[PATCH 5/8] io_uring: move cur_mm into io_submit_state

Pavel Begunkov
2020-01-24T21:42:03Z

cur_mm is only used per submission, so it could be place into
io_submit_state. There is the reasoning behind:
- it's more convenient, don't need to pass it down the call stack
- it's passed as a pointer, so in either case needs memory read/write
- now uses heap (ctx->submit_state) instead of stack
- set only once for non-IORING_SETUP_SQPOLL case.
- generates pretty similar code as @ctx is hot and always somewhere in a
register

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
fs/io_uring.c | 46 ++++++++++++++++++++++++++--------------------
1 file changed, 26 insertions(+), 20 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 951c2fc7b5b7..c0e72390d272 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -219,6 +219,8 @@ struct io_submit_state {

struct file *ring_file;
int ring_fd;
+
+ struct mm_struct *mm;
};

struct io_ring_ctx {
@@ -4834,8 +4836,7 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct io_kiocb *req,
}

static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
- struct file *ring_file, int ring_fd,
- struct mm_struct **mm, bool async)
+ struct file *ring_file, int ring_fd, bool async)
{
struct blk_plug plug;
struct io_kiocb *link = NULL;
@@ -4883,15 +4884,15 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
break;
}

- if (io_op_defs[req->opcode].needs_mm && !*mm) {
+ if (io_op_defs[req->opcode].needs_mm && !ctx->submit_state.mm) {
mm_fault = mm_fault || !mmget_not_zero(ctx->sqo_mm);
if (!mm_fault) {
use_mm(ctx->sqo_mm);
- *mm = ctx->sqo_mm;
+ ctx->submit_state.mm = ctx->sqo_mm;
}
}

- req->has_user = *mm != NULL;
+ req->has_user = (ctx->submit_state.mm != NULL);
req->in_async = async;
req->needs_fixed_file = async;
trace_io_uring_submit_sqe(ctx, req->opcode, req->user_data,
@@ -4918,7 +4919,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
static int io_sq_thread(void *data)
{
struct io_ring_ctx *ctx = data;
- struct mm_struct *cur_mm = NULL;
+ struct io_submit_state *submit = &ctx->submit_state;
const struct cred *old_cred;
mm_segment_t old_fs;
DEFINE_WAIT(wait);
@@ -4993,10 +4994,15 @@ static int io_sq_thread(void *data)
* adding ourselves to the waitqueue, as the unuse/drop
* may sleep.
*/
- if (cur_mm) {
- unuse_mm(cur_mm);
- mmput(cur_mm);
- cur_mm = NULL;
+ if (submit->mm) {
+ /*
+ * this thread is the only submitter, thus
+ * it's safe to change submit->mm without
+ * taking ctx->uring_lock
+ */
+ unuse_mm(submit->mm);
+ mmput(submit->mm);
+ submit->mm = NULL;
}

prepare_to_wait(&ctx->sqo_wait, &wait,
@@ -5027,16 +5033,17 @@ static int io_sq_thread(void *data)
}

mutex_lock(&ctx->uring_lock);
- ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
+ ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
mutex_unlock(&ctx->uring_lock);
if (ret > 0)
inflight += ret;
}

set_fs(old_fs);
- if (cur_mm) {
- unuse_mm(cur_mm);
- mmput(cur_mm);
+ if (submit->mm) {
+ unuse_mm(submit->mm);
+ mmput(submit->mm);
+ submit->mm = NULL;
}
revert_creds(old_cred);

@@ -5757,6 +5764,10 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
mmgrab(current->mm);
ctx->sqo_mm = current->mm;

+ ctx->submit_state.mm = NULL;
+ if (!(ctx->flags & IORING_SETUP_SQPOLL))
+ ctx->submit_state.mm = ctx->sqo_mm;
+
if (ctx->flags & IORING_SETUP_SQPOLL) {
ret = -EPERM;
if (!capable(CAP_SYS_ADMIN))
@@ -6369,8 +6380,6 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
wake_up(&ctx->sqo_wait);
submitted = to_submit;
} else if (to_submit) {
- struct mm_struct *cur_mm;
-
if (current->mm != ctx->sqo_mm ||
current_cred() != ctx->creds) {
ret = -EPERM;
@@ -6378,10 +6387,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
}

mutex_lock(&ctx->uring_lock);
- /* already have mm, so io_submit_sqes() won't try to grab it */
- cur_mm = ctx->sqo_mm;
- submitted = io_submit_sqes(ctx, to_submit, f.file, fd,
- &cur_mm, false);
+ submitted = io_submit_sqes(ctx, to_submit, f.file, fd, false);
mutex_unlock(&ctx->uring_lock);

if (submitted != to_submit)
--
2.24.0