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Subject[RFC PATCH 1/4] mm: madvise: read loop's step size beforehand in madvise_inject_error(), prepare for THP support.
Date
From: Zi Yan <zi.yan@cs.rutgers.edu>

The loop in madvise_inject_error() reads its step size from a page
after it is soft-offlined. It works because the page is:
1) a hugetlb page: the page size does not change;
2) a base page: the page size does not change;
3) a THP: soft-offline always splits THPs, thus, it is OK to use
PAGE_SIZE as step size.

It will be a problem when soft-offline supports THP migrations.
When a THP is migrated without split during soft-offlining, the THP
is split after migration, thus, before and after soft-offlining page
sizes do not match. This causes a THP to be unnecessarily soft-lined,
at most, 511 times, wasting free space.

Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
---
mm/madvise.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 47d8d8a25eae..49f6774db259 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -612,19 +612,22 @@ static long madvise_remove(struct vm_area_struct *vma,
static int madvise_inject_error(int behavior,
unsigned long start, unsigned long end)
{
- struct page *page;
+ struct page *page = NULL;
+ unsigned long page_size = PAGE_SIZE;

if (!capable(CAP_SYS_ADMIN))
return -EPERM;

- for (; start < end; start += PAGE_SIZE <<
- compound_order(compound_head(page))) {
+ for (; start < end; start += page_size) {
int ret;

ret = get_user_pages_fast(start, 1, 0, &page);
if (ret != 1)
return ret;

+ page_size = (PAGE_SIZE << compound_order(compound_head(page))) -
+ (PAGE_SIZE * (page - compound_head(page)));
+
if (PageHWPoison(page)) {
put_page(page);
continue;
@@ -637,6 +640,12 @@ static int madvise_inject_error(int behavior,
ret = soft_offline_page(page, MF_COUNT_INCREASED);
if (ret)
return ret;
+ /*
+ * Non hugetlb pages either have PAGE_SIZE
+ * or are split into PAGE_SIZE
+ */
+ if (!PageHuge(page))
+ page_size = PAGE_SIZE;
continue;
}
pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
@@ -645,6 +654,12 @@ static int madvise_inject_error(int behavior,
ret = memory_failure(page_to_pfn(page), 0, MF_COUNT_INCREASED);
if (ret)
return ret;
+ /*
+ * Non hugetlb pages either have PAGE_SIZE
+ * or are split into PAGE_SIZE
+ */
+ if (!PageHuge(page))
+ page_size = PAGE_SIZE;
}
return 0;
}
--
2.13.2
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