[PATCH] io_uring: increase max number of reg buffers

[PATCH] io_uring: increase max number of reg buffers

[Pavel Begunkov]

Since recent changes instead of storing a large array of struct
io_mapped_ubuf, we store pointers to them, that is 4 times slimmer and
we should not to so worry about restricting max number of registererd
buffer slots, increase the limit 4 times.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9ac5e278a91e..8f718d26f01c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -100,6 +100,8 @@
 #define IORING_MAX_RESTRICTIONS	(IORING_RESTRICTION_LAST + \
 				 IORING_REGISTER_LAST + IORING_OP_LAST)
 
+#define IORING_MAX_REG_BUFFERS	(1U << 14)
+
 #define SQE_VALID_FLAGS	(IOSQE_FIXED_FILE|IOSQE_IO_DRAIN|IOSQE_IO_LINK|	\
 				IOSQE_IO_HARDLINK | IOSQE_ASYNC | \
 				IOSQE_BUFFER_SELECT)
@@ -8390,7 +8392,7 @@ static int io_sqe_buffers_register(struct io_ring_ctx *ctx, void __user *arg,
 
 	if (ctx->user_bufs)
 		return -EBUSY;
-	if (!nr_args || nr_args > UIO_MAXIOV)
+	if (!nr_args || nr_args > IORING_MAX_REG_BUFFERS)
 		return -EINVAL;
 	ret = io_rsrc_node_switch_start(ctx);
 	if (ret)
-- 
2.31.1

Re: [PATCH] io_uring: increase max number of reg buffers

[Jens Axboe]

On 5/14/21 5:06 AM, Pavel Begunkov wrote:
> Since recent changes instead of storing a large array of struct
> io_mapped_ubuf, we store pointers to them, that is 4 times slimmer and
> we should not to so worry about restricting max number of registererd
> buffer slots, increase the limit 4 times.

Is this going to fall within the max kmalloc size?

-- 
Jens Axboe

Re: [PATCH] io_uring: increase max number of reg buffers

[Jens Axboe]

On 5/14/21 8:42 AM, Jens Axboe wrote:
> On 5/14/21 5:06 AM, Pavel Begunkov wrote:
>> Since recent changes instead of storing a large array of struct
>> io_mapped_ubuf, we store pointers to them, that is 4 times slimmer and
>> we should not to so worry about restricting max number of registererd
>> buffer slots, increase the limit 4 times.
> 
> Is this going to fall within the max kmalloc size?

Ah yes, it's just a u64 now, so should be fine. Might be worth considering
using vmalloc() for this in any case in the future, to make the allocation
more reliable. 128K of contig memory can sometimes be an issue in prod.

-- 
Jens Axboe

Re: [PATCH] io_uring: increase max number of reg buffers

[Pavel Begunkov]

On 5/14/21 3:42 PM, Jens Axboe wrote:
> On 5/14/21 5:06 AM, Pavel Begunkov wrote:
>> Since recent changes instead of storing a large array of struct
>> io_mapped_ubuf, we store pointers to them, that is 4 times slimmer and
>> we should not to so worry about restricting max number of registererd
>> buffer slots, increase the limit 4 times.
> 
> Is this going to fall within the max kmalloc size?
> 

2^14 * 8 bytes = 128KB or 32 pages, 

Even though 128KB may be not nice, but should be way below max
limit. On the other hand that's the same amount of memory we were
allowing prior to dynamic buffer changes.

-- 
Pavel Begunkov