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Subject: Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.
To:     Christoph Hellwig <hch@infradead.org>
Cc:     NeilBrown <neilb@suse.de>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>, linux-nfs@vger.kernel.org,
        Wang Yugui <wangyugui@e16-tech.com>,
        Ulli Horlacher <framstag@rus.uni-stuttgart.de>,
        linux-btrfs@vger.kernel.org
References: <20210613115313.BC59.409509F4@e16-tech.com>
 <20210310074620.GA2158@tik.uni-stuttgart.de>
 <162632387205.13764.6196748476850020429@noble.neil.brown.name>
 <edd94b15-90df-c540-b9aa-8eac89b6713b@toxicpanda.com>
 <YPBmGknHpFb06fnD@infradead.org>
From:   Josef Bacik <josef@toxicpanda.com>
Message-ID: <28bb883d-8d14-f11a-b37f-d8e71118f87f@toxicpanda.com>
Date:   Thu, 15 Jul 2021 13:11:29 -0400
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On 7/15/21 12:45 PM, Christoph Hellwig wrote:
> On Thu, Jul 15, 2021 at 10:09:37AM -0400, Josef Bacik wrote:
>> I'm going to restate what I think the problem is you're having just so I'm
>> sure we're on the same page.
>>
>> 1. We export a btrfs volume via nfsd that has multiple subvolumes.
>> 2. We run find, and when we stat a file, nfsd doesn't send along our bogus
>> st_dev, it sends it's own thing (I assume?).  This confuses du/find because
>> you get the same inode number with different parents.
>>
>> Is this correct?  If that's the case then it' be relatively straightforward
>> to add another callback into export_operations to grab this fsid right?
>> Hell we could simply return the objectid of the root since that's unique
>> across the entire file system.  We already do our magic FH encoding to make
>> sure we keep all this straight for NFS, another callback to give that info
>> isn't going to kill us.  Thanks,
> 
> Hell no.  btrfs is broken plain and simple, and we've been arguing about
> this for years without progress.  btrfs needs to stop claiming different
> st_dev inside the same mount, otherwise hell is going to break lose left
> right and center, and this is just one of the many cases where it does.
> 

Because there's no alternative.  We need a way to tell userspace they've 
wandered into a different inode namespace.  There's no argument that what we're 
doing is ugly, but there's never been a clear "do X instead".  Just a lot of 
whinging that btrfs is broken.  This makes userspace happy and is simple and 
straightforward.  I'm open to alternatives, but there have been 0 workable 
alternatives proposed in the last decade of complaining about it.  Thanks,

Josef