[PATCH 2/2] Check PG_swapbacked for swap cache pages

[Petr Tesarik]

When page cache is filtered out (dump level bitmap includes 2 or 4),
makedumpfile checks the PG_swapcache bit, but since kernel commit
6326fec1122cde256bd2a8c63f2606e08e44ce1d (v4.10-rc1~7) this bit is
an alias for PG_owner_priv_1, which is also used by filesystem
code (PG_checked) and Xen (PG_pinned and PG_foreign).

With these kernels, the PG_swapcache flag is valid only if
PG_swapbacked is set. A Linux kernel patch has already been
submitted to export the value of PG_swapbacked in VMCOREINFO.

Since there are released kernels in the wild which do not export the
value, a fallback is implemented. I considered these three situations:

  1. Kernels before v2.6.28-rc1~244:
     PG_swapbacked does not exist, so it must not be checked.
     Instead, check PG_swapcache, which is never overloaded for
     another purpose.

  2. Kernels between v2.6.28-rc1~244 and v4.10-rc1~7:
     It is sufficient to check only PG_swapcache, but PG_swapbacked
     may also be checked (it is always set if PG_swapcache is set).

  3. Kernels since v4.10-rc1~7:
     PG_swapbacked must be checked.

If PG_swapbacked value is known (exported or read from debuginfo),
it is always safe to use it (case 2 or 3). If PG_swapbacked is not
known, it is safe to ignore it for cases 1 and 2, but not 3.
Thankfully, the new value of PG_swapcache (since v4.10-rc1~7) is
less than PG_private (which is known), whereas the old value had
always been greater than PG_private. Moreover, the flags between
PG_private and PG_swapbacked haven't changed since v4.10-rc1~7, so
PG_swapbacked can fall back to PG_private + 6 if unknown.

Without this patch, all Xen dumps are unusable, because PG_pinned is
set for all page table pages.

Signed-off-by: Petr Tesarik <ptesarik at suse.com>
---
 makedumpfile.c | 19 ++++++++++++++++++-
 makedumpfile.h |  2 ++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/makedumpfile.c b/makedumpfile.c
index 175ba68..ec04a88 100644
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -252,7 +252,18 @@ isHugetlb(unsigned long dtor)
 static int
 is_cache_page(unsigned long flags)
 {
-	return isLRU(flags) || isSwapCache(flags);
+	if (isLRU(flags))
+		return TRUE;
+
+	/* PG_swapcache is valid only if:
+	 *   a. PG_swapbacked bit is set, or
+	 *   b. PG_swapbacked did not exist (kernels before 4.10-rc1).
+	 */
+	if ((NUMBER(PG_swapbacked) == NOT_FOUND_NUMBER || isSwapBacked(flags))
+	    && isSwapCache(flags))
+		return TRUE;
+
+	return FALSE;
 }
 
 static inline unsigned long
@@ -1735,6 +1746,7 @@ get_structure_info(void)
 	ENUM_NUMBER_INIT(PG_lru, "PG_lru");
 	ENUM_NUMBER_INIT(PG_private, "PG_private");
 	ENUM_NUMBER_INIT(PG_swapcache, "PG_swapcache");
+	ENUM_NUMBER_INIT(PG_swapbacked, "PG_swapbacked");
 	ENUM_NUMBER_INIT(PG_buddy, "PG_buddy");
 	ENUM_NUMBER_INIT(PG_slab, "PG_slab");
 	ENUM_NUMBER_INIT(PG_hwpoison, "PG_hwpoison");
@@ -1988,6 +2000,9 @@ get_value_for_old_linux(void)
 		NUMBER(PG_private) = PG_private_ORIGINAL;
 	if (NUMBER(PG_swapcache) == NOT_FOUND_NUMBER)
 		NUMBER(PG_swapcache) = PG_swapcache_ORIGINAL;
+	if (NUMBER(PG_swapbacked) == NOT_FOUND_NUMBER
+	    && NUMBER(PG_swapcache) < NUMBER(PG_private))
+		NUMBER(PG_swapbacked) = NUMBER(PG_private) + 6;
 	if (NUMBER(PG_slab) == NOT_FOUND_NUMBER)
 		NUMBER(PG_slab) = PG_slab_ORIGINAL;
 	if (NUMBER(PG_head_mask) == NOT_FOUND_NUMBER)
@@ -2264,6 +2279,7 @@ write_vmcoreinfo_data(void)
 	WRITE_NUMBER("PG_private", PG_private);
 	WRITE_NUMBER("PG_head_mask", PG_head_mask);
 	WRITE_NUMBER("PG_swapcache", PG_swapcache);
+	WRITE_NUMBER("PG_swapbacked", PG_swapbacked);
 	WRITE_NUMBER("PG_buddy", PG_buddy);
 	WRITE_NUMBER("PG_slab", PG_slab);
 	WRITE_NUMBER("PG_hwpoison", PG_hwpoison);
@@ -2658,6 +2674,7 @@ read_vmcoreinfo(void)
 	READ_NUMBER("PG_private", PG_private);
 	READ_NUMBER("PG_head_mask", PG_head_mask);
 	READ_NUMBER("PG_swapcache", PG_swapcache);
+	READ_NUMBER("PG_swapbacked", PG_swapbacked);
 	READ_NUMBER("PG_slab", PG_slab);
 	READ_NUMBER("PG_buddy", PG_buddy);
 	READ_NUMBER("PG_hwpoison", PG_hwpoison);
diff --git a/makedumpfile.h b/makedumpfile.h
index 6205ef3..fe306bb 100644
--- a/makedumpfile.h
+++ b/makedumpfile.h
@@ -155,6 +155,7 @@ test_bit(int nr, unsigned long addr)
 #define isPrivate(flags)	test_bit(NUMBER(PG_private), flags)
 #define isCompoundHead(flags)   (!!((flags) & NUMBER(PG_head_mask)))
 #define isSwapCache(flags)	test_bit(NUMBER(PG_swapcache), flags)
+#define isSwapBacked(flags)	test_bit(NUMBER(PG_swapbacked), flags)
 #define isHWPOISON(flags)	(test_bit(NUMBER(PG_hwpoison), flags) \
 				&& (NUMBER(PG_hwpoison) != NOT_FOUND_NUMBER))
 
@@ -1881,6 +1882,7 @@ struct number_table {
 	long	PG_head;
 	long	PG_head_mask;
 	long	PG_swapcache;
+	long	PG_swapbacked;
 	long	PG_buddy;
 	long	PG_slab;
 	long    PG_hwpoison;
-- 
2.13.6