linux-kernel - Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break

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Date:   Fri, 16 Mar 2018 12:10:05 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Chinner <david@...morbit.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break_layouts()

On Fri, Mar 16, 2018 at 12:04 PM, Christoph Hellwig <hch@....de> wrote:
> On Thu, Mar 15, 2018 at 08:52:29AM -0700, Dan Williams wrote:
>> In preparation for adding coordination between truncate operations and
>> busy dax-pages, extend xfs_break_layouts() to assume it must be called
>> with the mmap lock held. This locking scheme will be required for
>> coordinating the break of 'dax layouts' (non-idle dax (ZONE_DEVICE)
>> pages mapped into the file's address space).
>
> This requirement wasn't really there in the last series, why do we
> require it now?

It seems I misinterpreted your feedback.

>
> As far as I can tell all we'd need is to just drop this assert:
>
>> -     ASSERT(xfs_isilocked(ip, XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL));
>> +     ASSERT(xfs_isilocked(ip, XFS_IOLOCK_SHARED | XFS_IOLOCK_EXCL
>> +                             | XFS_MMAPLOCK_EXCL));
>
> entirely.
>
>>       while ((error = break_layout(inode, false) == -EWOULDBLOCK)) {
>>               xfs_iunlock(ip, *iolock);
>>               error = break_layout(inode, true);
>> -             *iolock = XFS_IOLOCK_EXCL;
>> +             *iolock &= ~XFS_IOLOCK_SHARED;
>> +             *iolock |= XFS_IOLOCK_EXCL;
>>               xfs_ilock(ip, *iolock);
>
> And take this one hunk from your patch.
>
> To enable the DAX use case.

Yeah, that looks good to me.