Re: [PATCH v7 14/14] xfs, dax: introduce xfs_break_dax_layouts()

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Mar 22, 2018 at 12:43 AM, Christoph Hellwig <hch@lst.de> wrote:
> On Wed, Mar 21, 2018 at 03:58:31PM -0700, Dan Williams wrote:
>> xfs_break_dax_layouts(), similar to xfs_break_leased_layouts(), scans
>> for busy / pinned dax pages and waits for those pages to go idle before
>> any potential extent unmap operation.
>>
>> dax_layout_busy_page() handles synchronizing against new page-busy
>> events (get_user_pages). It invalidates all mappings to trigger the
>> get_user_pages slow path which will eventually block on the xfs inode
>> lock held in XFS_MMAPLOCK_EXCL mode. If dax_layout_busy_page() finds a
>> busy page it returns it for xfs to wait for the page-idle event that
>> will fire when the page reference count reaches 1 (recall ZONE_DEVICE
>> pages are idle at count 1, see generic_dax_pagefree()).
>>
>> While waiting, the XFS_MMAPLOCK_EXCL lock is dropped in order to not
>> deadlock the process that might be trying to elevate the page count of
>> more pages before arranging for any of them to go idle. I.e. the typical
>> case of submitting I/O is that iov_iter_get_pages() elevates the
>> reference count of all pages in the I/O before starting I/O on the first
>> page. The process of elevating the reference count of all pages involved
>> in an I/O may cause faults that need to take XFS_MMAPLOCK_EXCL.
>>
>> Cc: Jan Kara <jack@suse.cz>
>> Cc: Dave Chinner <david@fromorbit.com>
>> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
>> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>>  fs/xfs/xfs_file.c |   59 ++++++++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 49 insertions(+), 10 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>> index 7f37fadf007e..d4573f93fddb 100644
>> --- a/fs/xfs/xfs_file.c
>> +++ b/fs/xfs/xfs_file.c
>> @@ -752,6 +752,38 @@ xfs_file_write_iter(
>>       return ret;
>>  }
>>
>> +static void
>> +xfs_wait_var_event(
>> +     struct inode            *inode,
>> +     uint                    iolock,
>> +     bool                    *did_unlock)
>> +{
>> +     struct xfs_inode        *ip = XFS_I(inode);
>> +
>> +     *did_unlock = true;
>> +     xfs_iunlock(ip, iolock);
>> +     schedule();
>> +     xfs_ilock(ip, iolock);
>> +}
>> +
>> +static int
>> +xfs_break_dax_layouts(
>> +     struct inode            *inode,
>> +     uint                    iolock,
>> +     bool                    *did_unlock)
>> +{
>> +     struct page             *page;
>> +
>> +     *did_unlock = false;
>> +     page = dax_layout_busy_page(inode->i_mapping);
>> +     if (!page)
>> +             return 0;
>> +
>> +     return ___wait_var_event(&page->_refcount,
>> +                     atomic_read(&page->_refcount) == 1, TASK_INTERRUPTIBLE,
>> +                     0, 0, xfs_wait_var_event(inode, iolock, did_unlock));
>> +}
>> +
>>  int
>>  xfs_break_layouts(
>>       struct inode            *inode,
>> @@ -766,16 +798,23 @@ xfs_break_layouts(
>>                               | (reason == BREAK_UNMAPI
>>                                       ? XFS_MMAPLOCK_EXCL : 0)));
>>
>> -     switch (reason) {
>> -     case BREAK_UNMAPI:
>> -             /* fall through */
>> -     case BREAK_WRITE:
>> -             error = xfs_break_leased_layouts(inode, iolock, &did_unlock);
>> -             break;
>> -     default:
>> -             error = -EINVAL;
>> -             break;
>> -     }
>> +     do {
>> +             switch (reason) {
>> +             case BREAK_UNMAPI:
>> +                     error = xfs_break_dax_layouts(inode, *iolock,
>> +                                     &did_unlock);
>> +                     /* fall through */
>> +             case BREAK_WRITE:
>> +                     if (error || did_unlock)
>> +                             break;
>> +                     error = xfs_break_leased_layouts(inode, iolock,
>> +                                     &did_unlock);
>> +                     break;
>> +             default:
>> +                     error = -EINVAL;
>> +                     break;
>> +             }
>> +     } while (error == 0 && did_unlock);
>
> Nitpick:  I think did_unlock should probably be called done in this context,
> and how about something like this:
>
>         for (;;) {
>                 switch (reason) {
>                 case BREAK_UNMAPI:
>                         error = xfs_break_dax_layouts(inode, *iolock, &done);
>                         if (error || done)
>                                 return error;
>                         /*fallthrough*/
>                 case BREAK_WRITE:
>                         error = xfs_break_leased_layouts(inode, iolock, &done);
>                         if (error || done)
>                                 return error;
>                         break;
>                 default:
>                         WARN_ON_ONCE(1);
>                         return -EINVAL;
>                 }
>         }

I can switch to the for() loop model, but it isn't 'done' flag, it's a
'retry' flag. I.e. if xfs_break_leased_layouts() dropped
XFS_MMAPLOCK_EXCL we need to retry xfs_break_dax_layouts(), and if
xfs_break_dax_layouts() slept we need to immediately retry it. So, how
about this?

for (;;) {
        switch (reason) {
        case BREAK_UNMAP:
                ASSERT(xfs_isilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL));

                error = xfs_break_dax_layouts(inode, *iolock, &retry);
                if (error)
                        return error;
                if (retry)
                        continue;
                /* fall through */
        case BREAK_WRITE:
                error = xfs_break_leased_layouts(inode, iolock, &retry);
                if (error || !retry)
                        return error;
                continue;
        default:
                WARN_ON_ONCE(1);
                return -EINVAL;
        }
}
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