linux-kernel - Re: [PATCH v3 5/5] block: use chunk_sectors when evaluating stacked atomic write limits

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Message-ID: <803d0903-2382-4219-b81e-9d676bd5de1f@oracle.com>
Date: Thu, 3 Jul 2025 17:01:00 +0100
From: John Garry <john.g.garry@...cle.com>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org, song@...nel.org,
        yukuai3@...wei.com, hch@....de, nilay@...ux.ibm.com,
        dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, linux-block@...r.kernel.org,
        ojaswin@...ux.ibm.com, martin.petersen@...cle.com
Subject: Re: [PATCH v3 5/5] block: use chunk_sectors when evaluating stacked
 atomic write limits

On 03/07/2025 16:36, Mikulas Patocka wrote:
>> I suppose theoretically it could happen, and I'm happy to change.
>>
>> However there seems to be precedent in assuming it won't:
>>
>> - in stripe_op_hints(), we hold chunk_size in an unsigned int
>> - in raid0_set_limits(), we hold mddev->chunk_sectors << 9 in lim.io_min,
>> which is an unsigned int type.
>>
>> Please let me know your thoughts on also changing these sort of instances. Is
>> it realistic to expect chunk_bytes > UINT_MAX?
>>
>> Thanks,
>> John
> dm-stripe can be created with a stripe size that is more than 0xffffffff
> bytes.
> 
> Though, the integer overflow already exists in the existing dm-stripe
> target:
> static void stripe_io_hints(struct dm_target *ti,
>                              struct queue_limits *limits)
> {
>          struct stripe_c *sc = ti->private;
>          unsigned int chunk_size = sc->chunk_size << SECTOR_SHIFT;
> 
>          limits->io_min = chunk_size;
>          limits->io_opt = chunk_size * sc->stripes;
> }
> What should we set there as io_min and io_opt if sc->chunk_size <<
> SECTOR_SHIFT overflows?


> Should we set nothing?

For io_min/opt, maybe reduce to a factor of the stripe size / width (and 
which fits in a unsigned int).

I am not sure if it is even sane to have such huge values in io_min and 
the bottom disk io_min should be used directly instead.

Martin Petersen might have a better idea.. he added those sysfs files :)

Thanks,
John