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

Dan Williams
2018-03-22T16:28:15Z

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;
}
}