Re: Widespread crashes in -next, bisected to 'mm: drop HASH_ADAPT'

[Michal Hocko]

On Sat 20-05-17 09:26:34, Michal Hocko wrote:
> On Fri 19-05-17 09:46:23, Guenter Roeck wrote:
> > Hi,
> > 
> > my qemu tests of next-20170519 show the following results:
> > 	total: 122 pass: 30 fail: 92
> > 
> > I won't bother listing all of the failures; they are available at
> > http://kerneltests.org/builders. I bisected one (openrisc, because
> > it gives me some console output before dying). It points to
> > 'mm: drop HASH_ADAPT' as the culprit. Bisect log is attached.
> > 
> > A quick glance suggests that 64 bit kernels pass and 32 bit kernels fail.
> > 32-bit x86 images fail and should provide an easy test case.
> 
> Hmm, this is quite unexpected as the patch is not supposed to change
> things much. It just removes the flag and perform the new hash scaling
> automatically for all requeusts which do not have any high limit.
> Some of those didn't have HASH_ADAPT before but that shouldn't change
> the picture much. The only thing that I can imagine is that what
> formerly failed for early memblock allocations is now suceeding and that
> depletes the early memory. Do you have any serial console from the boot?

OK, I guess I know what it going on here. Adaptive has scaling is not
really suited for 32b. ADAPT_SCALE_BASE is just too large for the word
size and so we end up in the endless loop. So the issue has been
introduced by the original "mm: adaptive hash table scaling" but my
patch made it more visible because [di]cache has tables most probably
suceeded in the early initialization which didn't have HASH_ADAPT.
The following should fix the hang. I am not yet sure about the maximum
size for the scaling and something even smaller would make sense to me
because kernel address space is just too small for such a large has
tables.
---
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a26e19c3e1ff..70c5fc1fb89a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7174,11 +7174,15 @@ static unsigned long __init arch_reserved_kernel_pages(void)
 /*
  * Adaptive scale is meant to reduce sizes of hash tables on large memory
  * machines. As memory size is increased the scale is also increased but at
- * slower pace.  Starting from ADAPT_SCALE_BASE (64G), every time memory
- * quadruples the scale is increased by one, which means the size of hash table
- * only doubles, instead of quadrupling as well.
+ * slower pace.  Starting from ADAPT_SCALE_BASE (64G on 64b systems and 32M
+ * on 32b), every time memory quadruples the scale is increased by one, which
+ * means the size of hash table only doubles, instead of quadrupling as well.
  */
+#if __BITS_PER_LONG == 64
 #define ADAPT_SCALE_BASE	(64ul << 30)
+#else
+#define ADAPT_SCALE_BASE	(32ul << 20)
+#endif
 #define ADAPT_SCALE_SHIFT	2
 #define ADAPT_SCALE_NPAGES	(ADAPT_SCALE_BASE >> PAGE_SHIFT)
 
-- 
Michal Hocko
SUSE Labs