[PATCH 6/8] fs: add IOCB flags related to passing back dio completions

Jens Axboe
2023-07-20T18:13:26Z

Async dio completions generally happen from hard/soft IRQ context, which
means that users like iomap may need to defer some of the completion
handling to a workqueue. This is less efficient than having the original
issuer handle it, like we do for sync IO, and it adds latency to the
completions.

Add IOCB_DIO_DEFER, which the issuer can set if it is able to safely
punt these completions to a safe context. If the dio handler is aware
of this flag, assign a callback handler in kiocb->dio_complete and
associated data io kiocb->private. The issuer will then call this handler
with that data from task context.

No functional changes in this patch.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
include/linux/fs.h | 34 ++++++++++++++++++++++++++++++++--
1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6867512907d6..2c589418a078 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -338,6 +338,20 @@ enum rw_hint {
#define IOCB_NOIO (1 << 20)
/* can use bio alloc cache */
#define IOCB_ALLOC_CACHE (1 << 21)
+/*
+ * IOCB_DIO_DEFER can be set by the iocb owner, to indicate that the
+ * iocb completion can be passed back to the owner for execution from a safe
+ * context rather than needing to be punted through a workqueue. If this
+ * flag is set, the completion handling may set iocb->dio_complete to a
+ * handler, which the issuer will then call from task context to complete
+ * the processing of the iocb. iocb->private should then also be set to
+ * the argument being passed to this handler. Note that while this provides
+ * a task context for the dio_complete() callback, it should only be used
+ * on the completion side for non-IO generating completions. It's fine to
+ * call blocking functions from this callback, but they should not wait for
+ * unrelated IO (like cache flushing, new IO generation, etc).
+ */
+#define IOCB_DIO_DEFER (1 << 22)

/* for use in trace events */
#define TRACE_IOCB_STRINGS \
@@ -351,7 +365,8 @@ enum rw_hint {
{ IOCB_WRITE, "WRITE" }, \
{ IOCB_WAITQ, "WAITQ" }, \
{ IOCB_NOIO, "NOIO" }, \
- { IOCB_ALLOC_CACHE, "ALLOC_CACHE" }
+ { IOCB_ALLOC_CACHE, "ALLOC_CACHE" }, \
+ { IOCB_DIO_DEFER, "DIO_DEFER" }

struct kiocb {
struct file *ki_filp;
@@ -360,7 +375,22 @@ struct kiocb {
void *private;
int ki_flags;
u16 ki_ioprio; /* See linux/ioprio.h */
- struct wait_page_queue *ki_waitq; /* for async buffered IO */
+ union {
+ /*
+ * Only used for async buffered reads, where it denotes the
+ * page waitqueue associated with completing the read. Valid
+ * IFF IOCB_WAITQ is set.
+ */
+ struct wait_page_queue *ki_waitq;
+ /*
+ * Can be used for O_DIRECT IO, where the completion handling
+ * is punted back to the issuer of the IO. May only be set
+ * if IOCB_DIO_DEFER is set by the issuer, and the issuer must
+ * then check for presence of this handler when ki_complete is
+ * invoked.
+ */
+ ssize_t (*dio_complete)(void *data);
+ };
};

static inline bool is_sync_kiocb(struct kiocb *kiocb)
--
2.40.1