Re: mm: race in put_and_wait_on_page_locked()

[Hugh Dickins]

On Tue, 5 Feb 2019, Artem Savkov wrote:
> On Mon, Feb 04, 2019 at 12:42:50PM -0800, Hugh Dickins wrote:
> > On Mon, 4 Feb 2019, Artem Savkov wrote:
> > 
> > > Hi Hugh,
> > > 
> > > Your recent patch 9a1ea439b16b "mm: put_and_wait_on_page_locked() while
> > > page is migrated" seems to have introduced a race into page migration
> > > process. I have a host that eagerly reproduces the following BUG under
> > > stress:
> > > 
> > > [  302.847402] page:f000000000021700 count:0 mapcount:0 mapping:c0000000b2710bb0 index:0x19
> > > [  302.848096] xfs_address_space_operations [xfs] 
> > > [  302.848100] name:"libc-2.28.so" 
> > > [  302.848244] flags: 0x3ffff800000006(referenced|uptodate)
> > > [  302.848521] raw: 003ffff800000006 5deadbeef0000100 5deadbeef0000200 0000000000000000
> > > [  302.848724] raw: 0000000000000019 0000000000000000 00000001ffffffff c0000000bc0b1000
> > > [  302.848919] page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
> > > [  302.849076] page->mem_cgroup:c0000000bc0b1000
> > > [  302.849269] ------------[ cut here ]------------
> > > [  302.849397] kernel BUG at include/linux/mm.h:546!
> > > [  302.849586] Oops: Exception in kernel mode, sig: 5 [#1]
> > > [  302.849711] LE SMP NR_CPUS=2048 NUMA pSeries
> > > [  302.849839] Modules linked in: pseries_rng sunrpc xts vmx_crypto virtio_balloon xfs libcrc32c virtio_net net_failover virtio_console failover virtio_blk
> > > [  302.850400] CPU: 3 PID: 8759 Comm: cc1 Not tainted 5.0.0-rc4+ #36
> > > [  302.850571] NIP:  c00000000039c8b8 LR: c00000000039c8b4 CTR: c00000000080a0e0
> > > [  302.850758] REGS: c0000000b0d7f7e0 TRAP: 0700   Not tainted  (5.0.0-rc4+)
> > > [  302.850952] MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 48024422  XER: 00000000
> > > [  302.851150] CFAR: c0000000003ff584 IRQMASK: 0 
> > > [  302.851150] GPR00: c00000000039c8b4 c0000000b0d7fa70 c000000001bcca00 0000000000000021 
> > > [  302.851150] GPR04: c0000000b044c628 0000000000000007 55555555555555a0 c000000001fc3760 
> > > [  302.851150] GPR08: 0000000000000007 0000000000000000 c0000000b0d7c000 c0000000b0d7f5ff 
> > > [  302.851150] GPR12: 0000000000004400 c00000003fffae80 0000000000000000 0000000000000000 
> > > [  302.851150] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > > [  302.851150] GPR20: c0000000689f5aa8 c00000002a13ee48 0000000000000000 c000000001da29b0 
> > > [  302.851150] GPR24: c000000001bf7d80 c0000000689f5a00 0000000000000000 0000000000000000 
> > > [  302.851150] GPR28: c000000001bf9e80 c0000000b0d7fab8 0000000000000001 f000000000021700 
> > > [  302.852914] NIP [c00000000039c8b8] put_and_wait_on_page_locked+0x398/0x3d0
> > > [  302.853080] LR [c00000000039c8b4] put_and_wait_on_page_locked+0x394/0x3d0
> > > [  302.853235] Call Trace:
> > > [  302.853305] [c0000000b0d7fa70] [c00000000039c8b4] put_and_wait_on_page_locked+0x394/0x3d0 (unreliable)
> > > [  302.853540] [c0000000b0d7fb10] [c00000000047b838] __migration_entry_wait+0x178/0x250
> > > [  302.853738] [c0000000b0d7fb50] [c00000000040c928] do_swap_page+0xd78/0xf60
> > > [  302.853997] [c0000000b0d7fbd0] [c000000000411078] __handle_mm_fault+0xbf8/0xe80
> > > [  302.854187] [c0000000b0d7fcb0] [c000000000411548] handle_mm_fault+0x248/0x450
> > > [  302.854379] [c0000000b0d7fd00] [c000000000078ca4] __do_page_fault+0x2d4/0xdf0
> > > [  302.854877] [c0000000b0d7fde0] [c0000000000797f8] do_page_fault+0x38/0xf0
> > > [  302.855057] [c0000000b0d7fe20] [c00000000000a7c4] handle_page_fault+0x18/0x38
> > > [  302.855300] Instruction dump:
> > > [  302.855432] 4bfffcf0 60000000 3948ffff 4bfffd20 60000000 60000000 3c82ff36 7fe3fb78 
> > > [  302.855689] fb210068 38843b78 48062f09 60000000 <0fe00000> 60000000 3b400001 3b600001 
> > > [  302.855950] ---[ end trace a52140e0f9751ae0 ]---
> > > 
> > > What seems to be happening is migrate_page_move_mapping() calling
> > > page_ref_freeze() on another cpu somewhere between __migration_entry_wait()
> > > taking a reference and wait_on_page_bit_common() calling page_put().
> > 
> > Thank you for reporting, Artem.
> > 
> > And see the mm thread https://marc.info/?l=linux-mm&m=154821775401218&w=2
> 
> Ah, thank you. Should have searched through linux-mm, not just lkml.
> 
> > That was on arm64, you are on power I think: both point towards xfs
> > (Cai could not reproduce it on ext4), but that should not be taken too
> > seriously - it could just be easier to reproduce on one than the other.
> > 
> > Your description in your last paragraph is what I imagined happening too.
> > And nothing wrong with that, except that the page_ref_freeze() should
> > have failed, but succeeded.  We believe that something has done an
> > improper put_page(), on a libc-2.28.so page that's normally always
> > in use, and the put_and_wait_on_page_locked() commit has exposed that
> > by making its migration possible when it was almost impossible before
> > (Cai has reproduced it without the put_and_wait_on_page_locked commit).
> 
> This is what I saw as well, only reproduces on xfs and page_ref_count == 0
> BUG through generic_file_buffered_read() when your patch is reverted.
> Wasn't sure that's the same issue though.
> 
> > I don't think any of us have made progress on this since the 25th.
> > I'll wrap up what I'm working on in the next hour or two, and switch
> > my attention to this. Even if put_and_wait_on_page_locked() happens to
> > be correct, and just makes a pre-existing bug much easier to hit, we
> > shall have to revert it from 5.0 if we cannot find the right answer
> > in the next week or so.  Which would be sad: I'll try to rescue it,
> > but don't have great confidence that I'll be successful.
> > 
> > I'll be looking through the source, thinking around it, and trying
> > to find a surplus put_page(). I don't have any experiments in mind
> > to try at this stage.
> > 
> > Something I shall not be doing, is verifying the correctness of the
> > low-level get_page_unless_zero() versus page_ref_freeze() protocol
> > on arm64 and power - nobody has reported on x86, and I do wonder if
> > there's a barrier missing somewhere, that could manifest in this way -
> > but I'm unlikely to be the one to find that (and also think that any
> > weakness there should have shown up long before now).
> 
> I tried reproducing it with 5.0-rc5 and failed. There is one patch that
> seems to be fixing an xfs page reference issue which to me sounds a lot
> like what you describe.  The patch is 8e47a457321c "iomap: get/put the
> page in iomap_page_create/release()". That would explain why
> page_ref_freeze() and all the expected_page_refs() checks succeed when
> they shouldn't.

Yes! I'm sure you've got it, that fits exactly, thank you so much Artem.
iomap_migrate_page() was very much on my search path yesterday, but I
was looking at latest source, had missed checking it for recent fixes.

> 
> Apart from no longer reproducing the bug I also see a drastic reduce in
> pgmigrate_fails in /proc/vmstat (from tens of thousands and
> being >pgmigrate_success, to just tens) so I assume it is possible for it
> to be just masking the problem by performing less retries. What do you think?

I'm a little surprised that it managed to get as far as so many fails
without crashing, when it had the refcounting wrong like that: but I'm
sure you've found the root fix, not something masking the bug.

> 
> Cai, can you please check if you can reproduce this issue in your
> environment with 5.0-rc5?

Yes, please do - practical confirmation more convincing than my certainty.

(Linus, I'll reply a little to yours later.)

Hugh