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Date:   Fri, 26 Apr 2019 05:45:53 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Dave Chinner <david@fromorbit.com>
Cc:     Jerome Glisse <jglisse@redhat.com>,
        lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [LSF/MM TOPIC] Direct block mapping through fs for device
Message-ID: <20190426124553.GB12339@infradead.org>
References: <20190426013814.GB3350@redhat.com>
 <20190426062816.GG1454@dread.disaster.area>
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On Fri, Apr 26, 2019 at 04:28:16PM +1000, Dave Chinner wrote:
> i.e. go look at how xfs_pnfs.c works to hand out block mappings to
> remote pNFS clients so they can directly access the underlying
> storage. Basically, anyone wanting to map blocks needs a file layout
> lease and then to manage the filesystem state over that range via
> these methods in the struct export_operations:
> 
>         int (*get_uuid)(struct super_block *sb, u8 *buf, u32 *len, u64 *offset);
>         int (*map_blocks)(struct inode *inode, loff_t offset,
>                           u64 len, struct iomap *iomap,
>                           bool write, u32 *device_generation);
>         int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
>                              int nr_iomaps, struct iattr *iattr);

Nipick:  get_uuid isn't needed for the least itself, it just works
around the fact that the original pNFS/block protocol is braindead.
The pNFS/SCSI prototocol already switches to a device UUID, and other
users that work locally shouldn't need it either.