linux-kernel - Re: [PATCH md-6.12 3/7] md: don't record new badblocks for faulty rdev

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Message-ID: <20240902105539.00007655@linux.intel.com>
Date: Mon, 2 Sep 2024 10:55:39 +0200
From: Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: mariusz.tkaczyk@...el.com, song@...nel.org, linux-raid@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH md-6.12 3/7] md: don't record new badblocks for faulty
 rdev

On Sat, 31 Aug 2024 09:14:39 +0800
Yu Kuai <yukuai1@...weicloud.com> wrote:

> Hi,
> 
> 在 2024/08/30 18:28, Mariusz Tkaczyk 写道:
> > On Fri, 30 Aug 2024 15:27:17 +0800
> > Yu Kuai <yukuai1@...weicloud.com> wrote:
> >   
> >> From: Yu Kuai <yukuai3@...wei.com>
> >>
> >> Faulty will be checked before issuing IO to the rdev, however, rdev can
> >> be faulty at any time, hence it's possible that rdev_set_badblocks()
> >> will be called for faulty rdev. In this case, mddev->sb_flags will be
> >> set and some other path can be blocked by updating super block.
> >>
> >> Since faulty rdev will not be accesed anymore, there is no need to
> >> record new babblocks for faulty rdev and forcing updating super block.
> >>
> >> Noted this is not a bugfix, just prevent updating superblock in some
> >> corner cases, and will help to slice a bug related to external
> >> metadata[1].
> >>
> >> [1]
> >> https://lore.kernel.org/all/f34452df-810b-48b2-a9b4-7f925699a9e7@linux.intel.com/
> >>
> >> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> >> ---
> >>   drivers/md/md.c | 4 ++++
> >>   1 file changed, 4 insertions(+)
> >>
> >> diff --git a/drivers/md/md.c b/drivers/md/md.c
> >> index 675d89597c7b..a3f7f407fe42 100644
> >> --- a/drivers/md/md.c
> >> +++ b/drivers/md/md.c
> >> @@ -9757,6 +9757,10 @@ int rdev_set_badblocks(struct md_rdev *rdev,
> >> sector_t s, int sectors, {
> >>   	struct mddev *mddev = rdev->mddev;
> >>   	int rv;
> >> +
> >> +	if (test_bit(Faulty, &rdev->flags))
> >> +		return 1;
> >> +  
> > 
> > Blame is volatile, this is why we need a comment here :)
> > Otherwise, someone may remove that.  
> 
> Perhaps something like following?
> 
> /*
>   * record new babblocks for faulty rdev will force unnecessary
>   * super block updating.
>   */
> 

Almost, we need to refer to external context because this is important to
mention where to expect issues:

/*
 * Recording new badblocks for faulty rdev will force unnecessary
 * super block updating. This is fragile for external management because
 * userspace daemon may trying to remove this device and deadlock may
 * occur. This will be probably solved in the mdadm, but it is safer to avoid
 * it.
 */

In my testing, I observed that it improves failing bios and device removal
path (recording badblock is simply expensive if there are many badblocks) so
the devices are removed faster but I don't have data here, this is what I saw.

Obviously, it is optimization.

Mariusz