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    SubjectRe: Latency writing to an mlocked ext4 mapping
    On Wed, Oct 19, 2011 at 7:17 PM, Andy Lutomirski <> wrote:
    > On Wed, Oct 19, 2011 at 6:15 PM, Andy Lutomirski <> wrote:
    >> On Wed, Oct 19, 2011 at 6:02 PM, Andreas Dilger <> wrote:
    >>> What kernel are you using?  A change to keep pages consistent during writeout was landed not too long ago (maybe Linux 3.0) in order to allow checksumming of the data.
    >> 3.0.6, with no relevant patches.  (I have a one-liner added to the tcp
    >> code that I'll submit sometime soon.)  Would this explain the latency
    >> in file_update_time or is that a separate issue?  file_update_time
    >> seems like a good thing to make fully asynchronous (especially if the
    >> file in question is a fifo, but I've already moved my fifos to tmpfs).
    > On, I got one instance of:
    > call_rwsem_down_read_failed ext4_map_blocks ext4_da_get_block_prep
    > __block_write_begin ext4_da_write_begin ext4_page_mkwrite do_wp_page
    > handle_pte_fault handle_mm_fault do_page_fault page_fault
    > but I'm not seeing the large numbers of the ext4_page_mkwrite trace
    > that I get on 3.0.6.  file_update_time is now by far the dominant
    > cause of latency.

    The culprit seems to be do_wp_page -> file_update_time ->
    mark_inode_dirty_sync. This surprises me for two reasons:

    - Why the _sync? Are we worried that data will be written out before
    the metadata? If so, surely there's a better way than adding latency

    - Why are we calling file_update_time at all? Presumably we also
    update the time when the page is written back (if not, that sounds
    like a bug, since the contents may be changed after something saw the
    mtime update), and, if so, why bother updating it on the first write?
    Anything that relies on this behavior is, I think, unreliable, because
    the page could be made writable arbitrarily early by another program
    that changes nothing.

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      Last update: 2011-10-20 08:03    [W:0.022 / U:0.724 seconds]
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