From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: [PATCH] jbd2: Limit number of reserved credits Date: Fri, 31 Jul 2015 12:46:04 +0200 Message-ID: <20150731104604.GB22869@quack.suse.cz> References: <1438329863-26422-1-git-send-email-lczerner@redhat.com> <20150731094639.GB16125@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Jan Kara , linux-ext4@vger.kernel.org To: =?utf-8?B?THVrw6HFoQ==?= Czerner Return-path: Received: from mx2.suse.de ([195.135.220.15]:34506 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751826AbbGaKqH (ORCPT ); Fri, 31 Jul 2015 06:46:07 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri 31-07-15 12:22:43, Luk=C3=A1=C5=A1 Czerner wrote: > On Fri, 31 Jul 2015, Jan Kara wrote: >=20 > > Date: Fri, 31 Jul 2015 11:46:39 +0200 > > From: Jan Kara > > To: Lukas Czerner > > Cc: linux-ext4@vger.kernel.org, jack@suse.cz > > Subject: Re: [PATCH] jbd2: Limit number of reserved credits > >=20 > > Hello, > >=20 > > 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 maxim= um > > > transaction size per operation (which is currently only possible = with > > > the former) we will spin forever in start_this_handle(). > > >=20 > > > Fix this by adding this limitation at the start of start_this_han= dle(). > > >=20 > > > This patch also removes the credit limitation 1/2 of maximum tran= saction > > > size, since we really only want to limit the number of reserved c= redits. > > > There is not much point to limit the credits if there is still sp= ace in > > > the journal. > > >=20 > > > This accidentally also fixes the online resize, where due to the > > > limitation of the journal credits we're unable to grow file syste= ms with > > > 1k block size and size between 16M and 32M. It has been partially= fixed > > > by 2c869b262a10ca99cb866d04087d75311587a30c, but not entirely. > > >=20 > > > Signed-off-by: Lukas Czerner > > > --- > > >=20 > > > 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 miss= ing > > > something, please let me know. Also do you have any specific test= case to > > > exercise transaction reservation support - I've only ran xfstests= =2E > > >=20 > > > fs/jbd2/transaction.c | 22 +++++++++++++--------- > > > 1 file changed, 13 insertions(+), 9 deletions(-) > > >=20 > > > 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 *jou= rnal, handle_t *handle, > > > int rsv_blocks =3D 0; > > > unsigned long ts =3D jiffies; > > > =20 > > > + if (handle->h_rsv_handle) > > > + rsv_blocks =3D handle->h_rsv_handle->h_buffer_credits; > > > + > > > /* > > > - * 1/2 of transaction can be reserved so we can practically han= dle > > > - * only 1/2 of maximum transaction size per operation > > > + * Limit the number of reserved credits to 1/2 of maximum trans= action > > > + * size and limit the number of total credits to not exceed max= imum > > > + * 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; > > > } > >=20 > > Well, the trouble with this is the following: The currently running > > transaction has X reserved credits and Y normal credits. We know X+= Y <=3D > > journal->j_max_transaction_buffers. Now you request additional A re= served > > and B normal credits. Suppose we cannot fit in the current transact= ion - > > 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. Howev= er, the > > new transaction will also have X reserved credits - you inherit res= erved > > credits from the previous transaction until they are converted to n= ormal > > 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 some= one > > converts his reserved credits. >=20 > Ok I understand, but isn't this true either way ? If anything the > limit might make it worse in that case because if >=20 > X+A+B is still > journal->j_max_transaction_buffers >=20 > 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 ? >=20 > 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. >=20 > Sorry if I am asking dumb questions, but I am trying to understand > how is this supposed to work. >=20 > 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 a= s 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... Honza >=20 > Thanks! > -Lukas >=20 >=20 > >=20 > > However these waits will create journal stalls causing possible per= formance > > 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). > >=20 > > So overall halving the maximum allowed credits seemed like the leas= t > > painful solution to the problem. > >=20 > > Honza > > > =20 > > > - if (handle->h_rsv_handle) > > > - rsv_blocks =3D handle->h_rsv_handle->h_buffer_credits; > > > - > > > alloc_transaction: > > > if (!journal->j_running_transaction) { > > > /* > > > --=20 > > > 1.8.3.1 > > >=20 > >=20 --=20 Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html