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Message-ID: <20131115102628.GD26002@suse.de>
Date: Fri, 15 Nov 2013 10:26:28 +0000
From: Mel Gorman <mgorman@...e.de>
To: Dave Hansen <dave@...1.net>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
dave.jiang@...el.com, akpm@...ux-foundation.org, dhillf@...il.com,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH 2/2] mm: thp: give transparent hugepage code a separate
copy_page
On Thu, Nov 14, 2013 at 03:34:00PM -0800, Dave Hansen wrote:
>
> Changes from v1:
> * removed explicit might_sleep() in favor of the one that we
> get from the cond_resched();
>
> --
>
> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> Right now, the migration code in migrate_page_copy() uses
> copy_huge_page() for hugetlbfs and thp pages:
>
> if (PageHuge(page) || PageTransHuge(page))
> copy_huge_page(newpage, page);
>
> So, yay for code reuse. But:
>
> void copy_huge_page(struct page *dst, struct page *src)
> {
> struct hstate *h = page_hstate(src);
>
> and a non-hugetlbfs page has no page_hstate(). This
> works 99% of the time because page_hstate() determines
> the hstate from the page order alone. Since the page
> order of a THP page matches the default hugetlbfs page
> order, it works.
>
> But, if you change the default huge page size on the
> boot command-line (say default_hugepagesz=1G), then
> we might not even *have* a 2MB hstate so page_hstate()
> returns null and copy_huge_page() oopses pretty fast
> since copy_huge_page() dereferences the hstate:
>
> void copy_huge_page(struct page *dst, struct page *src)
> {
> struct hstate *h = page_hstate(src);
> if (unlikely(pages_per_huge_page(h) > MAX_ORDER_NR_PAGES)) {
> ...
>
> This patch creates a copy_high_order_page() which can
> be used on THP pages.
>
> I believe the bug was introduced in b32967ff101:
> Author: Mel Gorman <mgorman@...e.de>
> Date: Mon Nov 19 12:35:47 2012 +0000
> mm: numa: Add THP migration for the NUMA working set scanning fault case.
>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
It is a mild pity that there are variants of this like copy_user_huge_page
for COW. They could be collapsed but the result API would not be pretty.
A rename of copy_huge_page to copy_hugetlbfs_page is justified to avoid
a repeat mistake. Alternatively, there seems to be little reason to add
hugetlbfs and thp specific apis when you could just do something like this
(untested)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0b7656e..784313a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -476,40 +476,6 @@ static int vma_has_reserves(struct vm_area_struct *vma, long chg)
return 0;
}
-static void copy_gigantic_page(struct page *dst, struct page *src)
-{
- int i;
- struct hstate *h = page_hstate(src);
- struct page *dst_base = dst;
- struct page *src_base = src;
-
- for (i = 0; i < pages_per_huge_page(h); ) {
- cond_resched();
- copy_highpage(dst, src);
-
- i++;
- dst = mem_map_next(dst, dst_base, i);
- src = mem_map_next(src, src_base, i);
- }
-}
-
-void copy_huge_page(struct page *dst, struct page *src)
-{
- int i;
- struct hstate *h = page_hstate(src);
-
- if (unlikely(pages_per_huge_page(h) > MAX_ORDER_NR_PAGES)) {
- copy_gigantic_page(dst, src);
- return;
- }
-
- might_sleep();
- for (i = 0; i < pages_per_huge_page(h); i++) {
- cond_resched();
- copy_highpage(dst + i, src + i);
- }
-}
-
static void enqueue_huge_page(struct hstate *h, struct page *page)
{
int nid = page_to_nid(page);
diff --git a/mm/migrate.c b/mm/migrate.c
index 9167b22..843b96d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -440,6 +440,49 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
return MIGRATEPAGE_SUCCESS;
}
+static void copy_gigantic_page(struct page *dst, struct page *src,
+ int nr_pages)
+{
+ int i;
+ struct page *dst_base = dst;
+ struct page *src_base = src;
+
+ for (i = 0; i < nr_pages; ) {
+ cond_resched();
+ copy_highpage(dst, src);
+
+ i++;
+ dst = mem_map_next(dst, dst_base, i);
+ src = mem_map_next(src, src_base, i);
+ }
+}
+
+void copy_huge_page(struct page *dst, struct page *src)
+{
+ int i;
+ int nr_pages;
+
+ if (PageHuge(src)) {
+ /* hugetlbfs page */
+ struct hstate *h = page_hstate(src);
+ nr_pages = pages_per_huge_page(h);
+
+ if (unlikely(nr_pages > MAX_ORDER_NR_PAGES)) {
+ copy_gigantic_page(dst, src, nr_pages);
+ return;
+ }
+ } else {
+ /* thp page */
+ BUG_ON(!PageTransHuge(src));
+ nr_pages = HPAGE_PMD_NR;
+ }
+
+ for (i = 0; i < nr_pages; i++) {
+ cond_resched();
+ copy_highpage(dst + i, src + i);
+ }
+}
+
/*
* Copy the page to its new location
*/
--
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