Re: [Xen-devel] help -- A question about ' XENFEAT_auto_translated_physmap':

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.08.16 at 15:19, <xuquan8@xxxxxxxxxx> wrote:
> On August 09, 2016 9:02 PM, < JBeulich@xxxxxxxx > wrote:
>> >>> On 09.08.16 at 14:36, <xuquan8@xxxxxxxxxx> wrote:
>> > Hi Jan,
>> >
>> > A question about ' XENFEAT_auto_translated_physmap':
>> >
>> > In linux code, in arch/x86/xen/mmu.c,
>>
>> I assume you know that I'm not a maintainer of the Linux code.
>>
>> >  __xen_pgd_walk()
>> > {
>> > ....
>> >
>> >         if (xen_feature(XENFEAT_auto_translated_physmap))
>> >                 return 0;
>> > ....
>> > }
>> >
>> >
>> >
>> > Why xen_feature(XENFEAT_auto_translated_physmap) is true, then return
>> > directly?
>> > If not return directly, is there any potential risk?
>>
>> Well, the function is specifically there for operations (pinning/unpinning) 
> which are required only for the not-auto-translated case.
>> Why would anyone want to traverse a page table tree just to do nothing on 
> each of the entries?
> 
> Jan, thank you!!
> As I am struggling with a dom0 crash, the kernel is old, 3.0.X..
> Now there is a crash in 
> 
> [<ffffffff80023465>] _pin_lock+0x165/0x2a0    <----*crash*
> 
> unable to handle kernel paging request at ffff8b1021826000

Which appears to have pretty little to do with the original question,
as here you appear to be dealing with a forward ported XenoLinux
kernel.

> static void _pin_lock(struct mm_struct *mm, int lock)
> {
> 171                 pgd_t *pgd = mm->pgd;
> 172                 unsigned g;
> 173
> 174                 for (g = 0; g <= ((TASK_SIZE_MAX-1) / PGDIR_SIZE); g++, 
> pgd++) {
> 175                         pud_t *pud;
> 176                         unsigned u;
> 177
> 178                         if (pgd_none(*pgd))
> 179                                 continue;
> 180                         pud = pud_offset(pgd, 0);
> 181                         for (u = 0; u < PTRS_PER_PUD; u++, pud++) {
> 182                                 pmd_t *pmd;
> 183                                 unsigned m;
> 184
> 185                                 if (pud_none(*pud))
> 186                                         continue;
> 187                                 pmd = pmd_offset(pud, 0);
> 188                                 for (m = 0; m < PTRS_PER_PMD; m++, pmd++) 
> {
> 189                                         spinlock_t *ptl;
> 190
> 191                                         if (pmd_none(*pmd))          
> <---------*crash*

Well, fine, it crashes here. But without further detail I don't think
you really expect anyone to be able to help (at the first glance
this looks like a follow-on effect from some earlier problem, as
presumably you've hit a ballooned out page, which obviously
shouldn't be referenced by any page tables)? And for this old a
kernel, and a non-upstream one, such a discussion presumably
doesn't belong on this list anyway. If this is a SUSE kernel, and
if the kernel is up-to-date wrt the distro it comes from, then either
open a ticket with our support folks, or (less reasonable, as there
never was an openSUSE version with a 3.0 based kernel) provide
much greater detail on a list like opensuse-virtual@xxxxxxxxxxxx.

Jan

> 192                                                 continue;
> 193                                         ptl = pte_lockptr(0, pmd);
> 194                                         if (lock)
> 195                                                 spin_lock(ptl);
> 196                                         else
> 197                                                 spin_unlock(ptl);
> 198                                 }
> 199                         }
> 200                 }
> 201         }
> 202 #endif
> }
> 
> 
> 
> Quan




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