| Message ID | 20240129154750.1245317-1-dwysocha@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | NFS: Fix nfs_netfs_issue_read() xarray locking for writeback interrupt | expand |
Dave Wysochanski <dwysocha@redhat.com> wrote: > - xas_lock(&xas); > + xas_lock_irqsave(&xas, flags); > xas_for_each(&xas, page, last) { You probably want to use RCU, not xas_lock(). The pages are locked and so cannot be evicted from the xarray. David
On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@redhat.com> wrote: > > Dave Wysochanski <dwysocha@redhat.com> wrote: > > > - xas_lock(&xas); > > + xas_lock_irqsave(&xas, flags); > > xas_for_each(&xas, page, last) { > > You probably want to use RCU, not xas_lock(). The pages are locked and so > cannot be evicted from the xarray. > I tried RCU originally and ran into a problem because NFS can schedule (see comment on line 328 below) 326 xas_lock_irqsave(&xas, flags); 327 xas_for_each(&xas, page, last) { 328 /* nfs_read_add_folio() may schedule() due to pNFS layout and other RPCs */ 329 xas_pause(&xas); 330 xas_unlock_irqrestore(&xas, flags); 331 err = nfs_read_add_folio(&pgio, ctx, page_folio(page)); 332 if (err < 0) { 333 netfs->error = err; 334 goto out; 335 } 336 xas_lock_irqsave(&xas, flags); 337 } 338 xas_unlock_irqrestore(&xas, flags);
On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote: > On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@redhat.com> wrote: > > > > Dave Wysochanski <dwysocha@redhat.com> wrote: > > > > > - xas_lock(&xas); > > > + xas_lock_irqsave(&xas, flags); > > > xas_for_each(&xas, page, last) { > > > > You probably want to use RCU, not xas_lock(). The pages are locked and so > > cannot be evicted from the xarray. > > > > I tried RCU originally and ran into a problem because NFS can schedule > (see comment on line 328 below) > > 326 xas_lock_irqsave(&xas, flags); > 327 xas_for_each(&xas, page, last) { > 328 /* nfs_read_add_folio() may schedule() due to pNFS > layout and other RPCs */ > 329 xas_pause(&xas); > 330 xas_unlock_irqrestore(&xas, flags); > 331 err = nfs_read_add_folio(&pgio, ctx, page_folio(page)); > 332 if (err < 0) { > 333 netfs->error = err; > 334 goto out; > 335 } > 336 xas_lock_irqsave(&xas, flags); > 337 } > 338 xas_unlock_irqrestore(&xas, flags); > Looking at it more closely, I think you might want to just use xa_for_each_start(). That will do the traversal using the rcu_read_lock under the hood, and you should be able to block on every iteration.
On Mon, Jan 29, 2024 at 12:44 PM Jeff Layton <jlayton@kernel.org> wrote: > > On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote: > > On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@redhat.com> wrote: > > > > > > Dave Wysochanski <dwysocha@redhat.com> wrote: > > > > > > > - xas_lock(&xas); > > > > + xas_lock_irqsave(&xas, flags); > > > > xas_for_each(&xas, page, last) { > > > > > > You probably want to use RCU, not xas_lock(). The pages are locked and so > > > cannot be evicted from the xarray. > > > > > > > I tried RCU originally and ran into a problem because NFS can schedule > > (see comment on line 328 below) > > > > 326 xas_lock_irqsave(&xas, flags); > > 327 xas_for_each(&xas, page, last) { > > 328 /* nfs_read_add_folio() may schedule() due to pNFS > > layout and other RPCs */ > > 329 xas_pause(&xas); > > 330 xas_unlock_irqrestore(&xas, flags); > > 331 err = nfs_read_add_folio(&pgio, ctx, page_folio(page)); > > 332 if (err < 0) { > > 333 netfs->error = err; > > 334 goto out; > > 335 } > > 336 xas_lock_irqsave(&xas, flags); > > 337 } > > 338 xas_unlock_irqrestore(&xas, flags); > > > > Looking at it more closely, I think you might want to just use > xa_for_each_start(). That will do the traversal using the rcu_read_lock > under the hood, and you should be able to block on every iteration. > Thanks Jeff. Yes, I agree after looking at this further, this is a good approach, and much cleaner. I'll work on a v2 patch (actually with xa_for_each_range as you suggested off list) and send after a bit of testing -- so far, so good. FWIW, my original usage of RCU was outside the whole loop. I ran into problems due to nfs_read_add_folio().
On Tue, 2024-01-30 at 09:56 -0500, David Wysochanski wrote: > On Mon, Jan 29, 2024 at 12:44 PM Jeff Layton <jlayton@kernel.org> wrote: > > > > On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote: > > > On Mon, Jan 29, 2024 at 12:15 PM David Howells <dhowells@redhat.com> wrote: > > > > > > > > Dave Wysochanski <dwysocha@redhat.com> wrote: > > > > > > > > > - xas_lock(&xas); > > > > > + xas_lock_irqsave(&xas, flags); > > > > > xas_for_each(&xas, page, last) { > > > > > > > > You probably want to use RCU, not xas_lock(). The pages are locked and so > > > > cannot be evicted from the xarray. > > > > > > > > > > I tried RCU originally and ran into a problem because NFS can schedule > > > (see comment on line 328 below) > > > > > > 326 xas_lock_irqsave(&xas, flags); > > > 327 xas_for_each(&xas, page, last) { > > > 328 /* nfs_read_add_folio() may schedule() due to pNFS > > > layout and other RPCs */ > > > 329 xas_pause(&xas); > > > 330 xas_unlock_irqrestore(&xas, flags); > > > 331 err = nfs_read_add_folio(&pgio, ctx, page_folio(page)); > > > 332 if (err < 0) { > > > 333 netfs->error = err; > > > 334 goto out; > > > 335 } > > > 336 xas_lock_irqsave(&xas, flags); > > > 337 } > > > 338 xas_unlock_irqrestore(&xas, flags); > > > > > > > Looking at it more closely, I think you might want to just use > > xa_for_each_start(). That will do the traversal using the rcu_read_lock > > under the hood, and you should be able to block on every iteration. > > > Thanks Jeff. Yes, I agree after looking at this further, this is a > good approach, and much cleaner. I'll work on a v2 patch (actually > with xa_for_each_range as you suggested off list) and send after > a bit of testing -- so far, so good. > > FWIW, my original usage of RCU was outside the whole loop. > I ran into problems due to nfs_read_add_folio(). > Makes sense. In principle you could do this by just dropping and acquiring the rcu_read_lock in the same places you do the spinlock in the original patch, but using xa_for_each_range is much simpler.
diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c index b05717fe0d4e..de7ec89bfe8d 100644 --- a/fs/nfs/fscache.c +++ b/fs/nfs/fscache.c @@ -308,6 +308,7 @@ static void nfs_netfs_issue_read(struct netfs_io_subrequest *sreq) struct nfs_open_context *ctx = sreq->rreq->netfs_priv; struct page *page; int err; + unsigned long flags; pgoff_t start = (sreq->start + sreq->transferred) >> PAGE_SHIFT; pgoff_t last = ((sreq->start + sreq->len - sreq->transferred - 1) >> PAGE_SHIFT); @@ -322,19 +323,19 @@ static void nfs_netfs_issue_read(struct netfs_io_subrequest *sreq) pgio.pg_netfs = netfs; /* used in completion */ - xas_lock(&xas); + xas_lock_irqsave(&xas, flags); xas_for_each(&xas, page, last) { /* nfs_read_add_folio() may schedule() due to pNFS layout and other RPCs */ xas_pause(&xas); - xas_unlock(&xas); + xas_unlock_irqrestore(&xas, flags); err = nfs_read_add_folio(&pgio, ctx, page_folio(page)); if (err < 0) { netfs->error = err; goto out; } - xas_lock(&xas); + xas_lock_irqsave(&xas, flags); } - xas_unlock(&xas); + xas_unlock_irqrestore(&xas, flags); out: nfs_pageio_complete_read(&pgio); nfs_netfs_put(netfs);
The loop inside nfs_netfs_issue_read() currently does not disable interrupts while iterating through pages in the xarray to submit for NFS read. This is not safe though since after taking xa_lock, another page in the mapping could be processed for writeback inside an interrupt, and deadlock can occur. To fix this, use the irqsave/irqrestore primitives for the xa_lock. The problem is easily reproduced with the following test: mount -o vers=3,fsc 127.0.0.1:/export /mnt/nfs dd if=/dev/zero of=/mnt/nfs/file1.bin bs=4096 count=1 echo 3 > /proc/sys/vm/drop_caches dd if=/mnt/nfs/file1.bin of=/dev/null umount /mnt/nfs On the console with a lockdep-enabled kernel a message similar to the following will be seen: ================================ WARNING: inconsistent lock state 6.7.0-lockdbg+ #10 Not tainted -------------------------------- inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage. test5/1708 [HC0[0]:SC0[0]:HE1:SE1] takes: ffff888127baa598 (&xa->xa_lock#4){+.?.}-{3:3}, at: nfs_netfs_issue_read+0x1b2/0x4b0 [nfs] {IN-SOFTIRQ-W} state was registered at: lock_acquire+0x144/0x380 _raw_spin_lock_irqsave+0x4e/0xa0 __folio_end_writeback+0x17e/0x5c0 folio_end_writeback+0x93/0x1b0 iomap_finish_ioend+0xeb/0x6a0 blk_update_request+0x204/0x7f0 blk_mq_end_request+0x30/0x1c0 blk_complete_reqs+0x7e/0xa0 __do_softirq+0x113/0x544 __irq_exit_rcu+0xfe/0x120 irq_exit_rcu+0xe/0x20 sysvec_call_function_single+0x6f/0x90 asm_sysvec_call_function_single+0x1a/0x20 pv_native_safe_halt+0xf/0x20 default_idle+0x9/0x20 default_idle_call+0x67/0xa0 do_idle+0x2b5/0x300 cpu_startup_entry+0x34/0x40 start_secondary+0x19d/0x1c0 secondary_startup_64_no_verify+0x18f/0x19b irq event stamp: 176891 hardirqs last enabled at (176891): [<ffffffffa67a0be4>] _raw_spin_unlock_irqrestore+0x44/0x60 hardirqs last disabled at (176890): [<ffffffffa67a0899>] _raw_spin_lock_irqsave+0x79/0xa0 softirqs last enabled at (176646): [<ffffffffa515d91e>] __irq_exit_rcu+0xfe/0x120 softirqs last disabled at (176633): [<ffffffffa515d91e>] __irq_exit_rcu+0xfe/0x120 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(&xa->xa_lock#4); <Interrupt> lock(&xa->xa_lock#4); *** DEADLOCK *** 2 locks held by test5/1708: #0: ffff888127baa498 (&sb->s_type->i_mutex_key#22){++++}-{4:4}, at: nfs_start_io_read+0x28/0x90 [nfs] #1: ffff888127baa650 (mapping.invalidate_lock#3){.+.+}-{4:4}, at: page_cache_ra_unbounded+0xa4/0x280 stack backtrace: CPU: 6 PID: 1708 Comm: test5 Kdump: loaded Not tainted 6.7.0-lockdbg+ #10 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-1.fc39 04/01/2014 Call Trace: dump_stack_lvl+0x5b/0x90 mark_lock+0xb3f/0xd20 __lock_acquire+0x77b/0x3360 _raw_spin_lock+0x34/0x80 nfs_netfs_issue_read+0x1b2/0x4b0 [nfs] netfs_begin_read+0x77f/0x980 [netfs] nfs_netfs_readahead+0x45/0x60 [nfs] nfs_readahead+0x323/0x5a0 [nfs] read_pages+0xf3/0x5c0 page_cache_ra_unbounded+0x1c8/0x280 filemap_get_pages+0x38c/0xae0 filemap_read+0x206/0x5e0 nfs_file_read+0xb7/0x140 [nfs] vfs_read+0x2a9/0x460 ksys_read+0xb7/0x140 Fixes: 000dbe0bec05 ("NFS: Convert buffered read paths to use netfs when fscache is enabled") Reviewed-by: Jeff Layton <jlayton@redhat.com> Signed-off-by: Dave Wysochanski <dwysocha@redhat.com> --- fs/nfs/fscache.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)