Message-ID: <alpine.LSU.2.11.2010071833100.2214@eggly.anvils>
Date:   Wed, 7 Oct 2020 20:21:25 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
cc:     Qian Cai <cai@....pm>, js1304@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 7/8] mm/mempolicy: use a standard migration target
 allocation callback

On Tue, 7 Jul 2020, Qian Cai wrote:
> On Tue, Jun 23, 2020 at 03:13:47PM +0900, js1304@...il.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > There is a well-defined migration target allocation callback.
> > Use it.
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > ---
...
> 
> migrate_pages() starts failing like this apparently using the new
> callback on NUMA systems,
> 
> [ 6147.019063][T45242] LTP: starting move_pages12
> [ 6147.475680][T64921] BUG: unable to handle page fault for address: ffffffffffffffe0
> [ 6147.483301][T64921] #PF: supervisor read access in kernel mode
> [ 6147.489170][T64921] #PF: error_code(0x0000) - not-present page
> [ 6147.495040][T64921] PGD 5df817067 P4D 5df817067 PUD 5df819067 PMD 0 
> [ 6147.501438][T64921] Oops: 0000 [#1] SMP KASAN NOPTI
> [ 6147.506348][T64921] CPU: 35 PID: 64921 Comm: move_pages12 Tainted: G           O      5.8.0-rc4-next-20200707 #1
> [ 6147.516586][T64921] Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
> [ 6147.525866][T64921] RIP: 0010:anon_vma_interval_tree_iter_first+0xa2/0x170
> avc_start_pgoff at mm/interval_tree.c:63
> (inlined by) __anon_vma_interval_tree_iter_first at mm/interval_tree.c:71
> (inlined by) anon_vma_interval_tree_iter_first at mm/interval_tree.c:95
> [ 6147.532787][T64921] Code: 00 00 00 00 fc ff df 48 8b 6d 08 48 8d 7d e0 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 b3 00 00 00 48 b8 00 00 00 00 00 fc ff df <48> 8b 6d e0 48 8d bd 98 00 00 00 48 89 f9 48 c1 e9 03 80 3c 01 00
> [ 6147.552370][T64921] RSP: 0018:ffffc9000bfdfa98 EFLAGS: 00010246
> [ 6147.558327][T64921] RAX: dffffc0000000000 RBX: ffff888524019b28 RCX: 1ffffffffffffffc
> [ 6147.566205][T64921] RDX: 00000000000003ff RSI: 0000000000000200 RDI: ffffffffffffffe0
> [ 6147.574084][T64921] RBP: 0000000000000000 R08: fffff94002b1c001 R09: fffff94002b1c001
> [ 6147.581962][T64921] R10: ffffea00158e0007 R11: fffff94002b1c000 R12: 0000000000000009
> [ 6147.589839][T64921] R13: ffffea00158e0008 R14: ffffea00158e0000 R15: ffffea00158e0000
> [ 6147.597717][T64921] FS:  00007f329cf18b80(0000) GS:ffff88881f7c0000(0000) knlGS:0000000000000000
> [ 6147.606557][T64921] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 6147.613037][T64921] CR2: ffffffffffffffe0 CR3: 000000081122a000 CR4: 00000000003506e0
> [ 6147.620914][T64921] Call Trace:
> [ 6147.624078][T64921]  rmap_walk_anon+0x141/0xa30
> rmap_walk_anon at mm/rmap.c:1864
> [ 6147.628639][T64921]  try_to_unmap+0x209/0x2d0
> try_to_unmap at mm/rmap.c:1763
> [ 6147.633026][T64921]  ? rmap_walk_locked+0x140/0x140
> [ 6147.637936][T64921]  ? page_remove_rmap+0x1190/0x1190
> [ 6147.643020][T64921]  ? page_not_mapped+0x10/0x10
> [ 6147.647668][T64921]  ? page_get_anon_vma+0x290/0x290
> [ 6147.652664][T64921]  ? page_mapcount_is_zero+0x10/0x10
> [ 6147.657838][T64921]  ? hugetlb_page_mapping_lock_write+0x97/0x180
> [ 6147.663972][T64921]  migrate_pages+0x1005/0x1fb0
> unmap_and_move_huge_page at mm/migrate.c:1383
> (inlined by) migrate_pages at mm/migrate.c:1468
> [ 6147.668617][T64921]  ? remove_migration_pte+0xac0/0xac0
> [ 6147.673875][T64921]  move_pages_and_store_status.isra.47+0xd7/0x1a0
> do_move_pages_to_node at mm/migrate.c:1595
> (inlined by) move_pages_and_store_status at mm/migrate.c:1683
> [ 6147.680181][T64921]  ? migrate_pages+0x1fb0/0x1fb0
> [ 6147.685002][T64921]  __x64_sys_move_pages+0xa5c/0x1100
> [ 6147.690176][T64921]  ? trace_hardirqs_on+0x20/0x1b5
> [ 6147.695084][T64921]  ? move_pages_and_store_status.isra.47+0x1a0/0x1a0
> [ 6147.701653][T64921]  ? rcu_read_lock_sched_held+0xaa/0xd0
> [ 6147.707088][T64921]  ? switch_fpu_return+0x196/0x400
> [ 6147.712083][T64921]  ? lockdep_hardirqs_on_prepare+0x38c/0x550
> [ 6147.717954][T64921]  ? do_syscall_64+0x24/0x310
> [ 6147.722513][T64921]  do_syscall_64+0x5f/0x310
> [ 6147.726897][T64921]  ? trace_hardirqs_off+0x12/0x1a0
> [ 6147.731894][T64921]  ? asm_exc_page_fault+0x8/0x30
> [ 6147.736714][T64921]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 6147.742495][T64921] RIP: 0033:0x7f329c3fe6ed
> [ 6147.746791][T64921] Code: Bad RIP value.
> [ 6147.750738][T64921] RSP: 002b:00007fff5b6b5f88 EFLAGS: 00000246 ORIG_RAX: 0000000000000117
> [ 6147.759055][T64921] RAX: ffffffffffffffda RBX: 00007f329cf18af8 RCX: 00007f329c3fe6ed
> [ 6147.766933][T64921] RDX: 00000000019b0ee0 RSI: 0000000000000400 RDI: 000000000000fd98
> [ 6147.774809][T64921] RBP: 0000000000000400 R08: 00000000019b3f00 R09: 0000000000000004
> [ 6147.782686][T64921] R10: 00000000019b2ef0 R11: 0000000000000246 R12: 0000000000000400
> [ 6147.790563][T64921] R13: 00000000019b0ee0 R14: 00000000019b2ef0 R15: 00000000019b3f00
> [ 6147.798440][T64921] Modules linked in: vfio_pci vfio_virqfd vfio_iommu_type1 vfio loop kvm_amd ses enclosure kvm irqbypass efivars acpi_cpufreq nls_ascii nls_cp437 vfat fat efivarfs ip_tables x_tables sd_mod smartpqi scsi_transport_sas tg3 mlx5_core libphy firmware_class dm_mirror dm_region_hash dm_log dm_mod [last unloaded: dummy_del_mod]
> [ 6147.828701][T64921] CR2: ffffffffffffffe0
> [ 6147.832736][T64921] ---[ end trace 40323b256f1c74a8 ]---
> [ 6147.838083][T64921] RIP: 0010:anon_vma_interval_tree_iter_first+0xa2/0x170
> [ 6147.845001][T64921] Code: 00 00 00 00 fc ff df 48 8b 6d 08 48 8d 7d e0 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 b3 00 00 00 48 b8 00 00 00 00 00 fc ff df <48> 8b 6d e0 48 8d bd 98 00 00 00 48 89 f9 48 c1 e9 03 80 3c 01 00
> [ 6147.864583][T64921] RSP: 0018:ffffc9000bfdfa98 EFLAGS: 00010246
> [ 6147.870539][T64921] RAX: dffffc0000000000 RBX: ffff888524019b28 RCX: 1ffffffffffffffc
> [ 6147.878417][T64921] RDX: 00000000000003ff RSI: 0000000000000200 RDI: ffffffffffffffe0
> [ 6147.886294][T64921] RBP: 0000000000000000 R08: fffff94002b1c001 R09: fffff94002b1c001
> [ 6147.894172][T64921] R10: ffffea00158e0007 R11: fffff94002b1c000 R12: 0000000000000009
> [ 6147.902049][T64921] R13: ffffea00158e0008 R14: ffffea00158e0000 R15: ffffea00158e0000
> [ 6147.909932][T64921] FS:  00007f329cf18b80(0000) GS:ffff88881f7c0000(0000) knlGS:0000000000000000
> [ 6147.918771][T64921] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 6147.925251][T64921] CR2: ffffffffffffffe0 CR3: 000000081122a000 CR4: 00000000003506e0
> [ 6147.933130][T64921] Kernel panic - not syncing: Fatal exception
> [ 6147.939493][T64921] Kernel Offset: 0x28c00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> [ 6147.951090][T64921] ---[ end Kernel panic - not syncing: Fatal exception ]---

I hit this too, trying LTP (yes, move_pages12) on 5.9-rc8.  I could not
then reproduce it, and suppose that Qian Cai was also unable to do so.

But it's fairly easy to explain, and not at all related to Joonsoo's
patch or patchset accused in this thread.

The faulting address ffffffffffffffe0 is not an -ESPIPE here,
it comes from a mov -0x20(%rcx),%rcx with NULL %rcx in my case: I've
been too impatient to unravel the interval tree defining to work out
exactly what that corresponds to, but I assume from an
anon_vma_interval_tree rb_entry container_of.

Mike, j'accuse... your 5.7 commit c0d0381ade79 ("hugetlbfs:
use i_mmap_rwsem for more pmd sharing synchronization"), in which
unmap_and_move_huge_page() now passes the TTU_RMAP_LOCKED flag to
try_to_unmap(), because it's already holding mapping->i_mmap_rwsem:
but that is not the right lock to secure an anon_vma lookup.

I intended to send a patch, passing TTU_RMAP_LOCKED only in the
!PageAnon case (and, see vma_adjust(), anon_vma lock conveniently
nests inside i_mmap_rwsem); but then wondered if i_mmap_rwsem was
needed in that case or not, so looked deeper into c0d0381ade79.

Hmm, not even you liked it!  But the worst of it looks simply
unnecessary to me, and I hope can be deleted - but better by you
than by me (in particular, you were trying to kill 1) and 2) birds
with one stone, and I've always given up on understanding hugetlb's
reservations: I suspect that side of it is irrelevant here,
but I wouldn't pretend to be sure).

How could you ever find a PageAnon page in a vma_shareable() area?

It is all rather confusing (vma_shareable() depending on VM_MAYSHARE,
whereas is_cow_mapping() on VM_SHARED and VM_MAYWRITE: they have to
be studied together with do_mmap()'s 
			vm_flags |= VM_SHARED | VM_MAYSHARE;
			if (!(file->f_mode & FMODE_WRITE))
				vm_flags &= ~(VM_MAYWRITE | VM_SHARED);

(And let me add to the confusion by admitting that, prior to 3.15's
cda540ace6a1 "mm: get_user_pages(write,force) refuse to COW in
shared areas", maybe it was possible to find a PageAnon there.)

But my belief (best confirmed by you running your tests with a
suitably placed BUG_ON or WARN_ON) is that you'll never find a
PageAnon in a vma_shareable() area, so will never need try_to_unmap()
to unshare a pagetable in the PageAnon case, so won't need i_mmap_rwsem
for PageAnon there, and _get_hugetlb_page_mapping() (your function that
deduces an address_space from an anon_vma) can just be deleted.

(And in passing, may I ask what hugetlb_page_mapping_lock_write()'s
hpage->_mapcount inc and dec are for?  You comment it as a hack,
but don't explain what needs that hack, and I don't see it.)

It does seem sad to be unsharing the page table in any case of
try_to_unmap(): the only reason for that I can see (if more care
were taken with TLB flushing) is the mmu_notifier situation -
I don't think we have any suitable mmu_notifier to alert all
the mms to be affected.

Hugh

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