Re: [Xen-devel] [PATCH 5/5] mm: Don't hold heap lock in alloc_heap_pages() longer than necessary

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

>> This patch has been applied to staging, but its got problems.  The
>> following crash is rather trivial to provoke:
>>
>> ~Andrew
>>
>> (d19) Test result: SUCCESS
>> (XEN) ----[ Xen-4.10-unstable  x86_64  debug=y   Tainted:    H ]----
>> (XEN) CPU:    5
>> (XEN) RIP:    e008:[<ffff82d0802252fc>] 
>> page_alloc.c#free_heap_pages+0x786/0x7a1
>> ...
>> (XEN) Pagetable walk from ffff82ffffffffe4:
>> (XEN)  L4[0x105] = 00000000abe5b063 ffffffffffffffff
>> (XEN)  L3[0x1ff] = 0000000000000000 ffffffffffffffff
> Some negative offset into somewhere, it seems. Upon second
> look I think the patch is simply wrong in its current shape:
> free_heap_pages() looks for page_state_is(..., free) when
> trying to merge chunks, while alloc_heap_pages() now sets
> PGC_state_inuse outside of the locked area. I'll revert it right
> away.

Yes, so we do need to update page state under heap lock. I'll then move
scrubbing (and checking) only to outside the lock.

I am curious though, what was the test to trigger this? I ran about 100
parallel reboots under memory pressure and never hit this.


-boris

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