Re: [PATCH] lib/iov_iter: Fix pipe handling in _copy_to_iter_mcsafe

[Dan Williams]

On Mon, Jul 2, 2018 at 9:58 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> On Sun, Jul 01, 2018 at 08:52:20AM -0700, Dan Williams wrote:
>> By mistake the ITER_PIPE early-exit / warning from copy_from_iter() was
>> cargo-culted in _copy_to_iter_mcsafe() rather than a machine-check-safe
>> version of copy_to_iter_pipe().
>>
>> Implement copy_pipe_to_iter_mcsafe() being careful to return the
>> indication of short copies due to a CPU exception.
>>
>> Without this regression-fix all splice reads to dax-mode files fail.
>>
>> Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Tony Luck <tony.luck@xxxxxxxxx>
>> Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>> Hi Ingo,
>>
>> I'm submitting this fix back through the tip tree since the regression
>> originated through tip/x86/dax.
>>
>>  lib/iov_iter.c |   37 +++++++++++++++++++++++++++++++++----
>>  1 file changed, 33 insertions(+), 4 deletions(-)
>
> Hey Dan,
>
> I retested the current linux/master with this patch applied, and XFS + DAX +
> generic/323 still dies for me:
>
>   run fstests generic/323 at 2018-07-02 10:51:35
>   BUG: unable to handle kernel paging request at 00007f16dc001000
>   PGD 80000000bb71a067 P4D 80000000bb71a067 PUD bb71b067 PMD bb6e8067 PTE 0
>   Oops: 0002 [#1] PREEMPT SMP PTI
>   CPU: 1 PID: 1598 Comm: aio-last-ref-he Not tainted
>   4.18.0-rc3-00001-g5174f2f2b6e5 #2
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>   rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.org 04/01/2014
>   RIP: 0010:__memcpy+0x12/0x20
>   Code: c3 e8 42 fb ff ff 48 8b 43 60 48 2b 43 50 88 43 4e 5b 5d c3 90 90 90
>   90 0f 1f 44 00 00 48 89 f8 48 89 d1 48 c1 e9 03 83 e2 07 <f3> 48 a5 89 d1 f3
>   a4 c3 66 0f 1f 44 00 00 48 89 f8 48 89 d1 f3 a4
>   RSP: 0018:ffffc90002783a60 EFLAGS: 00010246
>   RAX: 00007f16dc001000 RBX: ffff880151229000 RCX: 0000000000002000
>   RDX: 0000000000000000 RSI: ffff880151219000 RDI: 00007f16dc001000
>   RBP: ffffc90002783a68 R08: 0000004227a4083c R09: ffff880151219000
>   R10: ffffc90002783d40 R11: 0000000000000000 R12: 0000000000000000
>   R13: 0000000000010000 R14: ffffc90002783d18 R15: 0000000000010000
>   FS:  00007f16f1ec5700(0000) GS:ffff880114600000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 00007f16dc001000 CR3: 0000000035508000 CR4: 00000000000006e0
>   Call Trace:
>    ? copyout_mcsafe+0x3e/0x60
>    _copy_to_iter_mcsafe+0x9e/0x4c0
>    ? __lock_is_held+0x65/0xb0
>    pmem_copy_to_iter+0x17/0x20 [nd_pmem]
>    dax_copy_to_iter+0x49/0x70
>    dax_iomap_actor+0x1f8/0x280
>    ? dax_iomap_rw+0x100/0x100
>    iomap_apply+0xb5/0x130
>    ? dax_iomap_rw+0x100/0x100
>    dax_iomap_rw+0x95/0x100
>    ? dax_iomap_rw+0x100/0x100
>    xfs_file_dax_read+0x83/0x1f0
>    xfs_file_read_iter+0xac/0xc0
>    aio_read+0x11f/0x1a0
>    ? __might_fault+0x3e/0x90
>    io_submit_one+0x39d/0x5f0
>    ? io_submit_one+0x39d/0x5f0
>    __x64_sys_io_submit+0xa1/0x280
>    do_syscall_64+0x65/0x220
>    ? do_syscall_64+0x65/0x220
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> This failure looks identical to what I was hitting with the original bug
> report.

I see it now, my run was skipping generic/323.