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

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Message-ID: <8b5e009a-9e2a-4542-69fb-fc6d47287255@redhat.com>
Date: Thu, 3 Jul 2025 17:36:38 +0200 (CEST)
From: Mikulas Patocka <mpatocka@...hat.com>
To: John Garry <john.g.garry@...cle.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 Thu, 3 Jul 2025, John Garry wrote:

> > >   -/* Check stacking of first bottom device */
> > > -static bool blk_stack_atomic_writes_head(struct queue_limits *t,
> > > -				struct queue_limits *b)
> > > +static void blk_stack_atomic_writes_chunk_sectors(struct queue_limits *t)
> > >   {
> > > -	if (b->atomic_write_hw_boundary &&
> > > -	    !blk_stack_atomic_writes_boundary_head(t, b))
> > > -		return false;
> > > +	unsigned int chunk_bytes = t->chunk_sectors << SECTOR_SHIFT;
> > 
> > What about integer overflow?
> 
> 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?

Mikulas