• Darrick J. Wong's avatar
    xfs: mark speculative prealloc CoW fork extents unwritten · 5eda4300
    Darrick J. Wong authored
    Christoph Hellwig pointed out that there's a potentially nasty race when
    performing simultaneous nearby directio cow writes:
    
    "Thread 1 writes a range from B to c
    
    "                    B --------- C
                               p
    
    "a little later thread 2 writes from A to B
    
    "        A --------- B
                   p
    
    [editor's note: the 'p' denote cowextsize boundaries, which I added to
    make this more clear]
    
    "but the code preallocates beyond B into the range where thread
    "1 has just written, but ->end_io hasn't been called yet.
    "But once ->end_io is called thread 2 has already allocated
    "up to the extent size hint into the write range of thread 1,
    "so the end_io handler will splice the unintialized blocks from
    "that preallocation back into the file right after B."
    
    We can avoid this race by ensuring that thread 1 cannot accidentally
    remap the blocks that thread 2 allocated (as part of speculative
    preallocation) as part of t2's write preparation in t1's end_io handler.
    The way we make this happen is by taking advantage of the unwritten
    extent flag as an intermediate step.
    
    Recall that when we begin the process of writing data to shared blocks,
    we create a delayed allocation extent in the CoW fork:
    
    D: --RRRRRRSSSRRRRRRRR---
    C: ------DDDDDDD---------
    
    When a thread prepares to CoW some dirty data out to disk, it will now
    convert the delalloc reservation into an /unwritten/ allocated extent in
    the cow fork.  The da conversion code tries to opportunistically
    allocate as much of a (speculatively prealloc'd) extent as possible, so
    we may end up allocating a larger extent than we're actually writing
    out:
    
    D: --RRRRRRSSSRRRRRRRR---
    U: ------UUUUUUU---------
    
    Next, we convert only the part of the extent that we're actively
    planning to write to normal (i.e. not unwritten) status:
    
    D: --RRRRRRSSSRRRRRRRR---
    U: ------UURRUUU---------
    
    If the write succeeds, the end_cow function will now scan the relevant
    range of the CoW fork for real extents and remap only the real extents
    into the data fork:
    
    D: --RRRRRRRRSRRRRRRRR---
    U: ------UU--UUU---------
    
    This ensures that we never obliterate valid data fork extents with
    unwritten blocks from the CoW fork.
    Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    5eda4300
xfs_iomap.c 32.3 KB