[Cc linux-api]
On Fri 08-11-19 12:53:19, Mike Christie 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>
OK, this looks much better then explicitly exposing NOIO/NOFS to the
userspace. Restricting to CAP_SYS_RESOURCE makes sense as well but I am
not really an expert in that direction.
So for the proposed semantic this makes sense
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Please do not forget to add a man page update.
Keeping the rest of the email for the new CC
> ---
> 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))
> + error = 1;
> + else
> + error = 0;
> + break;
> default:
> error = -EINVAL;
> break;
> --
> 2.20.1
>
--
Michal Hocko
SUSE Labs
