}

@@ -1561,6 +1568,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,

migrate_prep();

+ down_read(&mm->mmap_sem);
for (i = start = 0; i < nr_pages; i++) {
const void __user *p;
unsigned long addr;
@@ -1628,6 +1636,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
if (!err)
err = err1;
out:
+ up_read(&mm->mmap_sem);
return err;
}

--
2.15.1
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