mm, page_alloc: more extensive free page checking with debug_pagealloc

The page allocator checks struct pages for expected state (mapcount,
flags etc) as pages are being allocated (check_new_page()) and freed
(free_pages_check()) to provide some defense against errors in page
allocator users.

Prior commits 479f854a207c ("mm, page_alloc: defer debugging checks of
pages allocated from the PCP") and 4db7548ccbd9 ("mm, page_alloc: defer
debugging checks of freed pages until a PCP drain") this has happened
for order-0 pages as they were allocated from or freed to the per-cpu
caches (pcplists).  Since those are fast paths, the checks are now
performed only when pages are moved between pcplists and global free
lists.  This however lowers the chances of catching errors soon enough.

In order to increase the chances of the checks to catch errors, the
kernel has to be rebuilt with CONFIG_DEBUG_VM, which also enables
multiple other internal debug checks (VM_BUG_ON() etc), which is
suboptimal when the goal is to catch errors in mm users, not in mm code
itself.

To catch some wrong users of the page allocator we have
CONFIG_DEBUG_PAGEALLOC, which is designed to have virtually no overhead
unless enabled at boot time.  Memory corruptions when writing to freed
pages have often the same underlying errors (use-after-free, double free)
as corrupting the corresponding struct pages, so this existing debugging
functionality is a good fit to extend by also perform struct page checks
at least as often as if CONFIG_DEBUG_VM was enabled.

Specifically, after this patch, when debug_pagealloc is enabled on boot,
and CONFIG_DEBUG_VM disabled, pages are checked when allocated from or
freed to the pcplists *in addition* to being moved between pcplists and
free lists.  When both debug_pagealloc and CONFIG_DEBUG_VM are enabled,
pages are checked when being moved between pcplists and free lists *in
addition* to when allocated from or freed to the pcplists.

When debug_pagealloc is not enabled on boot, the overhead in fast paths
should be virtually none thanks to the use of static key.

Link: http://lkml.kernel.org/r/20190603143451.27353-3-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index fa6d79281368..a35ab6c55192 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -19,12 +19,17 @@ config DEBUG_PAGEALLOC
 	  Depending on runtime enablement, this results in a small or large
 	  slowdown, but helps to find certain types of memory corruption.
 
+	  Also, the state of page tracking structures is checked more often as
+	  pages are being allocated and freed, as unexpected state changes
+	  often happen for same reasons as memory corruption (e.g. double free,
+	  use-after-free).
+
 	  For architectures which don't enable ARCH_SUPPORTS_DEBUG_PAGEALLOC,
 	  fill the pages with poison patterns after free_pages() and verify
-	  the patterns before alloc_pages().  Additionally,
-	  this option cannot be enabled in combination with hibernation as
-	  that would result in incorrect warnings of memory corruption after
-	  a resume because free pages are not saved to the suspend image.
+	  the patterns before alloc_pages(). Additionally, this option cannot
+	  be enabled in combination with hibernation as that would result in
+	  incorrect warnings of memory corruption after a resume because free
+	  pages are not saved to the suspend image.
 
 	  By default this option will have a small overhead, e.g. by not
 	  allowing the kernel mapping to be backed by large pages on some