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

[Michal Hocko]

On Mon 22-05-17 02:03:21, Guenter Roeck wrote:
> On 05/22/2017 01:45 AM, Michal Hocko wrote:
> >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)
> >
> I have seen another patch making it 64ull. Not sure if adaptive scaling
> on 32 bit systems really makes sense; unless there is a clear need I'd rather
> leave it alone.

I've just found out that my incoming emails sync didn't work since
friday. So I've missed those follow up emails. I will double check.
-- 
Michal Hocko
SUSE Labs