Currently, aio-nr is incremented in steps of 'num_possible_cpus() * 8'
for io_setup(nr_events, ..) with 'nr_events < num_possible_cpus() * 4':
ioctx_alloc()
...
nr_events = max(nr_events, num_possible_cpus() * 4);
nr_events *= 2;
...
ctx->max_reqs = nr_events;
...
aio_nr += ctx->max_reqs;
....
This limits the number of aio contexts actually available to much less
than aio-max-nr, and is increasingly worse with greater number of CPUs.
For example, with 64 CPUs, only 256 aio contexts are actually available
(with aio-max-nr = 65536) because the increment is 512 in that scenario.
Note: 65536 [max aio contexts] / (2*4*64) [increment per aio context]
is 128, but make it 256 (double) as counting against 'aio-max-nr * 2':
ioctx_alloc()
...
if (aio_nr + nr_events > (aio_max_nr * 2UL) ||
...
goto err_ctx;
...
This patch uses the original value of nr_events (from userspace) to
increment aio-nr and count against aio-max-nr, which resolves those.
The test-case and test-suite validation steps are included later in
this patch message, for documentation purposes.
Example on a system with 64 CPUs:
# cat /sys/devices/system/cpu/possible
0-63
# grep . /proc/sys/fs/aio-*
/proc/sys/fs/aio-max-nr:65536
/proc/sys/fs/aio-nr:0
test 1) number of aio contexts available with nr_events == 1
-------------------------------------------------------------
This test calls io_setup(1, ..) up to 65536 times, exiting on error.
- original kernel:
Only 256 aio contexts could be created successfully,
quickly falling into the aio-max-nr exceeded error path (-EAGAIN).
# ./io_setup 1 65536 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1, ): 256 calls with rc 0, last call with rc -11.
/proc/sys/fs/aio-nr:131072
One might notice the aio-nr value is twice the aio-max-nr limit,
an effect of how the current code handles that 'nr_events *= 2'.
- patched kernel:
Almost all of the limit of aio contexts could be allocated,
eventually falling into the insufficient resources error path (-ENOMEM):
# ./io_setup 1 65536 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1, ): 65516 calls with rc 0, last call with rc -12.
/proc/sys/fs/aio-nr:65516
Notice the aio-nr value is now _under_ the aio-max-nr limit.
test 2) increment value for nr_events == 1
-------------------------------------------
This test calls io_setup(1, ..) only 1 time, to show the increment:
- original kernel:
# ./io_setup 1 1 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1, ) : 1 calls with rc 0, last call with rc 0.
/proc/sys/fs/aio-nr:512
Notice the increment is 'num_online_cpus() * 8'.
- patched kernel:
# ./io_setup 1 1 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1, ): 1 calls with rc 0, last call with rc 0.
/proc/sys/fs/aio-nr:1
Notice the increment is exactly 1 (matches nr_events from userspace).
test 3) more aio contexts available with great-enough nr_events
----------------------------------------------------------------
The full aio-max-nr limit (65536) is available for greater nr_events.
This test calls io_setup(1024, ) exactly 64 times, without error.
- original kernel:
# ./io_setup 1024 64 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1024, ): 64 calls with rc 0, last call with rc 0.
/proc/sys/fs/aio-nr:131072
Notice the aio-nr value is twice the aio-max-nr limit.
- patched kernel:
# ./io_setup 1024 64 | grep -m1 . - /proc/sys/fs/aio-nr
(standard input):io_setup(1024, ): 64 calls with rc 0, last call with rc 0.
/proc/sys/fs/aio-nr:65536
Notice the aio-nr value is now _exactly_ the aio-max-nr limit.
Test-case: io_setup.c # gcc -o io_setup io_setup.c -laio
"""
#include <libaio.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
int nr_events, nr_calls, rc, i;
io_context_t *ioctx;
/* usage: io_setup <nr_events for io_setup()> <max calls to io_setup()> */
if (argc != 3)
return -1;
nr_events = atoi(argv[1]);
nr_calls = atoi(argv[2]);
ioctx = calloc(nr_calls, sizeof(*ioctx));
if (!ioctx)
return -2;
for (i = 0; i < nr_calls; i++)
if (rc = io_setup(nr_events, &ioctx[i]))
break;
printf("io_setup(%d, ): %d calls with rc 0, last call with rc %d.\n",
nr_events, i, rc);
fflush(stdout);
sleep(1);
return 0;
}
"""
Test-suite: libaio
# curl https://kojipkgs.fedoraproject.org//packages/libaio/0.3.110/7.fc26/src/libaio-0.3.110-7.fc26.src.rpm \
| rpm2cpio | cpio -mid
# tar xf libaio-0.3.110.tar.gz
# cd libaio-0.3.110
# make
# make check 2>&1 | grep '^test cases'
test cases/2.t completed PASSED.
test cases/3.t completed PASSED.
test cases/4.t completed PASSED.
test cases/5.t completed PASSED.
test cases/6.t completed PASSED.
test cases/7.t completed PASSED.
test cases/11.t completed PASSED.
test cases/12.t completed PASSED.
test cases/13.t completed PASSED.
test cases/14.t completed PASSED.
test cases/15.t completed PASSED.
test cases/16.t completed PASSED.
test cases/10.t completed PASSED.
test cases/8.t completed PASSED.
Signed-off-by: Mauricio Faria de Oliveira <[email protected]>
Reported-by: Lekshmi C. Pillai <[email protected]>
Tested-by: Lekshmi C. Pillai <[email protected]>
Tested-by: Paul Nguyen <[email protected]>
---
fs/aio.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)
diff --git a/fs/aio.c b/fs/aio.c
index f52d925ee259..3908480d7ccd 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -441,10 +441,9 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
#endif
};
-static int aio_setup_ring(struct kioctx *ctx)
+static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
{
struct aio_ring *ring;
- unsigned nr_events = ctx->max_reqs;
struct mm_struct *mm = current->mm;
unsigned long size, unused;
int nr_pages;
@@ -707,6 +706,12 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
int err = -ENOMEM;
/*
+ * Store the original nr_events -- what userspace passed to io_setup(),
+ * for counting against the global limit -- before it changes.
+ */
+ unsigned int 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.
*
@@ -724,14 +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);
@@ -753,7 +758,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
if (!ctx->cpu)
goto err;
- err = aio_setup_ring(ctx);
+ err = aio_setup_ring(ctx, nr_events);
if (err < 0)
goto err;
@@ -764,8 +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;
--
1.8.3.1