On 11/09/2019 01:47 PM, Ilya Dryomov wrote:
> On Fri, Nov 8, 2019 at 8:24 PM Mike Christie <mchristi@xxxxxxxxxx> wrote:
>>
>> There are several storage drivers like dm-multipath, iscsi, tcmu-runner,
>> amd nbd that have userspace components that can run in the IO path. For
>> example, iscsi and nbd's userspace deamons may need to recreate a socket
>> and/or send IO on it, and dm-multipath's daemon multipathd may need to
>> send SG IO or read/write IO to figure out the state of paths and re-set
>> them up.
>>
>> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
>> memalloc_*_save/restore functions to control the allocation behavior,
>> but for userspace we would end up hitting an allocation that ended up
>> writing data back to the same device we are trying to allocate for.
>> The device is then in a state of deadlock, because to execute IO the
>> device needs to allocate memory, but to allocate memory the memory
>> layers want execute IO to the device.
>>
>> Here is an example with nbd using a local userspace daemon that performs
>> network IO to a remote server. We are using XFS on top of the nbd device,
>> but it can happen with any FS or other modules layered on top of the nbd
>> device that can write out data to free memory. Here a nbd daemon helper
>> thread, msgr-worker-1, is performing a write/sendmsg on a socket to execute
>> a request. This kicks off a reclaim operation which results in a WRITE to
>> the nbd device and the nbd thread calling back into the mm layer.
>>
>> [ 1626.609191] msgr-worker-1 D 0 1026 1 0x00004000
>> [ 1626.609193] Call Trace:
>> [ 1626.609195] ? __schedule+0x29b/0x630
>> [ 1626.609197] ? wait_for_completion+0xe0/0x170
>> [ 1626.609198] schedule+0x30/0xb0
>> [ 1626.609200] schedule_timeout+0x1f6/0x2f0
>> [ 1626.609202] ? blk_finish_plug+0x21/0x2e
>> [ 1626.609204] ? _xfs_buf_ioapply+0x2e6/0x410
>> [ 1626.609206] ? wait_for_completion+0xe0/0x170
>> [ 1626.609208] wait_for_completion+0x108/0x170
>> [ 1626.609210] ? wake_up_q+0x70/0x70
>> [ 1626.609212] ? __xfs_buf_submit+0x12e/0x250
>> [ 1626.609214] ? xfs_bwrite+0x25/0x60
>> [ 1626.609215] xfs_buf_iowait+0x22/0xf0
>> [ 1626.609218] __xfs_buf_submit+0x12e/0x250
>> [ 1626.609220] xfs_bwrite+0x25/0x60
>> [ 1626.609222] xfs_reclaim_inode+0x2e8/0x310
>> [ 1626.609224] xfs_reclaim_inodes_ag+0x1b6/0x300
>> [ 1626.609227] xfs_reclaim_inodes_nr+0x31/0x40
>> [ 1626.609228] super_cache_scan+0x152/0x1a0
>> [ 1626.609231] do_shrink_slab+0x12c/0x2d0
>> [ 1626.609233] shrink_slab+0x9c/0x2a0
>> [ 1626.609235] shrink_node+0xd7/0x470
>> [ 1626.609237] do_try_to_free_pages+0xbf/0x380
>> [ 1626.609240] try_to_free_pages+0xd9/0x1f0
>> [ 1626.609245] __alloc_pages_slowpath+0x3a4/0xd30
>> [ 1626.609251] ? ___slab_alloc+0x238/0x560
>> [ 1626.609254] __alloc_pages_nodemask+0x30c/0x350
>> [ 1626.609259] skb_page_frag_refill+0x97/0xd0
>> [ 1626.609274] sk_page_frag_refill+0x1d/0x80
>> [ 1626.609279] tcp_sendmsg_locked+0x2bb/0xdd0
>> [ 1626.609304] tcp_sendmsg+0x27/0x40
>> [ 1626.609307] sock_sendmsg+0x54/0x60
>> [ 1626.609308] ___sys_sendmsg+0x29f/0x320
>> [ 1626.609313] ? sock_poll+0x66/0xb0
>> [ 1626.609318] ? ep_item_poll.isra.15+0x40/0xc0
>> [ 1626.609320] ? ep_send_events_proc+0xe6/0x230
>> [ 1626.609322] ? hrtimer_try_to_cancel+0x54/0xf0
>> [ 1626.609324] ? ep_read_events_proc+0xc0/0xc0
>> [ 1626.609326] ? _raw_write_unlock_irq+0xa/0x20
>> [ 1626.609327] ? ep_scan_ready_list.constprop.19+0x218/0x230
>> [ 1626.609329] ? __hrtimer_init+0xb0/0xb0
>> [ 1626.609331] ? _raw_spin_unlock_irq+0xa/0x20
>> [ 1626.609334] ? ep_poll+0x26c/0x4a0
>> [ 1626.609337] ? tcp_tsq_write.part.54+0xa0/0xa0
>> [ 1626.609339] ? release_sock+0x43/0x90
>> [ 1626.609341] ? _raw_spin_unlock_bh+0xa/0x20
>> [ 1626.609342] __sys_sendmsg+0x47/0x80
>> [ 1626.609347] do_syscall_64+0x5f/0x1c0
>> [ 1626.609349] ? prepare_exit_to_usermode+0x75/0xa0
>> [ 1626.609351] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> This patch adds a new prctl command that daemons can use after they have
>> done their initial setup, and before they start to do allocations that
>> are in the IO path. It sets the PF_MEMALLOC_NOIO and PF_LESS_THROTTLE
>> flags so both userspace block and FS threads can use it to avoid the
>> allocation recursion and try to prevent from being throttled while
>> writing out data to free up memory.
>>
>> Signed-off-by: Mike Christie <mchristi@xxxxxxxxxx>
>> ---
>> V3
>> - Drop NOFS, set PF_LESS_THROTTLE and rename prctl flag to reflect it
>> is more general and can support both FS and block devices. Both fuse
>> and block device daemons, nbd and tcmu-runner, have been tested to
>> confirm the more restrictive PF_MEMALLOC_NOIO also works for fuse.
>>
>> - Use CAP_SYS_RESOURCE instead of admin.
>>
>> V2:
>> - Use prctl instead of procfs.
>> - Add support for NOFS for fuse.
>> - Check permissions.
>>
>>
>> include/uapi/linux/capability.h | 1 +
>> include/uapi/linux/prctl.h | 4 ++++
>> kernel/sys.c | 26 ++++++++++++++++++++++++++
>> 3 files changed, 31 insertions(+)
>>
>> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
>> index 240fdb9a60f6..272dc69fa080 100644
>> --- a/include/uapi/linux/capability.h
>> +++ b/include/uapi/linux/capability.h
>> @@ -301,6 +301,7 @@ struct vfs_ns_cap_data {
>> /* Allow more than 64hz interrupts from the real-time clock */
>> /* Override max number of consoles on console allocation */
>> /* Override max number of keymaps */
>> +/* Control memory reclaim behavior */
>>
>> #define CAP_SYS_RESOURCE 24
>>
>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>> index 7da1b37b27aa..07b4f8131e36 100644
>> --- a/include/uapi/linux/prctl.h
>> +++ b/include/uapi/linux/prctl.h
>> @@ -234,4 +234,8 @@ struct prctl_mm_map {
>> #define PR_GET_TAGGED_ADDR_CTRL 56
>> # define PR_TAGGED_ADDR_ENABLE (1UL << 0)
>>
>> +/* Control reclaim behavior when allocating memory */
>> +#define PR_SET_IO_FLUSHER 57
>> +#define PR_GET_IO_FLUSHER 58
>> +
>> #endif /* _LINUX_PRCTL_H */
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index a611d1d58c7d..08c6b682fa99 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -2486,6 +2486,32 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>> return -EINVAL;
>> error = GET_TAGGED_ADDR_CTRL();
>> break;
>> + case PR_SET_IO_FLUSHER:
>> + if (!capable(CAP_SYS_RESOURCE))
>> + return -EPERM;
>> +
>> + if (arg3 || arg4 || arg5)
>> + return -EINVAL;
>> +
>> + if (arg2 == 1)
>> + current->flags |= PF_MEMALLOC_NOIO | PF_LESS_THROTTLE;
>> + else if (!arg2)
>> + current->flags &= ~(PF_MEMALLOC_NOIO | PF_LESS_THROTTLE);
>> + else
>> + return -EINVAL;
>> + break;
>> + case PR_GET_IO_FLUSHER:
>> + if (!capable(CAP_SYS_RESOURCE))
>> + return -EPERM;
>> +
>> + if (arg2 || arg3 || arg4 || arg5)
>> + return -EINVAL;
>> +
>> + if (current->flags & (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE))
>
> I think it needs to be conditioned on both flags instead of just one of
> them, for consistency with SET. Seems worth a define too, PF_IO_FLUSHER?
> Or something local to this file at least.
Yeah, that was a mistake. Will fix.
>
>> + error = 1;
>> + else
>> + error = 0;
>
> error = (current->flags & PF_IO_FLUSHER) == PF_IO_FLUSHER;
>
> Thanks,
>
> Ilya
>
