Re: [PATCH] futex: bugfix for futex-key conflict when futex usehugepage

[Mel Gorman]

On Tue, May 07, 2013 at 08:23:48PM +0800, Zhang Yi wrote:
> diff -uprN linux3.9-orig/kernel/futex.c linux3.9/kernel/futex.c
> --- linux3.9-orig/kernel/futex.c	2013-04-15 00:45:16.000000000 +0000
> +++ linux3.9/kernel/futex.c	2013-05-06 16:24:40.403525000 +0000
> @@ -215,6 +215,22 @@ static void drop_futex_key_refs(union fu
>  	}
>  }
> 
> +/*
> +* Get subpage index in compound page, and add it into futex_key.
> +*/
> +static void key_add_compound_idx(union futex_key *key,
> +				 struct page *head_page, struct page *page)
> +{
> +	int compound_idx;
> +
> +	if (compound_order(head_page) >= MAX_ORDER)
> +		compound_idx = page_to_pfn(page) - page_to_pfn(head_page);
> +	else
> +		compound_idx = page - head_page;
> +
> +	key->both.offset |= compound_idx << PAGE_SHIFT;
> +}
> +

This implicitely assumies it is dealing with a hugetlbfs page. Today, it
is the case that an inode-based futex with PageCompound is a hugetlbfs
page but that could change in the future if THP ever backs files. This
would then break again except it would be harder to fix because THP pages
can be collapsed underneath you after the futex key has been generated.

As this problem is hugetlbfs-specific should the fix be firmly in hugetlbfs
land? Something like the following untested and only partial diff? Is the
use of PageCompound in the futex path like this going to be problematic?

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 16e4e9a..f9c33d3 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -348,6 +348,17 @@ static inline int hstate_index(struct hstate *h)
 	return h - hstates;
 }
 
+pgoff_t __basepage_index(struct page *page);
+
+/* Return page->index in PAGE_SIZE units */
+static inline pgoff_t basepage_index(struct page *page)
+{
+	if (!PageCompound(page))
+		return page->index;
+
+	return __basepage_index(page);
+}
+
 #else
 struct hstate {};
 #define alloc_huge_page_node(h, nid) NULL
@@ -365,6 +376,10 @@ static inline unsigned int pages_per_huge_page(struct hstate *h)
 {
 	return 1;
 }
+static inline pgoff_t basepage_index(struct page *page)
+{
+	return page->index;
+}
 #define hstate_index_to_shift(index) 0
 #define hstate_index(h) 0
 #endif
diff --git a/kernel/futex.c b/kernel/futex.c
index b26dcfc..97beb5d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -61,6 +61,7 @@
 #include <linux/nsproxy.h>
 #include <linux/ptrace.h>
 #include <linux/sched/rt.h>
+#include <linux/hugetlb.h>
 
 #include <asm/futex.h>
 
@@ -365,7 +366,7 @@ again:
 	} else {
 		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
 		key->shared.inode = page_head->mapping->host;
-		key->shared.pgoff = page_head->index;
+		key->shared.pgoff = basepage_index(page_head);
 	}
 
 	get_futex_key_refs(key);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1a12f5b..ddbad35 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -690,6 +690,23 @@ int PageHuge(struct page *page)
 }
 EXPORT_SYMBOL_GPL(PageHuge);
 
+pgoff_t __basepage_index(struct page *page)
+{
+	struct page *page_head = compound_head(page);
+	pgoff_t index = page_index(page_head);
+	int compound_idx;
+
+	if (!PageHuge(page_head))
+		return page_index(page);
+
+	if (compound_order(page_head) >= MAX_ORDER)
+		compound_idx = page_to_pfn(page) - page_to_pfn(page_head);
+	else
+		compound_idx = page - head_page;
+
+	return (index << page_hstate(page_head)->order) + compound_idx;
+}
+
 static struct page *alloc_fresh_huge_page_node(struct hstate *h, int nid)
 {
 	struct page *page;
--
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