Re: [PATCH v2 09/12] md/raid5: Keep a reference to last stripe_head for batch

From: Logan Gunthorpe <logang@deltatee.com>
To: Guoqing Jiang <guoqing.jiang@linux.dev>,
	linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
	Song Liu <song@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Stephen Bates <sbates@raithlin.com>,
	Martin Oliveira <Martin.Oliveira@eideticom.com>,
	David Sloan <David.Sloan@eideticom.com>
Subject: Re: [PATCH v2 09/12] md/raid5: Keep a reference to last stripe_head for batch
Date: Wed, 27 Apr 2022 17:27:43 -0600	[thread overview]
Message-ID: <2dd3e7b3-ac85-6de2-3130-4fdd7b366739@deltatee.com> (raw)
In-Reply-To: <56208e2a-5035-eb8e-c468-70b4dae66d5c@linux.dev>

On 2022-04-26 19:36, Guoqing Jiang wrote:
> On 4/21/22 3:54 AM, Logan Gunthorpe wrote:
>>     /* we only do back search */
>> -static void stripe_add_to_batch_list(struct r5conf *conf, struct
>> stripe_head *sh)
>> +static void stripe_add_to_batch_list(struct r5conf *conf,
>> +        struct stripe_head *sh, struct stripe_head *last_sh)
> 
> Nit, from stripe_add_to_batch_list's view, I think "head_sh" makes more
> sense than
> "last_sh".

That made sense to me, but upon a closer look while making the change, I
think it's not a good idea:

stripe_add_to_batch_list() already has a stripe_head variable called
"head". If it now has an argument called "head_sh", it becomes very
confusing. This statement wouldn't make any sense:
+    if (last_sh && head_sector == last_sh->sector) {
+        head = last_sh;
+        atomic_inc(&head->count);
+    } else {

If it was changed to "head = head_sh" what would that even mean?

From stripe_add_to_batch_list's perspective, it is the "last" stripe
head. And it then decides whether the it is the correct stripe to use as
the head of the list to add to.

So I decline to make this change.

Thanks,

Logan