Hi all,
On Thu, Jan 4, 2018 at 7:33 PM, Jan Kara <jack@xxxxxxx> wrote:
>
> On Wed 03-01-18 20:56:32, Dan Williams wrote:
> > On Wed, Jan 3, 2018 at 2:04 AM, Jan Kara <jack@xxxxxxx> wrote:
> > > Hello,
> > >
> > > Over the years I have seen so far unexplained crashed in filesystem's
> > > (ext4, xfs) writeback path due to dirty pages without buffers attached to
> > > them (see [1] and [2] for relatively recent reports). This was confusing as
> > > reclaim takes care not to strip buffers from a dirty page and both
> > > filesystems do add buffers to a page when it is first written to - in
> > > ->page_mkwrite() and ->write_begin callbacks.
> > >
> > > Recently I have come across a code path that is probably leading to this
> > > inconsistent state and I'd like to discuss how to best fix the problem
> > > because it's not obvious to me. Consider the following race:
> > >
> > > CPU1 CPU2
> > >
> > > addr = mmap(file1, MAP_SHARED, ...);
> > > fd2 = open(file2, O_DIRECT | O_RDONLY);
> > > read(fd2, addr, len)
> > > do_direct_IO()
> > > page = dio_get_page()
> > > dio_refill_pages()
> > > iov_iter_get_pages()
> > > get_user_pages_fast()
> > > - page fault
> > > ->page_mkwrite()
> > > block_page_mkwrite()
> > > lock_page(page);
> > > - attaches buffers to page
> > > - makes sure blocks are allocated
> > > set_page_dirty(page)
> > > - install writeable PTE
> > > unlock_page(page);
> > > submit_page_section(page)
> > > - submits bio with 'page' as a buffer
> > > kswapd reclaims pages:
> > > ...
> > > shrink_page_list()
> > > trylock_page(page) - this is the
> > > page CPU1 has just faulted in
> > > try_to_unmap(page)
> > > pageout(page);
> > > clear_page_dirty_for_io(page);
> > > ->writepage()
> > > - let's assume page got written
> > > out fast enough, alternatively
> > > we could get to the same path as
> > > soon as the page IO completes
> > > if (page_has_private(page)) {
> > > try_to_release_page(page)
> > > - reclaims buffers from the
> > > page
> > > __remove_mapping(page)
> > > - fails as DIO code still
> > > holds page reference
> > > ...
> > >
> > > eventually read completes
> > > dio_bio_complete(bio)
> > > set_page_dirty_lock(page)
> > > Bummer, we've just marked the page as dirty without having buffers.
> > > Eventually writeback will find it and filesystem will complain...
> > >
> > > Am I missing something?
> > >
> > > The problem here is that filesystems fundamentally assume that a page can
> > > be written to only between ->write_begin - ->write_end (in this interval
> > > the page is locked), or between ->page_mkwrite - ->writepage and above is
> > > an example where this does not hold because when a page reference is
> > > acquired through get_user_pages(), page can get written to by the holder of
> > > the reference and dirtied even after it has been unmapped from page tables
> > > and ->writepage has been called. This is not only a cosmetic issue leading
> > > to assertion failure but it can also lead to data loss, data corruption, or
> > > other unpleasant surprises as filesystems assume page contents cannot be
> > > modified until either ->write_begin() or ->page_mkwrite gets called and
> > > those calls are serialized by proper locking with problematic operations
> > > such as hole punching etc.
> > >
> > > I'm not sure how to fix this problem. We could 'simulate' a writeable page
> > > fault in set_page_dirty_lock(). It is a bit ugly since we don't have a
> > > virtual address of the fault, don't hold mmap_sem, etc., possibly
> > > expensive, but it would make filesystems happy. Data stored by GUP user
> > > (e.g. read by DIO in the above case) could still get lost if someone e.g.
> > > punched hole under the buffer or otherwise messed with the underlying
> > > storage of the page while DIO was running but arguably users could expect
> > > such outcome.
> > >
> > > Another possible solution would be to make sure page is writeably mapped
> > > until GUP user drops its reference. That would be arguably cleaner but
> > > probably that would mean we have to track number of writeable GUP page
> > > references separately (no space space in struct page is a problem here) and
> > > block page_mkclean() until they are dropped. Also for long term GUP users
> > > like Infiniband or V4L we'd have to come up with some solution as we should
> > > not block page_mkclean() for so long.
> >
> > Do we need to block page_mkclean, or could we defer buffer reclaiming
> > to the last put of the page?
>
> As I wrote to Dave the problem is no so much with reclaiming of buffers but
> with the fact filesystems don't expect page can be dirtied after
> page_mkclean() is finished.
>
> > I think once we have the "register memory with lease" mechanism for
> > Infiniband we could expand it to the page cache case. The problem is
> > the regression this would cause with userspace that expects it can
> > maintain file backed memory registrations indefinitely.
> >
> > What are the implications of holding off page_mkclean or release
> > buffers indefinitely?
>
> Bad. You cannot write the page to disk until page_mkclean() finishes as
> page_mkclean() is part of clear_page_dirty_for_io(). And we really do need
> that functionality there e.g. to make sure tail of the last page in the
> file is properly zeroed out, storage with DIF/DIX can compute checksum of
> the data safely before submitting it to the device etc.
>
> > Is an indefinite / interruptible sleep waiting for the 'put' event of
> > a get_user_pages() page unacceptable? The current case that the file
> > contents will not be coherent with respect to in-flight RDMA, perhaps
> > waiting for that to complete is better than cleaning buffers from the
> > page prematurely.
>
> Yeah, indefinite sleep is really a no-go.
>
> > > As a side note DAX needs some solution for GUP users as well. The problems
> > > are similar there in nature, just much easier to hit. So at least a
> > > solution for long-term GUP users can (and I strongly believe should) be
> > > shared between standard and DAX paths.
> >
> > In the DAX case we rely on the fact that when the page goes idle we
> > only need to worry about the filesytem block map changing, the page
> > won't get reallocated somewhere else. We can't use page idle as an
> > event in this case, however, if the page reference count is one then
> > the DIO code can know that it has the page exclusively, so maybe DAX
> > and non-DAX can share the page count == 1 event notification.
>
> The races I describe do not need to involve truncate / hole punching. It is
> just enough to race with page writeback. So page references are of no use
> here. We would have to specifically track number of references acquired by
> GUP or something like that. So what I wanted to share with DAX is the
> long-term pin handling, the rest is unclear for now.
>
> Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
>
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The bug can be reliably reproduced in our platform with the
current upstream kernel(80aa76bcd364 Merge tag 'xfs-4.17-merge-4' of
git://git.kernel.org/pub/scm/fs/xfs/xfs-linux). I'm happy to
help to test and debug. The error message is as following:
kernel BUG at /home/gavin/work-kernel/fs/ext4/inode.c:2126!
invalid opcode: 0000 [#1] SMP PTI
Modules linked in: veth ipt_MASQUERADE nf_nat_masquerade_ipv4
nf_conntrack_netlink nfnetlink xfrm_user iptable_nat nf_conntrack_ipv4
nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype xt_conntrack nf_nat
nf_conntrack br_netfilter bridge stp llc overlay xt_multiport
iptable_filter ip_tables x_tables cachefiles fscache esp6_offload esp6
esp4_offload esp4 xfrm_algo nls_iso8859_1 intel_rapl sb_edac
x86_pkg_temp_thermal intel_powerclamp ipmi_ssif coretemp kvm_intel
nvidia_uvm(POE) mxm_wmi kvm joydev input_leds irqbypass intel_cstate
intel_rapl_perf mei_me ipmi_si shpchp lpc_ich mei acpi_power_meter
mac_hid wmi ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp
libiscsi scsi_transport_iscsi ipmi_devintf sunrpc ipmi_msghandler
autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov
async_memcpy
async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0
multipath linear i2c_algo_bit nvidia_drm(POE) ses crct10dif_pclmul
crc32_pclmul nvidia_modeset(POE) ttm ghash_clmulni_intel enclosure
hid_generic uas pcbc scsi_transport_sas drm_kms_helper usbhid
aesni_intel hid aes_x86_64 usb_storage syscopyarea crypto_simd
sysfillrect mlx5_core cryptd nvidia(POE) sysimgblt glue_helper ixgbe
mlxfw megaraid_sas fb_sys_fops devlink dca ahci ptp drm libahci
pps_core mdio
CPU: 54 PID: 8938 Comm: kworker/u161:0 Tainted: P OE
4.16.0-999-generic #201804102200
Workqueue: writeback wb_workfn (flush-8:0)
RIP: 0010:ext4_writepage+0x318/0x770
RSP: 0018:ffffb514e76cb7f8 EFLAGS: 00010246
RAX: 00500b4e8000026d RBX: 0000000000001000 RCX: ffff8c2cafba5000
RDX: ffff8bec4069a020 RSI: ffffb514e76cbc28 RDI: ffffe91efcc8bd80
RBP: ffffb514e76cb870 R08: 0000000000028115 R09: 00000000000280c0
R10: 0000000000000002 R11: ffff8c2dbffd4000 R12: ffffe91efcc8bd80
R13: ffff8bec40699ea8 R14: ffffb514e76cbc28 R15: ffffe91efcc8bd80
FS: 0000000000000000(0000) GS:ffff8becbfc80000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffcc8f6f080 CR3: 0000004b9ce0a006 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
? rmap_walk+0x41/0x60
? page_mkclean+0x9f/0xb0
? invalid_page_referenced_vma+0x80/0x80
__writepage+0x17/0x50
write_cache_pages+0x228/0x4a0
? __wb_calc_thresh+0x140/0x140
generic_writepages+0x61/0xa0
? _cond_resched+0x1a/0x50
? write_cache_pages+0x396/0x4a0
ext4_writepages+0x1fc/0xe00
? ext4_writepages+0x1fc/0xe00
? generic_writepages+0x6d/0xa0
? fprop_fraction_percpu+0x2f/0x80
do_writepages+0x1c/0x60
? do_writepages+0x1c/0x60
__writeback_single_inode+0x45/0x320
writeback_sb_inodes+0x266/0x580
__writeback_inodes_wb+0x92/0xc0
wb_writeback+0x282/0x310
wb_workfn+0x1a3/0x440
? wb_workfn+0x1a3/0x440
process_one_work+0x1db/0x3c0
worker_thread+0x4b/0x420
kthread+0x102/0x140
? rescuer_thread+0x380/0x380
? kthread_create_worker_on_cpu+0x70/0x70
ret_from_fork+0x35/0x40
Code: ff f6 c4 08 0f 85 58 ff ff ff e8 68 56 00 00 ba 00 10 00 00 31
f6 41 bd fb ff ff ff e8 a2 9e ff ff 4c 89 e7 e8 fa 39 ea ff eb 8a <0f>
0b c6 45 a8 00 e9 f0 fd ff ff 49 83 7c 24 10 00 0f 85 c7 03
RIP: ext4_writepage+0x318/0x770 RSP: ffffb514e76cb7f8
---[ end trace 59d4e1a4b221404b ]---
