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Subject: [RFC PATCH 00/10] pipe: Notification queue preparation [ver #2]
From:   David Howells <dhowells@redhat.com>
To:     torvalds@linux-foundation.org
Cc:     dhowells@redhat.com, Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        nicolas.dichtel@6wind.com, raven@themaw.net,
        Christian Brauner <christian@brauner.io>, dhowells@redhat.com,
        keyrings@vger.kernel.org, linux-usb@vger.kernel.org,
        linux-block@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Wed, 23 Oct 2019 21:17:04 +0100
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Here's a set of preparatory patches for building a general notification
queue on top of pipes.  It makes a number of significant changes:

 (1) It removes the nr_exclusive argument from __wake_up_sync_key() as this
     is always 1.  This prepares for step 2.

 (2) Adds wake_up_interruptible_sync_poll_locked() so that poll can be
     woken up from a function that's holding the poll waitqueue spinlock.

 (3) Change the pipe buffer ring to be managed in terms of unbounded head
     and tail indices rather than bounded index and length.  This means
     that reading the pipe only needs to modify one index, not two.

 (4) A selection of helper functions are provided to query the state of the
     pipe buffer, plus a couple to apply updates to the pipe indices.

 (5) The pipe ring is allowed to have kernel-reserved slots.  This allows
     many notification messages to be spliced in by the kernel without
     allowing userspace to pin too many pages if it writes to the same
     pipe.

 (6) Advance the head and tail indices inside the pipe waitqueue lock and
     use step 2 to poke poll without having to take the lock twice.

 (7) Rearrange pipe_write() to preallocate the buffer it is going to write
     into and then drop the spinlock.  This allows kernel notifications to
     then be added the ring whilst it is filling the buffer it allocated.
     The read side is stalled because the pipe mutex is still held.

 (8) Don't wake up readers on a pipe if there was already data in it when
     we added more.

 (9) Don't wake up writers on a pipe if the ring wasn't full before we
     removed a buffer.

The patches can be found here also:

=09http://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/log/?h=
=3Dpipe-experimental

PATCHES=09BENCHMARK=09BEST=09=09TOTAL BYTES=09AVG BYTES=09STDDEV
=3D=3D=3D=3D=3D=3D=3D=09=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=09=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=09=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=09=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=09=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
-=09pipe=09=09      307457969=09    36348556755=09      302904639=09       =
10622403
-=09splice=09=09      287117614=09    26933658717=09      224447155=09     =
 160777958
-=09vmsplice=09      435180375=09    51302964090=09      427524700=09      =
 19083037

rm-nrx=09pipe=09=09      311091179=09    37093181356=09      309109844=09  =
      7221622
rm-nrx=09splice=09=09      285628049=09    27916298942=09      232635824=09=
      158296431
rm-nrx=09vmsplice=09      417703153=09    47570362546=09      396419687=09 =
      33960822

wakesl=09pipe=09=09      310698731=09    36772541631=09      306437846=09  =
      8249347
wakesl=09splice=09=09      286193726=09    28600435451=09      238336962=09=
      141169318
wakesl=09vmsplice=09      436175803=09    50723895824=09      422699131=09 =
      40724240

ht=09pipe=09=09      305534565=09    36426079543=09      303550662=09      =
  5673885
ht=09splice=09=09      243632025=09    23319439010=09      194328658=09    =
  150479853
ht=09vmsplice=09      432825176=09    49101781001=09      409181508=09     =
  44102509

k-rsv=09pipe=09=09      308691523=09    36652267561=09      305435563=09   =
    12972559
k-rsv=09splice=09=09      244793528=09    23625172865=09      196876440=09 =
     125319143
k-rsv=09vmsplice=09      436119082=09    49460808579=09      412173404=09  =
     55547525

r-adv-t=09pipe=09=09      310094218=09    36860182219=09      307168185=09 =
       8081101
r-adv-t=09splice=09=09      285527382=09    27085052687=09      225708772=
=09      206918887
r-adv-t=09vmsplice=09      336885948=09    40128756927=09      334406307=09=
        5895935

r-cond=09pipe=09=09      308727804=09    36635828180=09      305298568=09  =
      9976806
r-cond=09splice=09=09      284467568=09    28445793054=09      237048275=09=
      200284329
r-cond=09vmsplice=09      449679489=09    51134833848=09      426123615=09 =
      66790875

w-preal=09pipe=09=09      307416578=09    36662086426=09      305517386=09 =
       6216663
w-preal=09splice=09=09      282655051=09    28455249109=09      237127075=
=09      194154549
w-preal=09vmsplice=09      437002601=09    47832160621=09      398601338=09=
       96513019

w-redun=09pipe=09=09      307279630=09    36329750422=09      302747920=09 =
       8913567
w-redun=09splice=09=09      284324488=09    27327152734=09      227726272=
=09      219735663
w-redun=09vmsplice=09      451141971=09    51485257719=09      429043814=09=
       51388217

w-ckful=09pipe=09=09      305055247=09    36374947350=09      303124561=09 =
       5400728
w-ckful=09splice=09=09      281575308=09    26841554544=09      223679621=
=09      215942886
w-ckful=09vmsplice=09      436653588=09    47564907110=09      396374225=09=
       82255342

The patches column indicates the point in the patchset at which the benchma=
rks
were taken:

=090=09No patches
=09rm-nrx=09"Remove the nr_exclusive argument from __wake_up_sync_key()"
=09wakesl=09"Add wake_up_interruptible_sync_poll_locked()"
=09ht=09"pipe: Use head and tail pointers for the ring, not cursor and leng=
th"
=09k-rsv=09"pipe: Allow pipes to have kernel-reserved slots"
=09r-adv-t=09"pipe: Advance tail pointer inside of wait spinlock in pipe_re=
ad()"
=09r-cond=09"pipe: Conditionalise wakeup in pipe_read()"
=09w-preal=09"pipe: Rearrange sequence in pipe_write() to preallocate slot"
=09w-redun=09"pipe: Remove redundant wakeup from pipe_write()"
=09w-ckful=09"pipe: Check for ring full inside of the spinlock in pipe_writ=
e()"

Changes:

 ver #2:

 (*) Split the notification patches out into a separate branch.

 (*) Removed the nr_exclusive parameter from __wake_up_sync_key().

 (*) Renamed the locked wakeup function.

 (*) Add helpers for empty, full, occupancy.

 (*) Split the addition of ->max_usage out into its own patch.

 (*) Fixed some bits pointed out by Rasmus Villemoes.

 ver #1:

 (*) Build on top of standard pipes instead of having a driver.

David
---
David Howells (10):
      pipe: Reduce #inclusion of pipe_fs_i.h
      Remove the nr_exclusive argument from __wake_up_sync_key()
      Add wake_up_interruptible_sync_poll_locked()
      pipe: Use head and tail pointers for the ring, not cursor and length
      pipe: Allow pipes to have kernel-reserved slots
      pipe: Advance tail pointer inside of wait spinlock in pipe_read()
      pipe: Conditionalise wakeup in pipe_read()
      pipe: Rearrange sequence in pipe_write() to preallocate slot
      pipe: Remove redundant wakeup from pipe_write()
      pipe: Check for ring full inside of the spinlock in pipe_write()


 fs/exec.c                  |    1=20
 fs/fuse/dev.c              |   31 +++--
 fs/ocfs2/aops.c            |    1=20
 fs/pipe.c                  |  225 ++++++++++++++++++++++---------------
 fs/splice.c                |  188 +++++++++++++++++++------------
 include/linux/pipe_fs_i.h  |   90 ++++++++++++++-
 include/linux/uio.h        |    4 -
 include/linux/wait.h       |   11 +-
 kernel/exit.c              |    2=20
 kernel/sched/wait.c        |   37 ++++--
 lib/iov_iter.c             |  266 +++++++++++++++++++++++++---------------=
----
 security/smack/smack_lsm.c |    1=20
 12 files changed, 541 insertions(+), 316 deletions(-)