Re: [PATCH] md/raid1: set max_sectors during early return from choose_slow_rdev()

[Paul E Luse]

On Thu, 11 Jul 2024 22:23:16 +0200
Mateusz Jończyk <mat.jonczyk@xxxxx> wrote:

> Linux 6.9+ is unable to start a degraded RAID1 array with one drive,
> when that drive has a write-mostly flag set. During such an attempt,
> the following assertion in bio_split() is hit:
> 

Nice catch and good patch :)  Kwai?

-Paul

> 	BUG_ON(sectors <= 0);
> 
> Call Trace:
> 	? bio_split+0x96/0xb0
> 	? exc_invalid_op+0x53/0x70
> 	? bio_split+0x96/0xb0
> 	? asm_exc_invalid_op+0x1b/0x20
> 	? bio_split+0x96/0xb0
> 	? raid1_read_request+0x890/0xd20
> 	? __call_rcu_common.constprop.0+0x97/0x260
> 	raid1_make_request+0x81/0xce0
> 	? __get_random_u32_below+0x17/0x70
> 	? new_slab+0x2b3/0x580
> 	md_handle_request+0x77/0x210
> 	md_submit_bio+0x62/0xa0
> 	__submit_bio+0x17b/0x230
> 	submit_bio_noacct_nocheck+0x18e/0x3c0
> 	submit_bio_noacct+0x244/0x670
> 
> After investigation, it turned out that choose_slow_rdev() does not
> set the value of max_sectors in some cases and because of it,
> raid1_read_request calls bio_split with sectors == 0.
> 
> Fix it by filling in this variable.
> 
> This bug was introduced in
> commit dfa8ecd167c1 ("md/raid1: factor out choose_slow_rdev() from
> read_balance()") but apparently hidden until
> commit 0091c5a269ec ("md/raid1: factor out helpers to choose the best
> rdev from read_balance()") shortly thereafter.
> 
> Cc: stable@xxxxxxxxxxxxxxx # 6.9.x+
> Signed-off-by: Mateusz Jończyk <mat.jonczyk@xxxxx>
> Fixes: dfa8ecd167c1 ("md/raid1: factor out choose_slow_rdev() from
> read_balance()") Cc: Song Liu <song@xxxxxxxxxx>
> Cc: Yu Kuai <yukuai3@xxxxxxxxxx>
> Cc: Paul Luse <paul.e.luse@xxxxxxxxxxxxxxx>
> Cc: Xiao Ni <xni@xxxxxxxxxx>
> Cc: Mariusz Tkaczyk <mariusz.tkaczyk@xxxxxxxxxxxxxxx>
> Link:
> https://lore.kernel.org/linux-raid/20240706143038.7253-1-mat.jonczyk@xxxxx/
> 
> --
> 
> Tested on both Linux 6.10 and 6.9.8.
> 
> Inside a VM, mdadm testsuite for RAID1 on 6.10 did not find any
> problems: ./test --dev=loop --no-error --raidtype=raid1
> (on 6.9.8 there was one failure, caused by external bitmap support not
> compiled in).
> 
> Notes:
> - I was reliably getting deadlocks when adding / removing devices
>   on such an array - while the array was loaded with fsstress with 20
>   concurrent processes. When the array was idle or loaded with
> fsstress with 8 processes, no such deadlocks happened in my tests.
>   This occurred also on unpatched Linux 6.8.0 though, but not on
>   6.1.97-rc1, so this is likely an independent regression (to be
>   investigated).
> - I was also getting deadlocks when adding / removing the bitmap on
> the array in similar conditions - this happened on Linux 6.1.97-rc1
>   also though. fsstress with 8 concurrent processes did cause it only
>   once during many tests.
> - in my testing, there was once a problem with hot adding an
>   internal bitmap to the array:
> 	mdadm: Cannot add bitmap while array is resyncing or
> reshaping etc. mdadm: failed to set internal bitmap.
>   even though no such reshaping was happening according to
> /proc/mdstat. This seems unrelated, though.
> ---
>  drivers/md/raid1.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 7b8a71ca66dd..82f70a4ce6ed 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -680,6 +680,7 @@ static int choose_slow_rdev(struct r1conf *conf,
> struct r1bio *r1_bio, len = r1_bio->sectors;
>  		read_len = raid1_check_read_range(rdev, this_sector,
> &len); if (read_len == r1_bio->sectors) {
> +			*max_sectors = read_len;
>  			update_read_sectors(conf, disk, this_sector,
> read_len); return disk;
>  		}
> 
> base-commit: 256abd8e550ce977b728be79a74e1729438b4948