linux-ext4 - Re: [PATCH 10/10 v5] ext4: remove bogus wait for unwritten extents in ext4_ind_direct

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130212125819.GE19583@quack.suse.cz>
Date:	Tue, 12 Feb 2013 13:58:19 +0100
From:	Jan Kara <jack@...e.cz>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org, Zheng Liu <wenqing.lz@...bao.com>,
	Theodore Ts'o <tytso@....edu>, Jan kara <jack@...e.cz>
Subject: Re: [PATCH 10/10 v5] ext4: remove bogus wait for unwritten extents
 in ext4_ind_direct_IO

On Fri 08-02-13 16:44:06, Zheng Liu wrote:
> From: Zheng Liu <wenqing.lz@...bao.com>
> 
> 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@...bao.com>
> Cc: "Theodore Ts'o" <tytso@....edu>
> Cc: Jan kara <jack@...e.cz>
> ---
>  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@...e.cz>
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html