diff mbox series

[v2] filemap: Move prefaulting out of hot write path

Message ID 20250228203722.CAEB63AC@davehans-spike.ostc.intel.com (mailing list archive)
State New
Headers show
Series [v2] filemap: Move prefaulting out of hot write path | expand

Commit Message

Dave Hansen Feb. 28, 2025, 8:37 p.m. UTC 
There is a generic anti-pattern that shows up in the VFS and several
filesystems where the hot write paths touch userspace twice when they
could get away with doing it once.

Dave Chinner suggested that they should all be fixed up[1]. I agree[2].
But, the series to do that fixup spans a bunch of filesystems and a lot
of people. This patch fixes common code that absolutely everyone uses.
It has measurable performance benefits[3].

I think this patch can go in and not be held up by the others.

I will post them separately to their separate maintainers for
consideration. But, honestly, I'm not going to lose any sleep if
the maintainers don't pick those up.

1. https://lore.kernel.org/all/Z5f-x278Z3wTIugL@dread.disaster.area/
2. https://lore.kernel.org/all/20250129181749.C229F6F3@davehans-spike.ostc.intel.com/
3. https://lore.kernel.org/all/202502121529.d62a409e-lkp@intel.com/

--

Changesfrom v1:
 * Update comment to talk more about how filesystem locking and
   atomic usercopies avoid deadlocks.

--

From: Dave Hansen <dave.hansen@linux.intel.com>

There is a bit of a sordid history here. I originally wrote
998ef75ddb57 ("fs: do not prefault sys_write() user buffer pages")
to fix a performance issue that showed up on early SMAP hardware.
But that was reverted with 00a3d660cbac because it exposed an
underlying filesystem bug.

This is a reimplementation of the original commit along with some
simplification and comment improvements.

The basic problem is that the generic write path has two userspace
accesses: one to prefault the write source buffer and then another to
perform the actual write. On x86, this means an extra STAC/CLAC pair.
These are relatively expensive instructions because they function as
barriers.

Keep the prefaulting behavior but move it into the slow path that gets
run when the write did not make any progress. This avoids livelocks
that can happen when the write's source and destination target the
same folio. Contrary to the existing comments, the fault-in does not
prevent deadlocks. That's accomplished by using an "atomic" usercopy
that disables page faults.

The end result is that the generic write fast path now touches
userspace once instead of twice.

0day has shown some improvements on a couple of microbenchmarks:

	https://lore.kernel.org/all/202502121529.d62a409e-lkp@intel.com/

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/yxyuijjfd6yknryji2q64j3keq2ygw6ca6fs5jwyolklzvo45s@4u63qqqyosy2/
Cc: Ted Ts'o <tytso@mit.edu>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>
---

 b/mm/filemap.c |   27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

Comments

Andrew Morton March 4, 2025, 2:37 a.m. UTC | #1 
On Fri, 28 Feb 2025 12:37:22 -0800 Dave Hansen <dave.hansen@linux.intel.com> wrote:

> There is a generic anti-pattern that shows up in the VFS and several
> filesystems where the hot write paths touch userspace twice when they
> could get away with doing it once.
> 
> Dave Chinner suggested that they should all be fixed up[1]. I agree[2].
> But, the series to do that fixup spans a bunch of filesystems and a lot
> of people. This patch fixes common code that absolutely everyone uses.
> It has measurable performance benefits[3].
> 
> I think this patch can go in and not be held up by the others.

I'll queue this in mm-hotfixes with a view to sneaking it into 6.14-rcX
as a basis for others to work against.
kernel test robot March 10, 2025, 8:45 a.m. UTC | #2 
Hello,

kernel test robot noticed a 3.6% improvement of will-it-scale.per_thread_ops on:


commit: 391ab5826c820c58d180534a7a727ff5668d4d61 ("[PATCH] [v2] filemap: Move prefaulting out of hot write path")
url: https://github.com/intel-lab-lkp/linux/commits/Dave-Hansen/filemap-Move-prefaulting-out-of-hot-write-path/20250301-043921
base: https://git.kernel.org/cgit/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/all/20250228203722.CAEB63AC@davehans-spike.ostc.intel.com/
patch subject: [PATCH] [v2] filemap: Move prefaulting out of hot write path

testcase: will-it-scale
config: x86_64-rhel-9.4
compiler: gcc-12
test machine: 104 threads 2 sockets (Skylake) with 192G memory
parameters:

	nr_task: 100%
	mode: thread
	test: pwrite1
	cpufreq_governor: performance






Details are as below:
-------------------------------------------------------------------------------------------------->


The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20250310/202503101621.e0858506-lkp@intel.com

=========================================================================================
compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
  gcc-12/performance/x86_64-rhel-9.4/thread/100%/debian-12-x86_64-20240206.cgz/lkp-skl-fpga01/pwrite1/will-it-scale

commit: 
  3dec9c0e67 ("foo")
  391ab5826c ("filemap: Move prefaulting out of hot write path")

3dec9c0e67aaf496 391ab5826c820c58d180534a7a7 
---------------- --------------------------- 
         %stddev     %change         %stddev
             \          |                \  
    182266 ±  3%      +9.4%     199333 ±  4%  meminfo.DirectMap4k
    765.67 ±  8%     -22.1%     596.83 ±  9%  perf-sched.wait_and_delay.count.__cond_resched.shmem_get_folio_gfp.shmem_write_begin.generic_perform_write.shmem_file_write_iter
     17510 ±  6%     +19.3%      20889 ±  8%  sched_debug.cpu.nr_switches.max
      3219 ±  5%     +11.2%       3578 ±  2%  sched_debug.cpu.nr_switches.stddev
  54561715            +3.6%   56543708        will-it-scale.104.threads
    524631            +3.6%     543689        will-it-scale.per_thread_ops
  54561715            +3.6%   56543708        will-it-scale.workload
 1.752e+10            -1.2%  1.731e+10        perf-stat.i.branch-instructions
      1.59            +0.0        1.63        perf-stat.i.branch-miss-rate%
      3.25            +1.8%       3.31        perf-stat.i.cpi
 8.828e+10            -1.5%  8.699e+10        perf-stat.i.instructions
      0.31            -1.8%       0.30        perf-stat.i.ipc
      1.58            +0.0        1.62        perf-stat.overall.branch-miss-rate%
      3.25            +1.8%       3.31        perf-stat.overall.cpi
      0.31            -1.7%       0.30        perf-stat.overall.ipc
    487316            -4.8%     464012        perf-stat.overall.path-length
 1.746e+10            -1.2%  1.725e+10        perf-stat.ps.branch-instructions
 8.798e+10            -1.5%   8.67e+10        perf-stat.ps.instructions
 2.659e+13            -1.3%  2.624e+13        perf-stat.total.instructions
     34.45            -5.9       28.57        perf-profile.calltrace.cycles-pp.generic_perform_write.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64.do_syscall_64
     48.17            -5.3       42.87 ±  2%  perf-profile.calltrace.cycles-pp.vfs_write.__x64_sys_pwrite64.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
     42.56            -5.3       37.29 ±  2%  perf-profile.calltrace.cycles-pp.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64.do_syscall_64.entry_SYSCALL_64_after_hwframe
     51.38            -4.9       46.46        perf-profile.calltrace.cycles-pp.__x64_sys_pwrite64.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
     54.26            -4.5       49.76        perf-profile.calltrace.cycles-pp.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
     62.52            -3.9       58.65        perf-profile.calltrace.cycles-pp.entry_SYSCALL_64_after_hwframe.__libc_pwrite
     13.30 ±  2%      -2.3       10.96        perf-profile.calltrace.cycles-pp.copy_page_from_iter_atomic.generic_perform_write.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64
     10.17 ±  2%      -2.0        8.18        perf-profile.calltrace.cycles-pp.rep_movs_alternative.copy_page_from_iter_atomic.generic_perform_write.shmem_file_write_iter.vfs_write
      0.59 ±  4%      -0.3        0.26 ±100%  perf-profile.calltrace.cycles-pp.__cond_resched.shmem_get_folio_gfp.shmem_write_begin.generic_perform_write.shmem_file_write_iter
      0.88 ±  3%      -0.1        0.74 ±  4%  perf-profile.calltrace.cycles-pp.folio_mark_accessed.shmem_get_folio_gfp.shmem_write_begin.generic_perform_write.shmem_file_write_iter
      0.67 ±  2%      -0.1        0.56        perf-profile.calltrace.cycles-pp.balance_dirty_pages_ratelimited_flags.generic_perform_write.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64
     99.51            +0.1       99.56        perf-profile.calltrace.cycles-pp.__libc_pwrite
      0.64 ±  5%      +0.1        0.71 ±  2%  perf-profile.calltrace.cycles-pp.fput.__x64_sys_pwrite64.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
      0.93 ±  2%      +0.1        1.02        perf-profile.calltrace.cycles-pp.folio_unlock.shmem_write_end.generic_perform_write.shmem_file_write_iter.vfs_write
      0.72            +0.1        0.82 ±  2%  perf-profile.calltrace.cycles-pp.ktime_get_coarse_real_ts64_mg.current_time.inode_needs_update_time.file_update_time.shmem_file_write_iter
      1.20            +0.2        1.38 ±  2%  perf-profile.calltrace.cycles-pp.syscall_exit_to_user_mode.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
      0.84 ±  2%      +0.2        1.04 ±  2%  perf-profile.calltrace.cycles-pp.noop_dirty_folio.shmem_write_end.generic_perform_write.shmem_file_write_iter.vfs_write
      1.48            +0.2        1.72 ±  2%  perf-profile.calltrace.cycles-pp.current_time.inode_needs_update_time.file_update_time.shmem_file_write_iter.vfs_write
      1.00 ±  2%      +0.3        1.26 ±  3%  perf-profile.calltrace.cycles-pp.folio_mark_dirty.shmem_write_end.generic_perform_write.shmem_file_write_iter.vfs_write
      1.99 ±  3%      +0.3        2.27 ±  4%  perf-profile.calltrace.cycles-pp.fdget.__x64_sys_pwrite64.do_syscall_64.entry_SYSCALL_64_after_hwframe.__libc_pwrite
      6.69            +0.3        7.02        perf-profile.calltrace.cycles-pp.entry_SYSCALL_64.__libc_pwrite
      2.24            +0.4        2.60 ±  3%  perf-profile.calltrace.cycles-pp.inode_needs_update_time.file_update_time.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64
      2.78 ±  2%      +0.4        3.21 ±  2%  perf-profile.calltrace.cycles-pp.file_update_time.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64.do_syscall_64
      4.34            +0.6        4.92        perf-profile.calltrace.cycles-pp.shmem_write_end.generic_perform_write.shmem_file_write_iter.vfs_write.__x64_sys_pwrite64
     12.01            +0.9       12.92        perf-profile.calltrace.cycles-pp.entry_SYSRETQ_unsafe_stack.__libc_pwrite
      2.89 ±  6%      +1.4        4.26 ±  6%  perf-profile.calltrace.cycles-pp.entry_SYSCALL_64_safe_stack.__libc_pwrite
     15.09            +1.5       16.61        perf-profile.calltrace.cycles-pp.syscall_return_via_sysret.__libc_pwrite
     34.58            -5.9       28.70        perf-profile.children.cycles-pp.generic_perform_write
     48.24            -5.3       42.94 ±  2%  perf-profile.children.cycles-pp.vfs_write
     42.95            -5.3       37.66 ±  2%  perf-profile.children.cycles-pp.shmem_file_write_iter
     51.54            -4.9       46.63        perf-profile.children.cycles-pp.__x64_sys_pwrite64
     54.37            -4.5       49.86        perf-profile.children.cycles-pp.do_syscall_64
     62.76            -3.9       58.90        perf-profile.children.cycles-pp.entry_SYSCALL_64_after_hwframe
     10.62 ±  2%      -2.3        8.30        perf-profile.children.cycles-pp.rep_movs_alternative
     13.47 ±  2%      -2.3       11.16        perf-profile.children.cycles-pp.copy_page_from_iter_atomic
      0.90 ±  3%      -0.1        0.76 ±  4%  perf-profile.children.cycles-pp.folio_mark_accessed
      0.69 ±  2%      -0.1        0.60        perf-profile.children.cycles-pp.balance_dirty_pages_ratelimited_flags
      0.29            -0.0        0.26 ±  2%  perf-profile.children.cycles-pp.testcase
      0.31 ±  3%      -0.0        0.29 ±  2%  perf-profile.children.cycles-pp.update_process_times
      0.50            -0.0        0.48        perf-profile.children.cycles-pp.rcu_all_qs
     99.67            +0.0       99.70        perf-profile.children.cycles-pp.__libc_pwrite
      0.64 ±  5%      +0.1        0.71 ±  2%  perf-profile.children.cycles-pp.fput
      0.43 ±  3%      +0.1        0.50 ±  2%  perf-profile.children.cycles-pp.folio_mapping
      0.94 ±  2%      +0.1        1.02        perf-profile.children.cycles-pp.folio_unlock
      0.74 ±  2%      +0.1        0.85 ±  2%  perf-profile.children.cycles-pp.ktime_get_coarse_real_ts64_mg
      1.23            +0.2        1.41 ±  2%  perf-profile.children.cycles-pp.syscall_exit_to_user_mode
      0.89            +0.2        1.10 ±  2%  perf-profile.children.cycles-pp.noop_dirty_folio
      1.54            +0.2        1.79 ±  2%  perf-profile.children.cycles-pp.current_time
      1.99 ±  3%      +0.3        2.27 ±  4%  perf-profile.children.cycles-pp.fdget
      1.08 ±  3%      +0.3        1.36 ±  3%  perf-profile.children.cycles-pp.folio_mark_dirty
      2.30            +0.4        2.66 ±  3%  perf-profile.children.cycles-pp.inode_needs_update_time
      2.86 ±  2%      +0.4        3.30 ±  2%  perf-profile.children.cycles-pp.file_update_time
      4.58            +0.6        5.17        perf-profile.children.cycles-pp.shmem_write_end
      1.73 ±  5%      +0.7        2.43 ±  5%  perf-profile.children.cycles-pp.entry_SYSCALL_64_safe_stack
     12.88            +1.0       13.84        perf-profile.children.cycles-pp.entry_SYSRETQ_unsafe_stack
      6.99            +1.0        7.95        perf-profile.children.cycles-pp.entry_SYSCALL_64
     15.22            +1.5       16.75        perf-profile.children.cycles-pp.syscall_return_via_sysret
     10.43 ±  2%      -2.3        8.08        perf-profile.self.cycles-pp.rep_movs_alternative
      3.26            -0.4        2.86        perf-profile.self.cycles-pp.generic_perform_write
      2.02            -0.2        1.78 ±  2%  perf-profile.self.cycles-pp.shmem_get_folio_gfp
      0.87 ±  3%      -0.1        0.74 ±  4%  perf-profile.self.cycles-pp.folio_mark_accessed
      0.53 ±  2%      -0.1        0.43 ±  2%  perf-profile.self.cycles-pp.balance_dirty_pages_ratelimited_flags
      0.25 ±  2%      -0.0        0.22 ±  2%  perf-profile.self.cycles-pp.testcase
      0.54            +0.0        0.59        perf-profile.self.cycles-pp.entry_SYSCALL_64_safe_stack
      0.79 ±  4%      +0.0        0.84 ±  2%  perf-profile.self.cycles-pp.__x64_sys_pwrite64
      0.51 ±  2%      +0.1        0.58 ±  2%  perf-profile.self.cycles-pp.fput
      0.38 ±  3%      +0.1        0.45 ±  2%  perf-profile.self.cycles-pp.folio_mapping
      0.74 ±  2%      +0.1        0.82        perf-profile.self.cycles-pp.folio_unlock
      0.73 ±  4%      +0.1        0.82 ±  4%  perf-profile.self.cycles-pp.inode_needs_update_time
      0.54 ±  6%      +0.1        0.64 ±  3%  perf-profile.self.cycles-pp.file_update_time
      0.72 ±  2%      +0.1        0.82 ±  2%  perf-profile.self.cycles-pp.ktime_get_coarse_real_ts64_mg
      0.79 ±  2%      +0.1        0.94 ±  3%  perf-profile.self.cycles-pp.current_time
      0.98 ±  2%      +0.2        1.14 ±  2%  perf-profile.self.cycles-pp.syscall_exit_to_user_mode
      0.85            +0.2        1.04 ±  2%  perf-profile.self.cycles-pp.noop_dirty_folio
      0.65 ±  3%      +0.2        0.85 ±  4%  perf-profile.self.cycles-pp.folio_mark_dirty
      1.11 ±  6%      +0.2        1.32 ±  2%  perf-profile.self.cycles-pp.do_syscall_64
      1.98 ±  3%      +0.3        2.26 ±  4%  perf-profile.self.cycles-pp.fdget
      2.14 ±  2%      +0.4        2.55 ±  3%  perf-profile.self.cycles-pp.__libc_pwrite
      1.63            +0.6        2.23 ±  3%  perf-profile.self.cycles-pp.shmem_write_begin
      8.54            +0.7        9.20        perf-profile.self.cycles-pp.entry_SYSCALL_64_after_hwframe
      6.09            +0.9        7.03 ±  2%  perf-profile.self.cycles-pp.entry_SYSCALL_64
     12.75            +1.0       13.70        perf-profile.self.cycles-pp.entry_SYSRETQ_unsafe_stack
     15.20            +1.5       16.72        perf-profile.self.cycles-pp.syscall_return_via_sysret




Disclaimer:
Results have been estimated based on internal Intel analysis and are provided
for informational purposes only. Any difference in system hardware or software
design or configuration may affect actual performance.
diff mbox series

Patch

diff -puN mm/filemap.c~generic_perform_write-1 mm/filemap.c
--- a/mm/filemap.c~generic_perform_write-1	2025-02-28 11:58:36.499615962 -0800
+++ b/mm/filemap.c	2025-02-28 12:13:27.845549365 -0800
@@ -4170,17 +4170,6 @@  retry:
 		bytes = min(chunk - offset, bytes);
 		balance_dirty_pages_ratelimited(mapping);
 
-		/*
-		 * Bring in the user page that we will copy from _first_.
-		 * Otherwise there's a nasty deadlock on copying from the
-		 * same page as we're writing to, without it being marked
-		 * up-to-date.
-		 */
-		if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
-			status = -EFAULT;
-			break;
-		}
-
 		if (fatal_signal_pending(current)) {
 			status = -EINTR;
 			break;
@@ -4198,6 +4187,12 @@  retry:
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_folio(folio);
 
+		/*
+		 * Faults here on mmap()s can recurse into arbitrary
+		 * filesystem code. Lots of locks are held that can
+		 * deadlock. Use an atomic copy to avoid deadlocking
+		 * in page fault handling.
+		 */
 		copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
 		flush_dcache_folio(folio);
 
@@ -4223,6 +4218,16 @@  retry:
 				bytes = copied;
 				goto retry;
 			}
+
+			/*
+			 * 'folio' is now unlocked and faults on it can be
+			 * handled. Ensure forward progress by trying to
+			 * fault it in now.
+			 */
+			if (fault_in_iov_iter_readable(i, bytes) == bytes) {
+				status = -EFAULT;
+				break;
+			}
 		} else {
 			pos += status;
 			written += status;