linux-ext4 - Re: [PATCH] jbd2: preserve original nofs flag during journal restart

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Message-ID: <20170517141357.GN18247@dhcp22.suse.cz>
Date:   Wed, 17 May 2017 16:13:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jbd2: preserve original nofs flag during journal restart

On Wed 17-05-17 05:33:01, Tahsin Erdogan wrote:
> When a transaction starts, start_this_handle() saves current
> PF_MEMALLOC_NOFS value so that it can be restored at journal stop time.
> Journal restart is a special case that calls start_this_handle() without
> stopping the transaction. start_this_handle() isn't aware that the
> original value is already stored so it overwrites it with current value.
> 
> For instance, a call sequence like below leaves PF_MEMALLOC_NOFS flag set
> at the end:
> 
>   jbd2_journal_start()
>   jbd2__journal_restart()
>   jbd2_journal_stop()
> 
> Make jbd2__journal_restart() restore the original value before calling
> start_this_handle().
> 
> Fixes: 81378da64de6 ("jbd2: mark the transaction context with the scope GFP_NOFS context")
> Signed-off-by: Tahsin Erdogan <tahsin@...gle.com>
> ---
>  fs/jbd2/transaction.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 9ee4832b6f8b..dfd6afebdfeb 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -680,6 +680,7 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
>  
>  	rwsem_release(&journal->j_trans_commit_map, 1, _THIS_IP_);
>  	handle->h_buffer_credits = nblocks;
> +	memalloc_nofs_restore(handle->saved_alloc_context);
>  	ret = start_this_handle(journal, handle, gfp_mask);
>  	return ret;
>  }

I remember Jack has mentioned something about nested transaction back
then when reviewing the patch. But I cannot remember or find a pointer
to that email. I have a vague recollection that there is a reference
counting for those transactions.

Anyway, Is this patch really correct? So let's say we are in
the transaction context already and then you disable the scope
NOFS protection, start_this_handle will allocate before it calls
memalloc_nofs_save and that would recurse to the filesystem.  If
anything wouldn't it be better to simply call memalloc_nofs_save only if
we start a new transaction? I thought we were doing that already but the
code is so convoluted I have hard time to wrap my head around it.

-- 
Michal Hocko
SUSE Labs