Re: [PATCH v7 08/14] xen/page_alloc: introduce preserved page flags macro

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2024 17:39, Carlo Nonato wrote:
> On Mon, Mar 25, 2024 at 8:19 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 22.03.2024 16:07, Carlo Nonato wrote:
>>> On Thu, Mar 21, 2024 at 5:23 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 21.03.2024 17:10, Julien Grall wrote:
>>>>> On 21/03/2024 16:07, Julien Grall wrote:
>>>>>> On 15/03/2024 10:58, Carlo Nonato wrote:
>>>>>>> PGC_static and PGC_extra needs to be preserved when assigning a page.
>>>>>>> Define a new macro that groups those flags and use it instead of or'ing
>>>>>>> every time.
>>>>>>>
>>>>>>> To make preserved flags even more meaningful, they are kept also when
>>>>>>> switching state in mark_page_free().
>>>>>>>
>>>>>>> Signed-off-by: Carlo Nonato <carlo.nonato@xxxxxxxxxxxxxxx>
>>>>>>
>>>>>> This patch is introducing a regression in OSStest (and possibly gitlab?):
>>>>>>
>>>>>> Mar 21 12:00:29.533676 (XEN) pg[0] MFN 2211c5 c=0x2c00000000000000 o=0
>>>>>> v=0xe40000010007ffff t=0x24
>>>>>> Mar 21 12:00:42.829785 (XEN) Xen BUG at common/page_alloc.c:1033
>>>>>> Mar 21 12:00:42.829829 (XEN) ----[ Xen-4.19-unstable  x86_64  debug=y
>>>>>> Not tainted ]----
>>>>>> Mar 21 12:00:42.829857 (XEN) CPU:    12
>>>>>> Mar 21 12:00:42.841571 (XEN) RIP:    e008:[<ffff82d04022fe1f>]
>>>>>> common/page_alloc.c#alloc_heap_pages+0x37f/0x6e2
>>>>>> Mar 21 12:00:42.841609 (XEN) RFLAGS: 0000000000010282   CONTEXT:
>>>>>> hypervisor (d0v8)
>>>>>> Mar 21 12:00:42.853654 (XEN) rax: ffff83023e3ed06c   rbx:
>>>>>> 000000000007ffff   rcx: 0000000000000028
>>>>>> Mar 21 12:00:42.853689 (XEN) rdx: ffff83047bec7fff   rsi:
>>>>>> ffff83023e3ea3e8   rdi: ffff83023e3ea3e0
>>>>>> Mar 21 12:00:42.865657 (XEN) rbp: ffff83047bec7c10   rsp:
>>>>>> ffff83047bec7b98   r8:  0000000000000000
>>>>>> Mar 21 12:00:42.877647 (XEN) r9:  0000000000000001   r10:
>>>>>> 000000000000000c   r11: 0000000000000010
>>>>>> Mar 21 12:00:42.877682 (XEN) r12: 0000000000000001   r13:
>>>>>> 0000000000000000   r14: ffff82e0044238a0
>>>>>> Mar 21 12:00:42.889652 (XEN) r15: 0000000000000000   cr0:
>>>>>> 0000000080050033   cr4: 0000000000372660
>>>>>> Mar 21 12:00:42.901651 (XEN) cr3: 000000046fe34000   cr2: 
>>>>>> 00007fb72757610b
>>>>>> Mar 21 12:00:42.901685 (XEN) fsb: 00007fb726def380   gsb:
>>>>>> ffff88801f200000   gss: 0000000000000000
>>>>>> Mar 21 12:00:42.913646 (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000
>>>>>> ss: e010   cs: e008
>>>>>> Mar 21 12:00:42.913680 (XEN) Xen code around <ffff82d04022fe1f>
>>>>>> (common/page_alloc.c#alloc_heap_pages+0x37f/0x6e2):
>>>>>> Mar 21 12:00:42.925645 (XEN)  d1 1c 00 e8 ad dd 02 00 <0f> 0b 48 85 c9
>>>>>> 79 36 0f 0b 41 89 cd 48 c7 47 f0
>>>>>> Mar 21 12:00:42.937649 (XEN) Xen stack trace from rsp=ffff83047bec7b98:
>>>>>> Mar 21 12:00:42.937683 (XEN)    0000000000000024 000000007bec7c20
>>>>>> 0000000000000001 ffff83046ccda000
>>>>>> Mar 21 12:00:42.949653 (XEN)    ffff82e000000021 0000000000000016
>>>>>> 0000000000000000 0000000000000000
>>>>>> Mar 21 12:00:42.949687 (XEN)    0000000000000000 0000000000000000
>>>>>> 0000000000000028 0000000000000021
>>>>>> Mar 21 12:00:42.961652 (XEN)    ffff83046ccda000 0000000000000000
>>>>>> 00007d2000000000 ffff83047bec7c48
>>>>>> Mar 21 12:00:42.961687 (XEN)    ffff82d0402302ff ffff83046ccda000
>>>>>> 0000000000000100 0000000000000000
>>>>>> Mar 21 12:00:42.973655 (XEN)    ffff82d0405f0080 00007d2000000000
>>>>>> ffff83047bec7c80 ffff82d0402f626c
>>>>>> Mar 21 12:00:42.985656 (XEN)    ffff83046ccda000 ffff83046ccda640
>>>>>> 0000000000000000 0000000000000000
>>>>>> Mar 21 12:00:42.985690 (XEN)    ffff83046ccda220 ffff83047bec7cb0
>>>>>> ffff82d0402f65a0 ffff83046ccda000
>>>>>> Mar 21 12:00:42.997662 (XEN)    0000000000000000 0000000000000000
>>>>>> 0000000000000000 ffff83047bec7cc0
>>>>>> Mar 21 12:00:43.009660 (XEN)    ffff82d040311f8a ffff83047bec7ce0
>>>>>> ffff82d0402bd543 ffff83046ccda000
>>>>>> Mar 21 12:00:43.009695 (XEN)    ffff83047bec7dc8 ffff83047bec7d08
>>>>>> ffff82d04032c524 ffff83046ccda000
>>>>>> Mar 21 12:00:43.021653 (XEN)    ffff83047bec7dc8 0000000000000002
>>>>>> ffff83047bec7d58 ffff82d040206750
>>>>>> Mar 21 12:00:43.033642 (XEN)    0000000000000000 ffff82d040233fe5
>>>>>> ffff83047bec7d48 0000000000000000
>>>>>> Mar 21 12:00:43.033678 (XEN)    0000000000000002 00007fb72767f010
>>>>>> ffff82d0405e9120 0000000000000001
>>>>>> Mar 21 12:00:43.045654 (XEN)    ffff83047bec7e70 ffff82d040240728
>>>>>> 0000000000000007 ffff83023e3b3000
>>>>>> Mar 21 12:00:43.045690 (XEN)    0000000000000246 ffff83023e2efa90
>>>>>> ffff83023e38e000 ffff83023e2efb40
>>>>>> Mar 21 12:00:43.057609 (XEN)    0000000000000007 ffff83023e3afb80
>>>>>> 0000000000000206 ffff83047bec7dc0
>>>>>> Mar 21 12:00:43.069662 (XEN)    0000001600000001 000000000000ffff
>>>>>> e75aaa8d0000000c ac0d6d864e487f62
>>>>>> Mar 21 12:00:43.069697 (XEN)    000000037fa48d76 0000000200000000
>>>>>> ffffffff000003ff 00000002ffffffff
>>>>>> Mar 21 12:00:43.081647 (XEN)    0000000000000000 00000000000001ff
>>>>>> 0000000000000000 0000000000000000
>>>>>> Mar 21 12:00:43.093646 (XEN) Xen call trace:
>>>>>> Mar 21 12:00:43.093677 (XEN)    [<ffff82d04022fe1f>] R
>>>>>> common/page_alloc.c#alloc_heap_pages+0x37f/0x6e2
>>>>>> Mar 21 12:00:43.093705 (XEN)    [<ffff82d0402302ff>] F
>>>>>> alloc_domheap_pages+0x17d/0x1e4
>>>>>> Mar 21 12:00:43.105652 (XEN)    [<ffff82d0402f626c>] F
>>>>>> hap_set_allocation+0x73/0x23c
>>>>>> Mar 21 12:00:43.105685 (XEN)    [<ffff82d0402f65a0>] F
>>>>>> hap_enable+0x138/0x33c
>>>>>> Mar 21 12:00:43.117646 (XEN)    [<ffff82d040311f8a>] F
>>>>>> paging_enable+0x2d/0x45
>>>>>> Mar 21 12:00:43.117679 (XEN)    [<ffff82d0402bd543>] F
>>>>>> hvm_domain_initialise+0x185/0x428
>>>>>> Mar 21 12:00:43.129652 (XEN)    [<ffff82d04032c524>] F
>>>>>> arch_domain_create+0x3e7/0x4c1
>>>>>> Mar 21 12:00:43.129687 (XEN)    [<ffff82d040206750>] F
>>>>>> domain_create+0x4cc/0x7e2
>>>>>> Mar 21 12:00:43.141665 (XEN)    [<ffff82d040240728>] F
>>>>>> do_domctl+0x1850/0x192d
>>>>>> Mar 21 12:00:43.141699 (XEN)    [<ffff82d04031a96a>] F
>>>>>> pv_hypercall+0x617/0x6b5
>>>>>> Mar 21 12:00:43.153656 (XEN)    [<ffff82d0402012ca>] F
>>>>>> lstar_enter+0x13a/0x140
>>>>>> Mar 21 12:00:43.153689 (XEN)
>>>>>> Mar 21 12:00:43.153711 (XEN)
>>>>>> Mar 21 12:00:43.153731 (XEN) ****************************************
>>>>>> Mar 21 12:00:43.165647 (XEN) Panic on CPU 12:
>>>>>> Mar 21 12:00:43.165678 (XEN) Xen BUG at common/page_alloc.c:1033
>>>>>> Mar 21 12:00:43.165703 (XEN) ****************************************
>>>>>> Mar 21 12:00:43.177633 (XEN)
>>>>>> Mar 21 12:00:43.177662 (XEN) Manual reset required ('noreboot' specified)
>>>>>>
>>>>>> The code around the BUG is:
>>>>>>
>>>>>>          /* Reference count must continuously be zero for free pages. */
>>>>>>          if ( (pg[i].count_info & ~PGC_need_scrub) != PGC_state_free )
>>>>>>          {
>>>>>>              printk(XENLOG_ERR
>>>>>>                     "pg[%u] MFN %"PRI_mfn" c=%#lx o=%u v=%#lx t=%#x\n",
>>>>>>                     i, mfn_x(page_to_mfn(pg + i)),
>>>>>>                     pg[i].count_info, pg[i].v.free.order,
>>>>>>                     pg[i].u.free.val, pg[i].tlbflush_timestamp);
>>>>>>              BUG();
>>>>>>          }
>>>>>>
>>>>>> Now that you are preserving some flags, you also want to modify the
>>>>>> condition. I haven't checked the rest of the code, so there might be
>>>>>> some adjustments necessary.
>>>>>
>>>>> Actually maybe the condition should not be adjusted. I think it would be
>>>>> wrong if a free pages has the flag PGC_extra set. Any thoughts?
>>>>
>>>> I agree, yet I'm inclined to say PGC_extra should have been cleared
>>>> before trying to free the page.
>>>
>>> So what to do now? Should I drop this commit?
>>
>> No, we need to get to the root of the issue. Since osstest has hit it quite
>> easily as it seems, I'm somewhat surprised you didn't hit it in your testing.
>> In any event, as per my earlier reply, my present guess is that your change
>> has merely uncovered a previously latent issue elsewhere.
> 
> Ok, what about removing PGC_extra in free_heap_pages() before the
> mark_page_free() call?

Question is: How would you justify such a change? IOW I'm not convinced
(yet) this wants doing there.

Jan