On Fri 08-02-13 16:44:06, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@xxxxxxxxxx>
>
> After converting unwritten extents from extent status tree in end_io, we
> can safely remove this bogus wait and don't worry about read stale data
> because we always try to lookup a block mapping in extent status tree
> firstly and unwritten extents in the tree has been converted at this
> time.
But you have to make sure the extent for the range really is in the
status tree (you didn't in the previous patch I think) and cannot be
reclaimed until the conversion is performed on disk.
Honza
> Before that commit, we need to flush unwritten ios before a dio read
> when dioread_nolock is enabled because in ext4_end_io_buffer_write and
> ext4_end_bio end_page_writeback() is called before converting unwritten
> extents in disk. So here is a window that a dio reader will read stale
> data as below if we don't wait for unwritten extents:
>
> dio read buffered write
> ->ext4_file_write
> ->ext4_da_write_begin
> ->ext4_da_write_end
> [buffered write has finished, but
> the data and metadata has not
> been flushed]
> ->generic_file_aio_read
> ->filemap_write_and_wait_range
> ->do_writepages
> ->ext4_da_writepages
> ->filemap_fdatawait_range
> ->wait_on_page_writeback
> ->ext4_end_bio
> ->end_page_writeback
> [unwritten extent has not been
> converted]
> ->ext4_ind_direct_IO
> [here we need to flush unwritten io]
>
> After that commit, we never need to wait for unwritten extents.
>
> dio read buffered write
> ->ext4_file_write
> ->ext4_da_write_begin
> ->ext4_da_write_end
> [buffered write has finished, but
> the data and metadata has not
> been flushed]
> ->generic_file_aio_read
> ->filemap_write_and_wait_range
> ->do_writepages
> ->ext4_da_writepages
> ->filemap_fdatawait_range
> ->wait_on_page_writeback
> ->ext4_end_bio
> ->ext4_es_convert_unwritten_extents
> ->end_page_writeback
> [unwritten extent has not been
> converted in disk, but they are
> converted in extent status tree]
> ->ext4_ind_direct_IO
> [here we will see the written
> extents in extent status tree]
>
>
> Signed-off-by: Zheng Liu <wenqing.lz@xxxxxxxxxx>
> Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> Cc: Jan kara <jack@xxxxxxx>
> ---
> fs/ext4/indirect.c | 5 -----
> 1 file changed, 5 deletions(-)
>
> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> index 20862f9..993247c 100644
> --- a/fs/ext4/indirect.c
> +++ b/fs/ext4/indirect.c
> @@ -807,11 +807,6 @@ ssize_t ext4_ind_direct_IO(int rw, struct kiocb *iocb,
>
> retry:
> if (rw == READ && ext4_should_dioread_nolock(inode)) {
> - if (unlikely(atomic_read(&EXT4_I(inode)->i_unwritten))) {
> - mutex_lock(&inode->i_mutex);
> - ext4_flush_unwritten_io(inode);
> - mutex_unlock(&inode->i_mutex);
> - }
> /*
> * Nolock dioread optimization may be dynamically disabled
> * via ext4_inode_block_unlocked_dio(). Check inode's state
> --
> 1.7.12.rc2.18.g61b472e
>
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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