linux-ext4 - Re: [PATCH 2/3] jbd2 : Fix journal start by passing a parameter to specify if the caller can deal with ENOMEM

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Date:	Wed, 25 May 2011 22:22:51 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Manish Katiyar <mkatiyar@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/3] jbd2 : Fix journal start by passing a parameter to
 specify if the caller can deal with ENOMEM

On Wed, May 25, 2011 at 10:13:33AM +0200, Jan Kara wrote:
> > 
> >    ok.. I will do it as a separate patch.
>   Well, patch 2/3 does not really make too much sense without it (errok
> parameter isn't used) so there's no reason to do it as a separate patch.
> Just add it to this patch please.

Agreed; right now this whole patch series is a no-op, since errok
isn't getting used for anything.  So fixing errok so it's passed to
start_this_handle() seems to be more in the category of "fix the
patch" more than anything else.

One more thing; perhaps we should be passing in a integer so we can
pass in a flag word.  That way you don't need to have a fail_ok
variant.  It's a lot more obvious if you have a call:

	  handle = ext4_journal_start(inode, 1, JBD2_FAIL_OK);

What we can also do is this:

	  handle = ext4_journal_start(inode, 1, JBD2_FAIL_OK | JBD2_TOPLEVEL);

What JBD2_TOPLEVEL means is that caller is from a top-level file
system function, such as ext4_symlink() or ext4_chmod(), such that
start_this_handle() can use GFP_KERNEL instead of GFP_NOFS.  GFP_NOFS
is needed for any function that might get called by the direct reclaim
path (i.e., the writepage() function).  But for the top-level
symlink() or chmod() function, it's actually OK to allocate memory
using GFP_KERNEL, since there's no potential recursion problem.

      		  		   	     	       - Ted
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