linux-kernel - Re: [PATCH] btrfs: raid56: do blk_check

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Message-ID: <32eaf406-aca7-a17a-813a-631616480e42@dorminy.me>
Date:   Sat, 19 Mar 2022 14:31:49 -0400
From:   Sweet Tea Dorminy <sweettea-kernel@...miny.me>
To:     Yusuf Khan <yusisamerican@...il.com>, linux-btrfs@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, dsterba@...e.com,
        josef@...icpanda.com, clm@...com
Subject: Re: [PATCH] btrfs: raid56: do blk_check_plugged check twice while
 writing

Hi Yusuf:

On 3/18/22 21:18, Yusuf Khan wrote:
> Do the check to see if the drive was connected twice in case that
> the first was a fluke.

Block plugging is not actually about checking the connectivity of the 
drive; it's about batching IO submission. Specifically, when a client of 
the block layer (a filesystem or DM device, for instance) knows that it 
needs to send a whole batch of IO to disk from a thread, it can request 
the thread's be temporarily delayed, with blk_start_plug() 
(block/blk-core.c has a lengthy great comment about it). Then it submits 
its batch of requests. Finally, it requests the plug be released. This 
is useful because it can allow better IO by releasing a large, 
potentially better ordered batch at once.

In this particular case, to my understanding, blk_check_plug() checks 
whether the current thread is plugged, and, if it is, it adds the rbio 
to a list; when the plug expires, perhaps from the thread going to 
sleep, the plug callback gets called, specifically btrfs_raid_unplug(), 
which sorts the pending IOs.

As such, it shouldn't be necessary to check whether the thread is 
plugged twice. Your change description is a little light on *why* you'd 
like to check twice; maybe I'm missing something.

Hope this helps,

Sweet Tea