[PATCH v1] makedumpfile: fix detection of typed (compound) pages (Linux 6.12)

[David Hildenbrand <david@redhat.com>]

Ever since kernel commit 4ffca5a96678c ("mm: support only one page_type
per page"), page types are no longer flags (PG_slab, PG_offline, ...)
stored in page->_mapcount, but values stored in the top byte.

Because we currently try deriving flags from the mapcount values to
check for slab and hugetlb pages, we get:
(1) "false positives" from isSlab(), making us not detect free (buddy)
    pages and offline pages anymore.
(2) "false positives" when detecting hugetlb pages.

In the common case we now simply dump all memory, and fail to exclude
offline and free (buddy) pages, assuming they are all slab pages,
which is bad.

We should just consistently compare the page->_mapcount with the unmodified
PAGE_*_MAPCOUNT_VALUE, like we already did for free (buddy) and offline
pages already. This also works for older kernels, because the kernel never
supported having multiple page types set on a single page, so there was
never the need to derive flags from the PAGE_*_MAPCOUNT_VALUE.

It is worth noting that the lower 24bit of the page->_mapcount field
can be used while a page type is set. This is, however, currently not
the case for any of the involved page types (slab, buddy, offline,
hugetlb). In the future, the kernel could either just tell us the types,
or provide a mask to be applied to the page->_mapcount ("bits to ignore")
when comparing the values. But, there might be bigger changes coming up
with the "memdesc" work in the kernel, where the type would not longer
be stored in page->_mapcount ... so for now we can keep it simple.

Link: https://github.com/makedumpfile/makedumpfile/issues/16
Cc: Masamitsu Yamazaki <yamazaki-msmt@nec.com>
Cc: Kazuhito Hagio <k-hagio-ab@nec.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 makedumpfile.c | 6 ++----
 makedumpfile.h | 2 --
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/makedumpfile.c b/makedumpfile.c
index b356eb3..bad3c48 100644
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -280,8 +280,7 @@ isSlab(unsigned long flags, unsigned int _mapcount)
 {
 	/* Linux 6.10 and later */
 	if (NUMBER(PAGE_SLAB_MAPCOUNT_VALUE) != NOT_FOUND_NUMBER) {
-		unsigned int PG_slab = ~NUMBER(PAGE_SLAB_MAPCOUNT_VALUE);
-		if ((_mapcount & (PAGE_TYPE_BASE | PG_slab)) == PAGE_TYPE_BASE)
+		if (_mapcount == (int)NUMBER(PAGE_SLAB_MAPCOUNT_VALUE))
 			return TRUE;
 	}
 
@@ -6549,11 +6548,10 @@ __exclude_unnecessary_pages(unsigned long mem_map,
 			 */
 			if (NUMBER(PAGE_HUGETLB_MAPCOUNT_VALUE) != NOT_FOUND_NUMBER) {
 				unsigned long _flags_1 = ULONG(addr + OFFSET(page.flags));
-				unsigned int PG_hugetlb = ~NUMBER(PAGE_HUGETLB_MAPCOUNT_VALUE);
 
 				compound_order = _flags_1 & 0xff;
 
-				if ((_mapcount & (PAGE_TYPE_BASE | PG_hugetlb)) == PAGE_TYPE_BASE)
+				if (_mapcount == (int)NUMBER(PAGE_HUGETLB_MAPCOUNT_VALUE))
 					compound_dtor = IS_HUGETLB;
 
 				goto check_order;
diff --git a/makedumpfile.h b/makedumpfile.h
index 7ed566d..2b3495e 100644
--- a/makedumpfile.h
+++ b/makedumpfile.h
@@ -164,8 +164,6 @@ test_bit(int nr, unsigned long addr)
 #define isAnon(mapping, flags, _mapcount) \
 	(((unsigned long)mapping & PAGE_MAPPING_ANON) != 0 && !isSlab(flags, _mapcount))
 
-#define PAGE_TYPE_BASE		(0xf0000000)
-
 #define PTOB(X)			(((unsigned long long)(X)) << PAGESHIFT())
 #define BTOP(X)			(((unsigned long long)(X)) >> PAGESHIFT())
 
-- 
2.47.0