[RESEND,v2,2/2] aio: use ctx->max_reqs only for counting against the global limit

Commit Message

Mauricio Faria de Oliveira May 24, 2017, 7:36 p.m. UTC

Decouple ctx->max_reqs and ctx->nr_events; each one represents
a different side of the same coin -- userspace and kernelspace,
respectively.

Briefly, ctx->max_reqs represents what is userspace/externally
accessible by userspace; and ctx->nr_events represents what is
kernelspace/internally needed by the percpu allocation scheme.

With the percpu scheme, the original value of ctx->max_reqs from
userspace is changed (but still used to count against aio_max_nr)
based on num_possible_cpus(), and it may increase significantly
on systems with great num_possible_cpus() for smaller nr_events.

This eventually prevents userspace applications from getting the
actual value of aio_max_nr in the total requested nr_events.

ctx->max_reqs

Patch

=============

The ctx->max_reqs value once again aligns with its description:

  * This is what userspace passed to io_setup(), it's not used for
  * anything but counting against the global max_reqs quota.

It stores the original value of nr_events that userspace passed
to io_setup() (it's not increased to make room for requirements
of the percpu allocation scheme) - and is used to increment and
decrement the 'aio_nr' value, and to check against 'aio_max_nr'.

So, regardless of how many additional nr_events are internally
required for the percpu allocation scheme (e.g. make it 4x the
number of possible CPUs, and double it), userspace can get all
of the 'aio-max-nr' value that is made available/visible to it.

Another benefit is a consistent value in '/proc/sys/fs/aio-nr':
the sum of all values as requested by userspace, and it's less
than or equal to '/proc/sys/fs/aio-max-nr' again (not 2x it).

ctx->nr_events
==============

The ctx->nr_events value is the actual size of the ringbuffer/
number of slots, which may be more than what userspace passed
to io_setup() (depending on the requested value for nr_events
and/or calculations made in aio_setup_ring()) - as determined
by the percpu allocation scheme for its correct/fast behavior.

Signed-off-by: Mauricio Faria de Oliveira <[e-mail]>
---
 fs/aio.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 7c3c01f352c1..4967b0e1ef1a 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -706,6 +706,12 @@  static struct kioctx *ioctx_alloc(unsigned nr_events)
 	int err = -ENOMEM;
 
 	/*
+	 * Store the original value of nr_events from userspace for counting
+	 * against the global limit (aio_max_nr).
+	 */
+	unsigned max_reqs = nr_events;
+
+	/*
 	 * We keep track of the number of available ringbuffer slots, to prevent
 	 * overflow (reqs_available), and we also use percpu counters for this.
 	 *
@@ -723,14 +729,14 @@  static struct kioctx *ioctx_alloc(unsigned nr_events)
 		return ERR_PTR(-EINVAL);
 	}
 
-	if (!nr_events || (unsigned long)nr_events > (aio_max_nr * 2UL))
+	if (!nr_events || (unsigned long)max_reqs > aio_max_nr)
 		return ERR_PTR(-EAGAIN);
 
 	ctx = kmem_cache_zalloc(kioctx_cachep, GFP_KERNEL);
 	if (!ctx)
 		return ERR_PTR(-ENOMEM);
 
-	ctx->max_reqs = nr_events;
+	ctx->max_reqs = max_reqs;
 
 	spin_lock_init(&ctx->ctx_lock);
 	spin_lock_init(&ctx->completion_lock);
@@ -763,8 +769,8 @@  static struct kioctx *ioctx_alloc(unsigned nr_events)
 
 	/* limit the number of system wide aios */
 	spin_lock(&aio_nr_lock);
-	if (aio_nr + nr_events > (aio_max_nr * 2UL) ||
-	    aio_nr + nr_events < aio_nr) {
+	if (aio_nr + ctx->max_reqs > aio_max_nr ||
+	    aio_nr + ctx->max_reqs < aio_nr) {
 		spin_unlock(&aio_nr_lock);
 		err = -EAGAIN;
 		goto err_ctx;