linux-kernel - Re: [PATCH v2 12/12] md/raid5: Pivot raid5_make

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Message-ID: <YmD9KBJtOjV+J5We@infradead.org>
Date:   Wed, 20 Apr 2022 23:43:52 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v2 12/12] md/raid5: Pivot raid5_make_request()

On Wed, Apr 20, 2022 at 01:54:25PM -0600, Logan Gunthorpe wrote:
>  struct stripe_request_ctx {
>  	bool do_flush;
>  	struct stripe_head *batch_last;
> +	sector_t disk_sector_done;
> +	sector_t start_disk_sector;

Very nitpicky, but why use two different naming styles for the sectors
here?

> +	bool first_wrap;
> +	sector_t last_sector;

And especially with the last_sector here a few comments explaining
what each of the sector values mean might be useful.

I'd also keep the two bool variables together for a better structure
layout.

> +	 * if new_sector is less than the starting sector. Clear the
> +	 * boolean once the start sector is hit for the second time.
> +	 * When first_wrap is set, ignore the disk_sector_done.
> +	 */
> +	if (ctx->start_disk_sector == MaxSector) {
> +		ctx->start_disk_sector = new_sector;
> +	} else if (new_sector < ctx->start_disk_sector) {
> +		ctx->first_wrap = true;
> +	} else if (new_sector == ctx->start_disk_sector) {
> +		ctx->first_wrap = false;
> +		ctx->start_disk_sector = 0;
> +		return STRIPE_SUCCESS;
> +	} else if (!ctx->first_wrap && new_sector <= ctx->disk_sector_done) {
> +		return STRIPE_SUCCESS;
> +	}
> +

I find this a bit confusing to read.  While trying to mentally untangle
it I came up with this version instead, but it could really use some
good comments explaining each of the checks as I found your big comment
to not quite match the logic easily.

	if (ctx->start_disk_sector == MaxSector) {
		/*
		 * First loop iteration, start our state machine.
		 * 
		ctx->start_disk_sector = new_sector;
	} else {
		/*
		 * We are done if we wrapped around to the same sector.
		 * (???)
		 */
		if (new_sector == ctx->start_disk_sector) {
			ctx->first_wrap = false;
			ctx->start_disk_sector = 0;
			return STRIPE_SUCCESS;
		}

		/*
		 * Sector before the start sector?  Keep going and wrap
		 * around.
		 */
		if (new_sector < ctx->start_disk_sector) {
			ctx->first_wrap = true;
		} else {
			// ???
			if (new_sector <= ctx->disk_sector_done &&
			   !ctx->first_wrap)
				return STRIPE_SUCCESS;
		}
	}