linux-ext4 - Re: [PATCH 06/29] dax: move the partition alignment check into fs_dax_get_by

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Date:   Wed, 24 Nov 2021 07:36:05 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>, dm-devel@...hat.com,
        linux-xfs@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-s390@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 06/29] dax: move the partition alignment check into
 fs_dax_get_by_bdev

On Tue, Nov 23, 2021 at 02:25:55PM -0800, Darrick J. Wong wrote:
> > +	if ((get_start_sect(bdev) * SECTOR_SIZE) % PAGE_SIZE ||
> > +	    (bdev_nr_sectors(bdev) * SECTOR_SIZE) % PAGE_SIZE) {
> 
> Do we have to be careful about 64-bit division here, or do we not
> support DAX on 32-bit?

I can't find anything in the Kconfig limiting DAX to 32-bit.  But
then again the existing code has divisions like this, so the compiler
is probably smart enough to turn them into shifts.

> > +		pr_info("%pg: error: unaligned partition for dax\n", bdev);
> 
> I also wonder if this should be ratelimited...?

This happens once (or maybe three times for XFS with rt and log devices)
at mount time, so I see no need for a ratelimit.