Re: [Xen-devel] [memory sharing] bug on get_page_and_type

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 15:43 +0000 on 02 Feb (1296661396), MaoXiaoyun wrote:
> Hi Tim:
> 
>          Thanks for  both your advice and quick reply. I will follow.
> 
>          So at last we should replace shr_lock with p2m_lock.
>          But more complicate, it seems both the
>          *check action* code and *nominate page* code need to be locked 
> ,right?
>          If so, quite a lot of  *check action* codes need to be locked.

Yes, I think you're right about that.  Unfortunately there are some very
long TOCTOU windows in those kind of p2m lookups, which will get more
important as the p2m gets more dynamic.  I don't want to have the
callers of p2m code touching the p2m lock directly so we may need a new
p2m interface to address it.

Tim.

>          Looking forward to your suggestion on the fix, I think Juihao and I 
> can help to do analyse and test.
> 
> > Date: Tue, 1 Feb 2011 14:28:16 +0000
> > From: Tim.Deegan@xxxxxxxxxx
> > To: George.Dunlap@xxxxxxxxxxxxx
> > CC: tinnycloud@xxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx; 
> > juihaochiang@xxxxxxxxx
> > Subject: Re: [Xen-devel] [memory sharing] bug on get_page_and_type
> >
> > At 12:47 +0000 on 31 Jan (1296478031), George Dunlap wrote:
> > > At first glance, it does look like a race, of the type that would
> > > normally be done by grabbing the p2m lock. I'd have to take a closer
> > > look to be sure, but I won't be able to for at least another day or
> > > two. Maybe Tim can comment.
> >
> > Yes, this is a race; it would be fixed by getting rid of the shr_lock
> > and using the p2m lock to cover page-sahring operations, which I think
> > is a good idea anyway.
> >
> > I think I'll have time to look at the locking properly some time this
> > month. Of course, I've been wrong before. :)
> > < BR>> Tim.
> >
> > > 2011/1/31 MaoXiaoyun <tinnycloud@xxxxxxxxxxx>:
> > > > Hi Georage:
> > > >
> > > > Thanks for your help, I think I really need to learn hg a bit.
> > > > Well, when looking into memory sharing code, a confusion
> > > > really brother me a lot.
> > > > I think maybe you can enlighten me.
> > > >
> > > > There has been *check action code* do like below steps
> > > > 1) Use gfn_to_mfn to get p2m_type_t p2mt
> > > > 2) check the type of p2mt,
> > > > 3) do some stuffes base if check on step 2 is failed
> > > >
> > > > While when *nominate page* to be shared, it does these follow steps
> > > > a) check page whether can be shared
> > > > b) call page_make_sharable to make sharable
> > > > c) if 2) is success, call p2m_change_type to update page type
> > > > p2m_ram_shared
> > > >
> > > > My confusion is if *nominate page* code reach step b, and before it go
> > > > to step c
> > > > *check action code* reach its step2, it will find page type is
> > > > not p2m_ram_shared
> > > > and go to step3. And later "nominate page" code go to step (c)
> > > >
> > > > Take xen/common/memory.c for example,
> > > > line 179 is much like step 2, it will goto 196 to clear the 
> > > > _PGC_allocated
> > > > flag
> > > > if check failed, so it might indicates, *nominate page* code change the 
> > > > page
> > > > type
> > > > into p2m_ram_shared, but the page actually has alreay been freed.
> > > >
> > > > Am I right?
> > > >
> > > >
> > > > 155 int guest_remove_page(struct domain *d, unsigned long gmfn)
> > > > 156 {
> > > > 157 struct page_info *page;
> > > > 158 #ifdef C ONFIG_X86
> > > > 159 p2m_type_t p2mt;
> > > > 160 #endif
> > > > 161 unsigned long mfn;
> > > > 162
> > > > 163 #ifdef CONFIG_X86
> > > > 164 mfn = mfn_x(gfn_to_mfn(d, gmfn, &p2mt));
> > > > 165 #else
> > > > 166 mfn = gmfn_to_mfn(d, gmfn);
> > > > 167 #endif
> > > > 168 if ( unlikely(!mfn_valid(mfn)) )
> > > > 169 {
> > > > 170 gdprintk(XENLOG_INFO, "Domain %u page number %lx invalid\n",
> > > > 171 d->domain_id, gmfn);
> > > > 172 return 0;
> > > > 173 }
> > > > 174
> > > > 175&nbsp ; page = mfn_to_page(mfn);
> > > > 176 #ifdef CONFIG_X86
> > > > 177 /* If gmfn is shared, just drop the guest reference (which may or
> > > > may not
> > > > 178 * free the page) */
> > > > 179 if(p2m_is_shared(p2mt))
> > > > 180 {
> > > > 181 put _page_and_type(page);
> > > > 182 guest_physmap_remove_page(d, gmfn, mfn, 0);
> > > > 183 return 1;
> > > > 184 }
> > > > 185
> > > > 186 #endif /* CONFIG_X86 */
> > > > 187 if ( unlikely(!get_page(page, d)) )
> > > > 188 {
> > > > 189 gdprintk(XENLOG_INFO, "Bad page free for domain %u\n",
> > > > d->domain_id);
> > > > 190 return 0;
> > > > 189 gdprintk(XENLOG_INFO, "Bad page free for domain %u\n",
> > > > d->domain_id);
> > > > 190 return 0;
> > > > 191 }
> > > > 192
> > > > 193 if ( test_and_clear_bit(_PGT_pinned, &page->u.inuse.type_info) )
> > > > 194 put_page_and_type(page);
> > > > 195
> > > > 196 if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
> > > > 197 put_page(page);
> > > > 198
> > > > 199 guest_physmap_remove_page (d, gmfn, mfn, 0);
> > > > 200
> > > > 201 put_page(page);
> > > > 202
> > > > 203 return 1;
> > > > 204 }
> > > >
> > > >> Date: Mon, 31 Jan 2011 10:49:37 +0000
> > > >> Subject: Re: [Xen-devel] [memory sharing] bug on get_page_and_type
> > > >> From: George.Dunlap@xxxxxxxxxxxxx
> > > >> To: tinnycloud@xxxxxxxxxxx
> > > >> CC: xen-devel@xxxxxxxxxxxxxxxxxxx; tim.deegan@xxxxxxxxxx;
> > > >> juihaochiang@xxxxxxxxx
> > > >>
> > > >> Xiaoyun,
> > > >>
> > > >> Thanks for all of your work getting page sharing working. When
> > > >> submitting patches, please break them down into individual chunks,
> > > >> each of which does one thing. Each patch should also include a
> > > >> comment saying what the patch does and why, and a Signed-off-by line
> > > ; >> indicating that you certify that the copyright holder (possibly you)
> > > >> is placing the code under the GPL.
> > > >>
> > > >> Using mercurial queues:
> > > >> http://mercurial.selenic.com/wiki/MqExtension
> > > >> and the mercurial patchbomb extension:
> > > >> http://mercurial.selenic.com/wiki/PatchbombExtension
> > > >> are particularly handy for this process.
> > > > &g t;
> > > >> Quick comment: The ept-locking part of the patch is going to be
> > > >> NACK-ed, as it will cause circular locking dependencies with the
> > > >> hap_lock. c/s 22526:7a5ee380 has an explanation of the circular
> > > >> dependency, and a fix which doesn't introduce a circular dependency.
> > > >> (It may need to be adapted a bit to apply to 4.0-testing)
> > > >>
> > > >> -George
> > > >>
> > > >> On Mon, Jan 31, 2011 at 10:13 AM, tinnycloud <tinnycloud@xxxxxxxxxxx>
> > > >> wrote:
> > > >> > Hi:
> > > >> >
> > > >> >
> > > >> > Attached is the whole patch suit for latest xen-4.0-testing,changeset
> > > >> > 21443,
> > > >> >
> > > >> > It comes from George, Tim and JuiHao, also, has some extra debug 
> > > >> > info.
> > > >> >
> > > >> >
> > > >> >
> > > >> > On most occasion, memory sharing works fine, but still exists a bug.
> > > >> >
> > > >> > I?ve been tracing this problem for a while.
> > > >> >
> > > >> > &nbs p; There is a bug on get_page_and_type() in
> > > >> > mem_sharing_share_pages()
> > > >> >
> > > >> >
> > > >&g t; >
> > > >> >
> > > >> > --------------------------------------------mem_sharing_share_pages()-----------------------
> > > >> >
> > > >> > 789 if(!get_page_and_type(spage, dom_cow, PGT_shared_page)){
> > > >> >
> > > >> > 790 mem_sharing_debug_gfn(cd, gfn->gfn);
> > > >> >
> > > >> > 791 mem_sharing_debug_gfn(sd, sgfn->gfn);
> > > >> >
> > > >> > 792 printk("c->dying %d s->dying %d spage %p se->mfn %lx\n",
> > > >> > cd->is_dying, sd->is_dying, spage, se->mfn);
> > > >> >
> > > >> > 793 printk("Debug page: MFN=%lx is ci=%lx, ti=%lx,
> > > >> > owner_id=%d\n",
> > > >> >
> > > >> > 794 mfn_x(page_to_mfn(spage)),
> > > >> >
> > > >> > 795 spage->count_info,
> &g t; > >> >
> > > >> > 796 spage->u.inuse.type_info,
> > > >> >
> > > >> > 797 page_get_owner(spage)->domain_id);
> > > >> >
> > > >> > 798 BUG();
> > > >> >
> > > >> > 799 }
> > > >> >
> > > >> >
> > > >> >
> > > >> >
> > > >> >
> > > >> > Below painc log contains the debug info from line 790-798.
> > > >> >
> > > >> > We saw that 180000000000000 which is PGC_state_free,
> > > >> >
> > > >> > So it looks like a shared page has been freed unexpectly.
> > > >> >
> > > >> >
> > > >> >
> > > >> >
> > > >> >
> > > >> > (XEN) teardown 64
> > > >> >
> > > >> > (XEN) teardow n 66
> > > >> >
> > > >> > blktap_sysfs_destroy
> > > >> >
> > > >> > blktap_sysfs_create: adding attributes for dev ffff880109d0c200
> > > >> >
> > > >> > blktap_sysfs_destroy
> > > >> >
> > > >> > __ratelimit: 1 callbacks suppressed
> > > >> >
> > > >> > blktap_sysfs_destroy
> > > >> >
> > > >> > (XEN) Debug for domain=83, gfn=1e8dc, Debug page: MFN=1306dc is
> > > >> > ci=8000000000000005, ti=8400000000000001, owner_id=32755
> > > >> >
> > > >> > (XEN) Debug for domain=79, gfn=1dc95, Invalid MFN=ffffffffffffffff
> > > >> >
> > > >> > (XEN) c->dying 0 s->dying 0 spage ffff82f60a0f12a0 se->mfn 507895
> > > >> >
> > > >> > (XEN) Debug page: MFN=507895 is ci= 1800000000 00000,
> > > >> > ti=8400000000000001,
> > > >> > owner_id=32755
> > > >> >
> > > >> > (XEN) Xen BUG at mem_sharing.c:798
> > > >> >
> > > >> > (XEN) ----[ Xen-4.0.2-rc2-pre x86_64 debug=n Not tainted ]----
> > > >> >
> > > >> > (XEN) CPU: 0
> > > >> >
> > > >> > (XEN) RIP: e008:[<ffff82c4801c3760>]
> > > >> > mem_sharing_share_pages+0x5b0/0x5d0
> > > >> >
> > > >> > (XEN) RFLAGS: 0000000000010286 CONTEXT: hypervisor
> > > >> >
> > > >> > (XEN) rax: 0000000000000000 rbx: ffff83044ef76000 rcx:
> > > >> > 0000000000000092
> > > >> >
> > > >> > (XEN) rdx: 000000000000000a rsi: 000000000000000a rdi:
> > > >> > ffff82c4802237c4
> > > >> >
> > > >> > (XEN) rbp: ffff83030d99e310 rsp: ffff82c48035fc48 r8:
> > > >> > 0000000000000001
> > > >> >
> > > >> > (XEN) r9: 0000000000000001 r10: 00000000fffffff8 r11:
> > > >> > 0000000000000005
> > > >> >
> > > >> > (XEN) r12: ffff83050b558000 r13: ffff830626ec5740 r14:
> > > >> > 0000000000347967
> > > >> >
> > > >> > (XEN) r15: ffff83030d99e068 cr0: 0000000080050033 cr4:
> > > >> > 00000000000026f0
> > > >> >
> > > >> > (XEN) cr3: 000000010e642000 cr2: 00002abdc4809000
> > > >> >
> > > >> > (XEN) ds: 0000 es: 0000 fs: 0000 gs: 0000 ss: e010 cs: e008
> > > >> >
> > > >> > (XEN) Xen stack trace from rsp=ffff82c48035fc48:
> > > >> >
> > > >> > (XEN) 000000000001e8dc ffff83030d99e068 00000000005078 95
> > > >> > ffff83030d99e050
> > > >> >
> > > >> > (XEN) ffff82f60a0f12a0 ffff82f60260db80 ffff83030d99e300
> > > >> > ffff830626afbbd0
> > > >> >
> > > >> > (XEN)&nb sp; 0000000080372980 ffff82c48035fe38 ffff83023febe000
> > > >> > 00000000008f7000
> > > >> >
> > > >> > (XEN) 0000000000305000 0000000000000006 0000000000000006
> > > >> > ffff82c4801c44a4
> > > >> >
> > > >> > (XEN) ffff82c480258188 ffff82c48011cb89 000000000001e8dc
> > > >> > fffffffffffffff3
> > > >> >
> > > >> > (XEN) ffff82c48035fe28 ffff82c480148503 000003d932971281
> > > >> > 0000000000000080
> > > >> >
> > > >> > (XEN) 000003d9329611d6 ffff82c48018aedf ffff82c4803903a8
> > > >> > ffff82c48035fd38
> > > >> >
> > > >> > (XEN) ffff82c480390380 ffff82c48035fe28 0000000000000002
> > > >> > 0000000000000000
> > > >> >
> > > >> > (XEN) 0000000000000002 0000000000000000 ffff83023febe000
> > > >> > ffff83023ff80080
> > > >> >
> > > >> > (XEN) ffff82c48035fe58 0000000000000000 ffff82c48022ca80
> > > >> > 0000000000000000
> > > >> >
> > > >> > (XEN) 0000000000000002 ffff82c48018a 452 0000000000000000
> > > >> > ffff82c48016f6bb
> > > >> >
> > > >> > (XEN) ffff82c48035fe58 ffff82c4801538ea 000000023ab79067
> > > >> > fffffffffffffff3
> > > >> >
> > > >> > (XEN) ffff82c48035fe28 00000000008f7000 0000000000305000
> > > >> > 0000000000000006
> > > >> >
> > > >> > (XEN) 0 000000000000006 ffff82c4801042b3 0000000000000000
> > > >> > ffff82c48010d2a5
> > > >> >
> > > >> > (XEN) ffff830477fb51e8 0000000000000000 00007ff000000200
> > > >> > ffff8300bf76e000
> > > >> >
> > > >> > (XEN) 0000000600000039 0000000000000000 00007fc09d744003
> > > >> > 0000000000325258
> > > >> >
> > > >> > (XEN) 0000000000347967 ffffffffff600429 000000004d463b17
> > > >> > 0000000000062fe2
> > > >> >
> > > >> > (XEN) 0000000000000000 00007fc09d744070 00007fc09d744000
> > > >> > 00007fffcfec0d20
> > > >> >
> > > >> > (XEN) 00007fc09d744078 0000000000430fd8 00007fffcfec0d88
> > > >> > 00000000019f1ca8
> > > >> >
> > > >> > (XEN) 0000f05c00000000 00007fc09f4c2718 0000000000000000
> > &g t; >> > 0000000000000246
> > > >> >
> > > >> > (XEN) Xen call trace:
> > > >> >
> > > >> > (XEN) [<ffff82c4801c3760>] mem_sharing_share_pages+0x5b0/0x5d0
> > > >> >
> > > >> > (XEN) [<ffff82c4801c44a4>] mem_sharing_domctl+0xe4/0x130
> > > >> >
> > > >> > (XEN) [<ffff82c48011cb89>] cpumask_raise_softirq+0x89/0xa0
> > > >> >
> > > >> > (XEN) [<ffff82c480148503>] arch_do_domctl+0x14f3/0x22d0
> > > >> >
> > > >> > (XEN) [<ffff82c48018aedf>] handle_hpet_broadcast+0x16f/0x1d0
> > > >> >
> > > >> > (XEN) [<ffff82c48018a452>] hpet_legacy_irq_tick+0x42/0x50
> > > >> >
> > > >> > (XEN) [<ffff82c48016f6bb>] timer_interrupt+0xb/0x130
> > > >> >
> > > >> > (XEN) [<ffff82c4801538ea> ] ack_edge_ioapic_irq+0x2a/0x70
> > > >> >
> > > >> > (XEN) [<ffff82c4801042b3>] do_domctl+0x163/0xfe0
> > > >> >
> > > >> > (XEN) [<ffff82c48010d2a5>] do_grant_table_op+0x75/0x1ad0
> > > >> >
> > > >> > (XEN) [<ffff82c4801e7169>] syscall_enter+0xa9/0xae
> > > >> >
> > > >> > (XEN)
> > > >> >
> > > >> > (XEN)
> > > >> >
> > > >> > (XEN) ****************************************
> > > >> >
> > > >> > (XEN) Panic on CPU 0:
> > > >> >
> > > >> > (XEN) Xen BUG at mem_sharing.c:798
> > > >> >
> > > >> > (XEN) ****************************************
> > > >> >
> > > >> > (XEN)
> > > >> >
> > > & gt;> > (XEN) Manual reset required ('noreboot' specified)
> > > >> >
> > > >> >
> > > >> >
> > > >> > _______________________________________________
> > > >> > Xen-devel mailing list
> > > >> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > >> > http://lists.xensource.com/xen- devel
> > > >> >
> > > >> >
> > > >
> > > > _______________________________________________
> > > > Xen-devel mailing list
> > > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > > http://lists.xensource.com/xen-devel
> > > >
> > > >
> >
> > --
> > Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> > Principal Software Engineer, Xen Platform Team
> > Citrix Systems UK Ltd. (Company #02937203, SL9 0BG)

> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel


-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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