Re: [GIT PULL] bcachefs

[Jens Axboe]

On 6/27/23 10:01?PM, Kent Overstreet wrote:
> On Tue, Jun 27, 2023 at 09:16:31PM -0600, Jens Axboe wrote:
>> On 6/27/23 2:15?PM, Kent Overstreet wrote:
>>>> to ktest/tests/xfstests/ and run it with -bcachefs, otherwise it kept
>>>> failing because it assumed it was XFS.
>>>>
>>>> I suspected this was just a timing issue, and it looks like that's
>>>> exactly what it is. Looking at the test case, it'll randomly kill -9
>>>> fsstress, and if that happens while we have io_uring IO pending, then we
>>>> process completions inline (for a PF_EXITING current). This means they
>>>> get pushed to fallback work, which runs out of line. If we hit that case
>>>> AND the timing is such that it hasn't been processed yet, we'll still be
>>>> holding a file reference under the mount point and umount will -EBUSY
>>>> fail.
>>>>
>>>> As far as I can tell, this can happen with aio as well, it's just harder
>>>> to hit. If the fput happens while the task is exiting, then fput will
>>>> end up being delayed through a workqueue as well. The test case assumes
>>>> that once it's reaped the exit of the killed task that all files are
>>>> released, which isn't necessarily true if they are done out-of-line.
>>>
>>> Yeah, I traced it through to the delayed fput code as well.
>>>
>>> I'm not sure delayed fput is responsible here; what I learned when I was
>>> tracking this down has mostly fell out of my brain, so take anything I
>>> say with a large grain of salt. But I believe I tested with delayed_fput
>>> completely disabled, and found another thing in io_uring with the same
>>> effect as delayed_fput that wasn't being flushed.
>>
>> I'm not saying it's delayed_fput(), I'm saying it's the delayed putting
>> io_uring can end up doing. But yes, delayed_fput() is another candidate.
> 
> Sorry - was just working through my recollections/initial thought
> process out loud

No worries, it might actually be a combination and this is why my
io_uring side patch didn't fully resolve it. Wrote a simple reproducer
and it seems to reliably trigger it, but is fixed with an flush of the
delayed fput list on mount -EBUSY return. Still digging...

>>>> For io_uring specifically, it may make sense to wait on the fallback
>>>> work. The below patch does this, and should fix the issue. But I'm not
>>>> fully convinced that this is really needed, as I do think this can
>>>> happen without io_uring as well. It just doesn't right now as the test
>>>> does buffered IO, and aio will be fully sync with buffered IO. That
>>>> means there's either no gap where aio will hit it without O_DIRECT, or
>>>> it's just small enough that it hasn't been hit.
>>>
>>> I just tried your patch and I still have generic/388 failing - it
>>> might've taken a bit longer to pop this time.
>>
>> Yep see the same here. Didn't have time to look into it after sending
>> that email today, just took a quick stab at writing a reproducer and
>> ended up crashing bcachefs:
> 
> You must have hit an error before we finished initializing the
> filesystem, the list head never got initialized. Patch for that will be
> in the testing branch momentarily.

I'll pull that in. In testing just now, I hit a few more leaks:

unreferenced object 0xffff0000e55cf200 (size 128):
  comm "mount", pid 723, jiffies 4294899134 (age 85.868s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000001d69062c>] slab_post_alloc_hook.isra.0+0xb4/0xbc
    [<00000000c503def2>] __kmem_cache_alloc_node+0xd0/0x178
    [<00000000cde48528>] __kmalloc+0xac/0xd4
    [<000000006cb9446a>] kmalloc_array.constprop.0+0x18/0x20
    [<000000008341b32c>] bch2_fs_alloc+0x73c/0xbcc
    [<000000003b8339fd>] bch2_fs_open+0x19c/0x430
    [<00000000aef40a23>] bch2_mount+0x194/0x45c
    [<0000000005e49357>] legacy_get_tree+0x2c/0x54
    [<00000000f5813622>] vfs_get_tree+0x28/0xd4
    [<00000000ea6972ec>] path_mount+0x5d0/0x6c8
    [<00000000468ec307>] do_mount+0x80/0xa4
    [<00000000ea5d305d>] __arm64_sys_mount+0x150/0x168
    [<00000000da6d98cb>] invoke_syscall.constprop.0+0x70/0xb8
    [<000000008f20c487>] do_el0_svc+0xbc/0xf0
    [<00000000a1018c2c>] el0_svc+0x74/0x9c
    [<00000000fc46d579>] el0t_64_sync_handler+0xa8/0x134
unreferenced object 0xffff0000e55cf580 (size 128):
  comm "mount", pid 723, jiffies 4294899134 (age 85.868s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000001d69062c>] slab_post_alloc_hook.isra.0+0xb4/0xbc
    [<00000000c503def2>] __kmem_cache_alloc_node+0xd0/0x178
    [<00000000cde48528>] __kmalloc+0xac/0xd4
    [<0000000097f806f1>] __prealloc_shrinker+0x3c/0x60
    [<000000008ff20762>] register_shrinker+0x14/0x34
    [<000000007fa7e36c>] bch2_fs_btree_cache_init+0xf8/0x150
    [<000000005135a635>] bch2_fs_alloc+0x7ac/0xbcc
    [<000000003b8339fd>] bch2_fs_open+0x19c/0x430
    [<00000000aef40a23>] bch2_mount+0x194/0x45c
    [<0000000005e49357>] legacy_get_tree+0x2c/0x54
    [<00000000f5813622>] vfs_get_tree+0x28/0xd4
    [<00000000ea6972ec>] path_mount+0x5d0/0x6c8
    [<00000000468ec307>] do_mount+0x80/0xa4
    [<00000000ea5d305d>] __arm64_sys_mount+0x150/0x168
    [<00000000da6d98cb>] invoke_syscall.constprop.0+0x70/0xb8
    [<000000008f20c487>] do_el0_svc+0xbc/0xf0
unreferenced object 0xffff0000e55cf480 (size 128):
  comm "mount", pid 723, jiffies 4294899134 (age 85.868s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000001d69062c>] slab_post_alloc_hook.isra.0+0xb4/0xbc
    [<00000000c503def2>] __kmem_cache_alloc_node+0xd0/0x178
    [<00000000cde48528>] __kmalloc+0xac/0xd4
    [<0000000097f806f1>] __prealloc_shrinker+0x3c/0x60
    [<000000008ff20762>] register_shrinker+0x14/0x34
    [<000000003d050c32>] bch2_fs_btree_key_cache_init+0x88/0x90
    [<00000000d9f351c0>] bch2_fs_alloc+0x7c0/0xbcc
    [<000000003b8339fd>] bch2_fs_open+0x19c/0x430
    [<00000000aef40a23>] bch2_mount+0x194/0x45c
    [<0000000005e49357>] legacy_get_tree+0x2c/0x54
    [<00000000f5813622>] vfs_get_tree+0x28/0xd4
    [<00000000ea6972ec>] path_mount+0x5d0/0x6c8
    [<00000000468ec307>] do_mount+0x80/0xa4
    [<00000000ea5d305d>] __arm64_sys_mount+0x150/0x168
    [<00000000da6d98cb>] invoke_syscall.constprop.0+0x70/0xb8
    [<000000008f20c487>] do_el0_svc+0xbc/0xf0

>>> I wonder if there might be a better way of solving this though? For aio,
>>> when a process is exiting we just synchronously tear down the ioctx,
>>> including waiting for outstanding iocbs.
>>
>> aio is pretty trivial, because the only async it supports is O_DIRECT
>> on regular files which always completes in finite time. io_uring has to
>> cancel etc, so we need to do a lot more.
> 
> ahh yes, buffered IO would complicate things
> 
>> But the concept of my patch should be fine, but I think we must be
>> missing a case. Which is why I started writing a small reproducer
>> instead. I'll pick it up again tomorrow and see what is going on here.
> 
> Ok. Soon as you've got a patch I'll throw it at my CI, or I can point my
> CI at your branch if you have one.

I should have something later today, don't feel like I fully understand
all of it just yet.

-- 
Jens Axboe