Re: [Xen-devel] [Bug 198497] handle_mm_fault / xen_pmd_val / radix_tree_lookup_slot Null pointer

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.04.18 at 17:25, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 20/04/18 16:20, Jason Andryuk wrote:
>> Adding xen-devel and the Linux Xen maintainers.
>>
>> Summary: Some Xen users (and maybe others) are hitting a BUG in
>> __radix_tree_lookup() under do_swap_page() - example backtrace is
>> provided at the end.  Matthew Wilcox provided a band-aid patch that
>> prints errors like the following instead of triggering the bug.
>>
>> Skylake 32bit PAE Dom0:
>> Bad swp_entry: 80000000
>> mm/swap_state.c:683: bad pte d3a39f1c(8000000400000000)
>>
>> Ivy Bridge 32bit PAE Dom0:
>> Bad swp_entry: 40000000
>> mm/swap_state.c:683: bad pte d3a05f1c(8000000200000000)
>>
>> Other 32bit DomU:
>> Bad swp_entry: 4000000
>> mm/swap_state.c:683: bad pte e2187f30(8000000200000000)
>>
>> Other 32bit:
>> Bad swp_entry: 2000000
>> mm/swap_state.c:683: bad pte ef3a3f38(8000000100000000)
>>
>> The Linux bugzilla has more info
>> https://bugzilla.kernel.org/show_bug.cgi?id=198497 
>>
>> This may not be exclusive to Xen Linux, but most of the reports are on
>> Xen.  Matthew wonders if Xen might be stepping on the upper bits of a
>> pte.
> 
> Yes - Xen does use the upper bits of a PTE, but only 1 in release
> builds, and a second in debug builds.  I don't understand where you're
> getting the 3rd bit in there.

The former supposedly is _PAGE_GUEST_KERNEL, which we use for 64-bit
guests only. Above talk is of 32-bit guests only.

In addition both this and _PAGE_GNTTAB are used on present PTEs only,
while above talk is about swap entries.

Jan



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