Re: [PATCH v2 2/3] jbd2: Add extra parameter in start_this_handle() to control allocation flags.

[Jan Kara <jack@suse.cz>]

On Fri 27-05-11 23:20:57, Manish Katiyar wrote:
> changes from v1 -> v2 :
> *) Update start_this_handle to take extra parameter to specify whether
> to retry the allocation or not.
> *) Added jbd allocation flags for callers to control the transaction allocation
> behavior. Callers can pass JBD2_TOPLEVEL if allocation needs to be done using GFP_KERNEL.
  The above changelog should be below where (*) is.

Also - this is mainly for Ted: I've looked at where we JBD2_TOPLEVEL could
actually be enabled and the results are: Pretty much nowhere.

The problem is that with ext4, we need i_mutex in io completion path to
end page writeback. So we cannot do GFP_KERNEL allocation whenever we hold
i_mutex because mm might wait in direct reclaim for IO to complete and that
cannot happen until we release i_mutex. And pretty much every write path in
ext4 holds i_mutex.

So JBD2_TOPLEVEL looks like a useless excercise to me and I'd just don't do
it.

> Pass extra flags in journal routines to specify if its ok to
> fail in the journal transaction allocation. Passing JBD2_FAIL_OK means caller is
> ok with journal start failures and can handle ENOMEM.
> 
> Update ocfs2 and ext4 routines to pass JBD2_NO_FAIL for the updated journal
> interface by default, to retain the existing behavior.
> 
> Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
> ---
>  fs/ext4/ext4_jbd2.h   |    2 +-
>  fs/ext4/super.c       |    2 +-
>  fs/jbd2/transaction.c |   44 ++++++++++++++++----------------------------
>  fs/ocfs2/journal.c    |    8 ++++----
>  include/linux/jbd2.h  |   13 +++++++++----
>  5 files changed, 31 insertions(+), 38 deletions(-)
  (*) HERE


> +/* JBD2 transaction allocation flags */
> +#define JBD2_NO_FAIL	0x00000001
> +#define JBD2_FAIL_OK	0x00000002
> +#define JBD2_TOPLEVEL	0x00000004
> +
  I guess there's no need for JBD2_FAIL_OK - if NOFAIL is not set, we can
fail. Otherwise the patch looks OK.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR