On Wed, May 9, 2018 at 5:27 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Tue 24-04-18 16:33:50, 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.
>>
>> Although XFS_MMAPLOCK_EXCL is dropped while waiting, XFS_IOLOCK_EXCL is
>> held while sleeping. We need this to prevent starvation of the truncate
>> path as continuous submission of direct-I/O could starve the truncate
>> path indefinitely if the lock is dropped.
>>
>> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
>> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Reported-by: Jan Kara <jack@xxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>
> Looks good to me except some nits below. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
> for as much as it is worth with XFS code ;)
>
>> +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_dax_page(inode, did_unlock));
>> +}
>> +
>> int
>> xfs_break_layouts(
>> struct inode *inode,
>> @@ -729,17 +760,23 @@ xfs_break_layouts(
>>
>> ASSERT(xfs_isilocked(XFS_I(inode), XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL));
>>
>> - switch (reason) {
>> - case BREAK_UNMAP:
>> - ASSERT(xfs_isilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL));
>> - /* fall through */
>> - case BREAK_WRITE:
>> - error = xfs_break_leased_layouts(inode, iolock, &retry);
>> - break;
>> - default:
>> - WARN_ON_ONCE(1);
>> - return -EINVAL;
>> - }
>> + do {
>> + switch (reason) {
>> + case BREAK_UNMAP:
>> + ASSERT(xfs_isilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL));
>
> Maybe move the assertion to xfs_break_dax_layouts()?
Makes sense, sure.
>
>> +
>> + error = xfs_break_dax_layouts(inode, *iolock, &retry);
>> + /* fall through */
>> + case BREAK_WRITE:
>> + if (error || retry)
>> + break;
>
> The error handling IMHO belongs above the 'fall through' comment above.
Ok, yes I think this location is a stale holdover.
>
>> + error = xfs_break_leased_layouts(inode, iolock, &retry);
>> + break;
>> + default:
>> + WARN_ON_ONCE(1);
>> + return -EINVAL;
>> + }
>> + } while (error == 0 && retry);
>
> As a general 'taste' comment, I prefer if the 'retry' is always initialized
> to 'false' at the beginning of the loop body in these kinds of loops. That
> way it is obvious we are doing the right thing when looking at the loop
> body and we don't have to verify that each case statement initializes
> 'retry' properly (in fact I'd remove the initialization from
> xfs_break_dax_layouts() and xfs_break_leased_layouts()). But this is more a
> matter of taste and consistency with other code in the area so I defer to
> XFS maintainers for a final opinion. Darrick?
I'm fine with making this change, I'll proceed unless / until Darrick says no.
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