Re: [PATCH 8/8] iomap: support IOCB_DIO_DEFER

[Dave Chinner <david@fromorbit.com>]

On Thu, Jul 20, 2023 at 12:13:10PM -0600, Jens Axboe wrote:
> If IOCB_DIO_DEFER is set, utilize that to set kiocb->dio_complete handler
> and data for that callback. Rather than punt the completion to a
> workqueue, we pass back the handler and data to the issuer and will get a
> callback from a safe task context.
....
> @@ -288,12 +319,17 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		 * after IO completion such as unwritten extent conversion) and
>  		 * the underlying device either supports FUA or doesn't have
>  		 * a volatile write cache. This allows us to avoid cache flushes
> -		 * on IO completion.
> +		 * on IO completion. If we can't use stable writes and need to
> +		 * sync, disable in-task completions as dio completion will
> +		 * need to call generic_write_sync() which will do a blocking
> +		 * fsync / cache flush call.
>  		 */
>  		if (!(iomap->flags & (IOMAP_F_SHARED|IOMAP_F_DIRTY)) &&
>  		    (dio->flags & IOMAP_DIO_STABLE_WRITE) &&
>  		    (bdev_fua(iomap->bdev) || !bdev_write_cache(iomap->bdev)))
>  			use_fua = true;
> +		else if (dio->flags & IOMAP_DIO_NEED_SYNC)
> +			dio->flags &= ~IOMAP_DIO_DEFER_COMP;
>  	}
>  
>  	/*
> @@ -319,6 +355,13 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		pad = pos & (fs_block_size - 1);
>  		if (pad)
>  			iomap_dio_zero(iter, dio, pos - pad, pad);
> +
> +		/*
> +		 * If need_zeroout is set, then this is a new or unwritten
> +		 * extent. These need extra handling at completion time, so
> +		 * disable in-task deferred completion for those.
> +		 */
> +		dio->flags &= ~IOMAP_DIO_DEFER_COMP;
>  	}

I don't think these are quite right. They miss the file extension
case that I pointed out in an earlier patch (i.e. where IOCB_HIPRI
gets cleared).

Fundamentally, I don't like have three different sets of logic which
all end up being set/cleared for the same situation - polled bios
and defered completion should only be used in situations where
inline iomap completion can be run.

IOWs, I think the iomap_dio_bio_iter() code needs to first decide
whether IOMAP_DIO_INLINE_COMP can be set, and if it cannot be set,
we then clear both IOCB_HIPRI and IOMAP_DIO_DEFER_COMP, because
neither should be used for an IO that can not do inline completion.

i.e. this all comes down to something like this:

-	/*
-	 * We can only poll for single bio I/Os.
-	 */
-	if (need_zeroout ||
-	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode)))
-		dio->iocb->ki_flags &= ~IOCB_HIPRI;
+	/*
+	 * We can only do inline completion for pure overwrites that
+	 * don't require additional IO at completion. This rules out
+	 * writes that need zeroing or extent conversion, extend
+	 * the file size, or issue journal IO or cache flushes
+	 * during completion processing.
+	 */
+	if (need_zeroout ||
+	    ((dio->flags & IOMAP_DIO_NEED_SYNC) && !use_fua) ||
+	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode)))
+		dio->flags &= ~IOMAP_DIO_INLINE_COMP;
+
+	/*
+	 * We can only used polled for single bio IOs or defer
+	 * completion for IOs that will run inline completion.
+	 */
+	if (!(dio->flags & IOMAP_DIO_INLINE_COMP) {
+		dio->iocb->ki_flags &= ~IOCB_HIPRI;
+		dio->flags &= ~IOMAP_DIO_DEFER_COMP;
+	}

This puts the iomap inline completion decision logic all in one
place in the submission code and clearly keys the fast path IO
completion cases to the inline completion paths.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com