[PATCH v11 18/20] nfs/localio: use dedicated workqueues for filesystem read and write

Mike Snitzer
2024-07-02T16:28:56Z

From: Trond Myklebust <trond.myklebust@hammerspace.com>

For localio access, don't call filesystem read() and write() routines
directly. This solves two problems:

1) localio writes need to use a normal (non-memreclaim) unbound
workqueue. This avoids imposing new requirements on how underlying
filesystems process frontend IO, which would cause a large amount
of work to address all filesystem. Without this change, when XFS
starts getting low on space, XFS flushes work on a non-memreclaim
work queue, which causes a priority inversion problem:

00573 workqueue: WQ_MEM_RECLAIM writeback:wb_workfn is flushing !WQ_MEM_RECLAIM xfs-sync/vdc:xfs_flush_inodes_worker
00573 WARNING: CPU: 6 PID: 8525 at kernel/workqueue.c:3706 check_flush_dependency+0x2a4/0x328
00573 Modules linked in:
00573 CPU: 6 PID: 8525 Comm: kworker/u71:5 Not tainted 6.10.0-rc3-ktest-00032-g2b0a133403ab #18502
00573 Hardware name: linux,dummy-virt (DT)
00573 Workqueue: writeback wb_workfn (flush-0:33)
00573 pstate: 400010c5 (nZcv daIF -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
00573 pc : check_flush_dependency+0x2a4/0x328
00573 lr : check_flush_dependency+0x2a4/0x328
00573 sp : ffff0000c5f06bb0
00573 x29: ffff0000c5f06bb0 x28: ffff0000c998a908 x27: 1fffe00019331521
00573 x26: ffff0000d0620900 x25: ffff0000c5f06ca0 x24: ffff8000828848c0
00573 x23: 1fffe00018be0d8e x22: ffff0000c1210000 x21: ffff0000c75fde00
00573 x20: ffff800080bfd258 x19: ffff0000cad63400 x18: ffff0000cd3a4810
00573 x17: 0000000000000000 x16: 0000000000000000 x15: ffff800080508d98
00573 x14: 0000000000000000 x13: 204d49414c434552 x12: 1fffe0001b6eeab2
00573 x11: ffff60001b6eeab2 x10: dfff800000000000 x9 : ffff60001b6eeab3
00573 x8 : 0000000000000001 x7 : 00009fffe491154e x6 : ffff0000db775593
00573 x5 : ffff0000db775590 x4 : ffff0000db775590 x3 : 0000000000000000
00573 x2 : 0000000000000027 x1 : ffff600018be0d62 x0 : dfff800000000000
00573 Call trace:
00573 check_flush_dependency+0x2a4/0x328
00573 __flush_work+0x184/0x5c8
00573 flush_work+0x18/0x28
00573 xfs_flush_inodes+0x68/0x88
00573 xfs_file_buffered_write+0x128/0x6f0
00573 xfs_file_write_iter+0x358/0x448
00573 nfs_local_doio+0x854/0x1568
00573 nfs_initiate_pgio+0x214/0x418
00573 nfs_generic_pg_pgios+0x304/0x480
00573 nfs_pageio_doio+0xe8/0x240
00573 nfs_pageio_complete+0x160/0x480
00573 nfs_writepages+0x300/0x4f0
00573 do_writepages+0x12c/0x4a0
00573 __writeback_single_inode+0xd4/0xa68
00573 writeback_sb_inodes+0x470/0xcb0
00573 __writeback_inodes_wb+0xb0/0x1d0
00573 wb_writeback+0x594/0x808
00573 wb_workfn+0x5e8/0x9e0
00573 process_scheduled_works+0x53c/0xd90
00573 worker_thread+0x370/0x8c8
00573 kthread+0x258/0x2e8
00573 ret_from_fork+0x10/0x20

2) Some filesystem writeback routines can end up taking up a lot of
stack space (particularly XFS). Instead of risking running over
due to the extra overhead from the NFS stack, we should just call
these routines from a workqueue job. Since we need to do this to
address 1) above we're able to avoid possibly blowing the stack
"for free".

Use of dedicated workqueues improves performance over using the
system_unbound_wq.

Also, the creds of the client task are used to override_creds() in
both nfs_local_call_read() and nfs_local_call_write() -- otherwise the
workqueue could have elevated capabilities (which caller may not).

Lastly, care is taken to set PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO in
both nfs_do_local_read() and nfs_do_local_write().

Dave Chinner detailed the need for these flags with:
"PF_LOCAL_THROTTLE prevents deadlocks in balance_dirty_pages() by
lifting the dirty ratio for this thread a little, hence giving it
priority over the upper filesystem. i.e. the upper filesystem will
throttle incoming writes first, then the back end IO submission
thread can still submit new front end IOs to the lower filesystem
and they won't block in balance_dirty_pages() because the lower
filesystem has a higher limit. hence the lower filesystem can always
drain the dirty pages on the upper filesystem, and the system won't
deadlock in balance_dirty_pages().

The PF_MEMALLOC_NOIO flag prevents the lower filesystem IO from
causing memory reclaim to re-enter filesystems or IO devices and so
prevents deadlocks from occuring where IO that cleans pages is
waiting on IO to complete."

Message-Id: <ZoHuXHMEuMrem73H@dread.disaster.area>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
fs/nfs/inode.c | 57 ++++++++++++++++---------
fs/nfs/internal.h | 1 +
fs/nfs/localio.c | 103 +++++++++++++++++++++++++++++++++++-----------
3 files changed, 119 insertions(+), 42 deletions(-)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index f9923cbf6058..aac8c5302503 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -2394,35 +2394,54 @@ static void nfs_destroy_inodecache(void)
kmem_cache_destroy(nfs_inode_cachep);
}

+struct workqueue_struct *nfslocaliod_workqueue;
struct workqueue_struct *nfsiod_workqueue;
EXPORT_SYMBOL_GPL(nfsiod_workqueue);

/*
- * start up the nfsiod workqueue
- */
-static int nfsiod_start(void)
-{
- struct workqueue_struct *wq;
- dprintk("RPC: creating workqueue nfsiod\n");
- wq = alloc_workqueue("nfsiod", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
- if (wq == NULL)
- return -ENOMEM;
- nfsiod_workqueue = wq;
- return 0;
-}
-
-/*
- * Destroy the nfsiod workqueue
+ * Destroy the nfsiod workqueues
*/
static void nfsiod_stop(void)
{
struct workqueue_struct *wq;

wq = nfsiod_workqueue;
- if (wq == NULL)
- return;
- nfsiod_workqueue = NULL;
- destroy_workqueue(wq);
+ if (wq != NULL) {
+ nfsiod_workqueue = NULL;
+ destroy_workqueue(wq);
+ }
+#if IS_ENABLED(CONFIG_NFS_LOCALIO)
+ wq = nfslocaliod_workqueue;
+ if (wq != NULL) {
+ nfslocaliod_workqueue = NULL;
+ destroy_workqueue(wq);
+ }
+#endif /* CONFIG_NFS_LOCALIO */
+}
+
+/*
+ * Start the nfsiod workqueues
+ */
+static int nfsiod_start(void)
+{
+ dprintk("RPC: creating workqueue nfsiod\n");
+ nfsiod_workqueue = alloc_workqueue("nfsiod", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
+ if (nfsiod_workqueue == NULL)
+ return -ENOMEM;
+#if IS_ENABLED(CONFIG_NFS_LOCALIO)
+ /*
+ * localio writes need to use a normal (non-memreclaim) workqueue.
+ * When we start getting low on space, XFS goes and calls flush_work() on
+ * a non-memreclaim work queue, which causes a priority inversion problem.
+ */
+ dprintk("RPC: creating workqueue nfslocaliod\n");
+ nfslocaliod_workqueue = alloc_workqueue("nfslocaliod", WQ_UNBOUND, 0);
+ if (unlikely(nfslocaliod_workqueue == NULL)) {
+ nfsiod_stop();
+ return -ENOMEM;
+ }
+#endif /* CONFIG_NFS_LOCALIO */
+ return 0;
}

unsigned int nfs_net_id;
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index d352040e3232..9251a357d097 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -440,6 +440,7 @@ int nfs_check_flags(int);

/* inode.c */
extern struct workqueue_struct *nfsiod_workqueue;
+extern struct workqueue_struct *nfslocaliod_workqueue;
extern struct inode *nfs_alloc_inode(struct super_block *sb);
extern void nfs_free_inode(struct inode *);
extern int nfs_write_inode(struct inode *, struct writeback_control *);
diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index efa01d732206..7039a181ff89 100644
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -44,6 +44,13 @@ struct nfs_local_fsync_ctx {
};
static void nfs_local_fsync_work(struct work_struct *work);

+struct nfs_local_io_args {
+ struct nfs_local_kiocb *iocb;
+ const struct cred *cred;
+ struct work_struct work;
+ struct completion *done;
+};
+
/*
* We need to translate between nfs status return values and
* the local errno values which may not be the same.
@@ -301,30 +308,55 @@ nfs_local_read_done(struct nfs_local_kiocb *iocb, long status)
status > 0 ? status : 0, hdr->res.eof);
}

-static int
-nfs_do_local_read(struct nfs_pgio_header *hdr, struct file *filp,
- const struct rpc_call_ops *call_ops)
+static void nfs_local_call_read(struct work_struct *work)
{
- struct nfs_local_kiocb *iocb;
+ struct nfs_local_io_args *args =
+ container_of(work, struct nfs_local_io_args, work);
+ struct nfs_local_kiocb *iocb = args->iocb;
+ struct file *filp = iocb->kiocb.ki_filp;
+ const struct cred *save_cred;
struct iov_iter iter;
ssize_t status;

+ current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
+ save_cred = override_creds(args->cred);
+
+ nfs_local_iter_init(&iter, iocb, READ);
+
+ status = filp->f_op->read_iter(&iocb->kiocb, &iter);
+ WARN_ON_ONCE(status == -EIOCBQUEUED);
+
+ nfs_local_read_done(iocb, status);
+ nfs_local_pgio_release(iocb);
+
+ revert_creds(save_cred);
+ complete(args->done);
+}
+
+static int nfs_do_local_read(struct nfs_pgio_header *hdr, struct file *filp,
+ const struct rpc_call_ops *call_ops)
+{
+ struct nfs_local_io_args args;
+ DECLARE_COMPLETION_ONSTACK(done);
+ struct nfs_local_kiocb *iocb;
+
dprintk("%s: vfs_read count=%u pos=%llu\n",
__func__, hdr->args.count, hdr->args.offset);

iocb = nfs_local_iocb_alloc(hdr, filp, GFP_KERNEL);
if (iocb == NULL)
return -ENOMEM;
- nfs_local_iter_init(&iter, iocb, READ);

nfs_local_pgio_init(hdr, call_ops);
hdr->res.eof = false;

- status = filp->f_op->read_iter(&iocb->kiocb, &iter);
- WARN_ON_ONCE(status == -EIOCBQUEUED);
-
- nfs_local_read_done(iocb, status);
- nfs_local_pgio_release(iocb);
+ args.iocb = iocb;
+ args.done = &done;
+ args.cred = current_cred();
+ INIT_WORK_ONSTACK(&args.work, nfs_local_call_read);
+ queue_work(nfslocaliod_workqueue, &args.work);
+ wait_for_completion(&done);
+ destroy_work_on_stack(&args.work);

return 0;
}
@@ -456,14 +488,41 @@ nfs_local_write_done(struct nfs_local_kiocb *iocb, long status)
nfs_local_pgio_done(hdr, status);
}

-static int
-nfs_do_local_write(struct nfs_pgio_header *hdr, struct file *filp,
- const struct rpc_call_ops *call_ops)
+static void nfs_local_call_write(struct work_struct *work)
{
- struct nfs_local_kiocb *iocb;
+ struct nfs_local_io_args *args =
+ container_of(work, struct nfs_local_io_args, work);
+ struct nfs_local_kiocb *iocb = args->iocb;
+ struct file *filp = iocb->kiocb.ki_filp;
+ const struct cred *save_cred;
struct iov_iter iter;
ssize_t status;

+ current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
+ save_cred = override_creds(args->cred);
+
+ nfs_local_iter_init(&iter, iocb, WRITE);
+
+ file_start_write(filp);
+ status = filp->f_op->write_iter(&iocb->kiocb, &iter);
+ file_end_write(filp);
+ WARN_ON_ONCE(status == -EIOCBQUEUED);
+
+ nfs_local_write_done(iocb, status);
+ nfs_local_vfs_getattr(iocb);
+ nfs_local_pgio_release(iocb);
+
+ revert_creds(save_cred);
+ complete(args->done);
+}
+
+static int nfs_do_local_write(struct nfs_pgio_header *hdr, struct file *filp,
+ const struct rpc_call_ops *call_ops)
+{
+ struct nfs_local_io_args args;
+ DECLARE_COMPLETION_ONSTACK(done);
+ struct nfs_local_kiocb *iocb;
+
dprintk("%s: vfs_write count=%u pos=%llu %s\n",
__func__, hdr->args.count, hdr->args.offset,
(hdr->args.stable == NFS_UNSTABLE) ? "unstable" : "stable");
@@ -471,7 +530,6 @@ nfs_do_local_write(struct nfs_pgio_header *hdr, struct file *filp,
iocb = nfs_local_iocb_alloc(hdr, filp, GFP_NOIO);
if (iocb == NULL)
return -ENOMEM;
- nfs_local_iter_init(&iter, iocb, WRITE);

switch (hdr->args.stable) {
default:
@@ -486,14 +544,13 @@ nfs_do_local_write(struct nfs_pgio_header *hdr, struct file *filp,

nfs_set_local_verifier(hdr->inode, hdr->res.verf, hdr->args.stable);

- file_start_write(filp);
- status = filp->f_op->write_iter(&iocb->kiocb, &iter);
- file_end_write(filp);
- WARN_ON_ONCE(status == -EIOCBQUEUED);
-
- nfs_local_write_done(iocb, status);
- nfs_local_vfs_getattr(iocb);
- nfs_local_pgio_release(iocb);
+ args.iocb = iocb;
+ args.done = &done;
+ args.cred = current_cred();
+ INIT_WORK_ONSTACK(&args.work, nfs_local_call_write);
+ queue_work(nfslocaliod_workqueue, &args.work);
+ wait_for_completion(&done);
+ destroy_work_on_stack(&args.work);

return 0;
}
--
2.44.0