Date:	Fri, 31 Jul 2015 12:56:55 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Jan Kara <jack@...e.cz>
cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] jbd2: Limit number of reserved credits

On Fri, 31 Jul 2015, Jan Kara wrote:

> Date: Fri, 31 Jul 2015 12:46:04 +0200
> From: Jan Kara <jack@...e.cz>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
> Subject: Re: [PATCH] jbd2: Limit number of reserved credits
> 
> On Fri 31-07-15 12:22:43, Lukáš Czerner wrote:
> > On Fri, 31 Jul 2015, Jan Kara wrote:
> > 
> > > Date: Fri, 31 Jul 2015 11:46:39 +0200
> > > From: Jan Kara <jack@...e.cz>
> > > To: Lukas Czerner <lczerner@...hat.com>
> > > Cc: linux-ext4@...r.kernel.org, jack@...e.cz
> > > Subject: Re: [PATCH] jbd2: Limit number of reserved credits
> > > 
> > >   Hello,
> > > 
> > > On Fri 31-07-15 10:04:23, Lukas Czerner wrote:
> > > > Currently there is no limitation on number of reserved credits we can
> > > > ask for. If we ask for more reserved credits than 1/2 of maximum
> > > > transaction size, or if total number of credits exceeds the maximum
> > > > transaction size per operation (which is currently only possible with
> > > > the former) we will spin forever in start_this_handle().
> > > > 
> > > > Fix this by adding this limitation at the start of start_this_handle().
> > > > 
> > > > This patch also removes the credit limitation 1/2 of maximum transaction
> > > > size, since we really only want to limit the number of reserved credits.
> > > > There is not much point to limit the credits if there is still space in
> > > > the journal.
> > > > 
> > > > This accidentally also fixes the online resize, where due to the
> > > > limitation of the journal credits we're unable to grow file systems with
> > > > 1k block size and size between 16M and 32M. It has been partially fixed
> > > > by 2c869b262a10ca99cb866d04087d75311587a30c, but not entirely.
> > > > 
> > > > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > > > ---
> > > > 
> > > > Honzo I think that this should be enough to remove the limitation of 1/2 of
> > > > maximum transaction size for regular credits, but I might be missing
> > > > something, please let me know. Also do you have any specific test case to
> > > > exercise transaction reservation support - I've only ran xfstests.
> > > > 
> > > >  fs/jbd2/transaction.c | 22 +++++++++++++---------
> > > >  1 file changed, 13 insertions(+), 9 deletions(-)
> > > > 
> > > > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > > > index f3d0617..491a328 100644
> > > > --- a/fs/jbd2/transaction.c
> > > > +++ b/fs/jbd2/transaction.c
> > > > @@ -262,20 +262,24 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
> > > >  	int		rsv_blocks = 0;
> > > >  	unsigned long ts = jiffies;
> > > >  
> > > > +	if (handle->h_rsv_handle)
> > > > +		rsv_blocks = handle->h_rsv_handle->h_buffer_credits;
> > > > +
> > > >  	/*
> > > > -	 * 1/2 of transaction can be reserved so we can practically handle
> > > > -	 * only 1/2 of maximum transaction size per operation
> > > > +	 * Limit the number of reserved credits to 1/2 of maximum transaction
> > > > +	 * size and limit the number of total credits to not exceed maximum
> > > > +	 * transaction size per operation.
> > > >  	 */
> > > > -	if (WARN_ON(blocks > journal->j_max_transaction_buffers / 2)) {
> > > > -		printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n",
> > > > -		       current->comm, blocks,
> > > > -		       journal->j_max_transaction_buffers / 2);
> > > > +	if ((rsv_blocks > journal->j_max_transaction_buffers / 2) ||
> > > > +	    (rsv_blocks + blocks > journal->j_max_transaction_buffers)) {
> > > > +		printk(KERN_ERR "JBD2: %s wants too many credits "
> > > > +		       "credits:%d rsv_credits:%d max:%d\n",
> > > > +		       current->comm, blocks, rsv_blocks,
> > > > +		       journal->j_max_transaction_buffers);
> > > > +		WARN_ON(1);
> > > >  		return -ENOSPC;
> > > >  	}
> > > 
> > > Well, the trouble with this is the following: The currently running
> > > transaction has X reserved credits and Y normal credits. We know X+Y <=
> > > journal->j_max_transaction_buffers. Now you request additional A reserved
> > > and B normal credits. Suppose we cannot fit in the current transaction -
> > > i.e., X+Y+A+B > journal->j_max_transaction_buffers. The only thing we can do
> > > is to push running transaction to commit and start a new one. However, the
> > > new transaction will also have X reserved credits - you inherit reserved
> > > credits from the previous transaction until they are converted to normal
> > > credits. So if X+A+B is still > journal->j_max_transaction_buffers, you
> > > still cannot start current handle and you'd have to wait until someone
> > > converts his reserved credits.
> > 
> > Ok I understand, but isn't this true either way ? If anything the
> > limit might make it worse in that case because if
> > 
> > X+A+B is still > journal->j_max_transaction_buffers
> > 
> > in the new case without the limit then it's definitely true for the
> > case with the limit as well. The number of reserved credits is
> > limited in both cases so it's not really a factor, is it ?
> > 
> > Yes in the limitless case it might happen that we have so much
> > normal credits that we can't fit in the reserved credits so we have
> > to commit and start a new one, but that's true in both cases only
> > with the limit it will happen sooner and possible more often because
> > we just have less space to work with.
> > 
> > Sorry if I am asking dumb questions, but I am trying to understand
> > how is this supposed to work.
> > 
> > And above all that limitation we're talking about is a hard limit
> > which you're not supposed to hit ever. Only if something is really
> > wrong and is asking for a handle with way too much credits...that's
> > not what can normally happen. So what's the problem again ?
> 
> Thanks for correcting me! I was conflating two different conditions in the
> transaction handling code. So with the change you propose, it would be only
> possible that starting of large handles would keep pushing transactions to
> commit because it couldn't fit the handle into the running transaction
> because of reserved credits. So if we wanted to relieve the condition as
> you suggest, we'd also need to modify the logic in
> add_transaction_credits() to wait on j_wait_reserved in case number of
> reserved credits of current trans + number of credits requested for the handle
> is too big. But that looks doable...

Ah, right. Thanks, I'll resend the patch.

-Lukas

> 
> 								Honza
> 
> > 
> > Thanks!
> > -Lukas
> > 
> > 
> > > 
> > > However these waits will create journal stalls causing possible performance
> > > issues and also introduce a lock dependency - suddently you are not allowed
> > > to acquire locks ranking above transaction start before starting a reserved
> > > handle (as these locks can be held by processes being stuck waiting for
> > > reserved credits to convert).
> > > 
> > > So overall halving the maximum allowed credits seemed like the least
> > > painful solution to the problem.
> > > 
> > > 								Honza
> > > >  
> > > > -	if (handle->h_rsv_handle)
> > > > -		rsv_blocks = handle->h_rsv_handle->h_buffer_credits;
> > > > -
> > > >  alloc_transaction:
> > > >  	if (!journal->j_running_transaction) {
> > > >  		/*
> > > > -- 
> > > > 1.8.3.1
> > > > 
> > > 
> 

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