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    SubjectRe: Direct IO for fat
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    Date
    Jan Kara <jack@suse.cz> writes:

    >> FAT has to fill the hole completely, but DIO doesn't seems to do.
    >>
    >> e.g.
    >> fd = open("file", O_WRONLY | O_CREAT | O_TRUNC);
    >> write(fd, buf, 512);
    >> lseek(fd, 10000, SEEK_SET);
    >> write(fd, buf, 512);
    >>
    >> We need to allocate the blocks on 512 ~ 10000, and fill it with zero.
    >> However, I think DIO doesn't fill it. If I'm missing something, please
    >> let me know, I'll kill that check.
    > I know. DIO doesn't do it. But the point is that if blockdev_direct_IO
    > finds out it should allocate new blocks, it exits without allocating them.
    > Then in __generic_file_aio_write_nolock() if we find out that we did not
    > write everything in generic_file_direct_write(), we just call
    > generic_file_buffered_write() to write the unwritten part.
    > Hence, in case you describe above, the second write() finds out that
    > block is not allocated and eventually everything falls back to calling
    > generic_file_buffered_write() which calls prepare_write() and everything is
    > happy.

    I see. But sorry, I can't see where is preventing it... Finally, I
    think we do the following on second write(2).

    This is write, so create == 1, and ->lock_type == DIO_LOCKING,
    and dio->block_in_file > ->i_size, so DIO callback fat_get_block() with
    create == 1.

    Then fat_get_block() seems to allocate block without fill hole,
    because it can't know caller is prepre_write or not...
    Well, anyway I'll test it on weekend. Thanks.

    -> blockdev_direct_IO()
    -> direct_io_worker()
    -> do_direct_IO()
    -> get_more_blocks()

    create = dio->rw & WRITE;
    if (dio->lock_type == DIO_LOCKING) {
    if (dio->block_in_file < (i_size_read(dio->inode) >>
    dio->blkbits))
    create = 0;
    } else if (dio->lock_type == DIO_NO_LOCKING) {
    create = 0;
    }

    /*
    * For writes inside i_size we forbid block creations: only
    * overwrites are permitted. We fall back to buffered writes
    * at a higher level for inside-i_size block-instantiating
    * writes.
    */
    ret = (*dio->get_block)(dio->inode, fs_startblk,
    map_bh, create);
    --
    OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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