Re: [PATCH] f2fs: handle decompress only post processing in softirq

[Eric Biggers]

[+Cc Nathan Huckleberry who is looking into a similar problem in dm-verity]

On Mon, Jun 13, 2022 at 08:56:12AM -0700, Daeho Jeong wrote:
> From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> 
> Now decompression is being handled in workqueue and it makes read I/O
> latency non-deterministic, because of the non-deterministic scheduling
> nature of workqueues. So, I made it handled in softirq context only if
> possible.
> 
> Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
> ---
>  fs/f2fs/compress.c | 145 +++++++++++++++++++++++++--------------------
>  fs/f2fs/data.c     |  50 ++++++++++------
>  fs/f2fs/f2fs.h     |  10 ++--
>  3 files changed, 119 insertions(+), 86 deletions(-)
[...]
>  static void f2fs_read_end_io(struct bio *bio)
> @@ -281,16 +283,28 @@ static void f2fs_read_end_io(struct bio *bio)
>  	}
>  
>  	if (bio->bi_status) {
> -		f2fs_finish_read_bio(bio);
> +		f2fs_finish_read_bio(bio, true);
>  		return;
>  	}
>  
> -	if (ctx && (ctx->enabled_steps & (STEP_DECRYPT | STEP_DECOMPRESS))) {
> -		INIT_WORK(&ctx->work, f2fs_post_read_work);
> -		queue_work(ctx->sbi->post_read_wq, &ctx->work);
> -	} else {
> -		f2fs_verify_and_finish_bio(bio);
> +	if (ctx) {
> +		unsigned int enabled_steps = ctx->enabled_steps &
> +					(STEP_DECRYPT | STEP_DECOMPRESS);
> +
> +		/*
> +		 * If we have only decompression step between decompression and
> +		 * decrypt, we don't need post processing for this.
> +		 */
> +		if (enabled_steps == STEP_DECOMPRESS) {
> +			f2fs_handle_step_decompress(ctx, true);

One question: is this (the bio endio callback) actually guaranteed to be
executed from a softirq?  If you look at dm-crypt's support for workqueue-less
decryption, for example, it explicitly checks 'in_hardirq() || irqs_disabled()'
and schedules a tasklet if either of those is the case.

- Eric