Re: [PATCH v9 9/9] xfs, dax: introduce xfs_break_dax_layouts()

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(a)fromorbit.com&gt; Cc: "Darrick J. Wong" <darrick.wong(a)oracle.com&gt; Cc: Ross Zwisler <ross.zwisler(a)linux.intel.com&gt; Reported-by: Jan Kara <jack(a)suse.cz&gt; Cc: Christoph Hellwig <hch(a)lst.de&gt; Signed-off-by: Dan Williams <dan.j.williams(a)intel.com&gt;

+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));

+ + error = xfs_break_dax_layouts(inode, *iolock, &retry); + /* fall through */ + case BREAK_WRITE: + if (error || retry) + break;

+ error = xfs_break_leased_layouts(inode, iolock, &retry); + break; + default: + WARN_ON_ONCE(1); + return -EINVAL; + } + } while (error == 0 && retry);