From: Jan Kara Subject: Re: [PATCH-v2] JBD: Fix race between free buffer and commit trasanction Date: Wed, 28 May 2008 20:55:55 +0200 Message-ID: <20080528185554.GA16018@duck.suse.cz> References: <1210971693.3608.46.camel@localhost.localdomain> <20080518223739.GB11006@atrey.karlin.mff.cuni.cz> <1211227158.3663.25.camel@localhost.localdomain> <20080519132553.de9b78b0.akpm@linux-foundation.org> <1211234829.3663.39.camel@localhost.localdomain> <1211306575.3664.19.camel@localhost.localdomain> <20080520235303.GB23521@atrey.karlin.mff.cuni.cz> <1211390093.5571.16.camel@BVR-FS.beaverton.ibm.com> <20080524224447.GE20563@atrey.karlin.mff.cuni.cz> <1211998739.3791.20.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , pbadari@us.ibm.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, Jens Axboe To: Mingming Cao Return-path: Received: from styx.suse.cz ([82.119.242.94]:48785 "EHLO mail.suse.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751340AbYE1Sz5 (ORCPT ); Wed, 28 May 2008 14:55:57 -0400 Content-Disposition: inline In-Reply-To: <1211998739.3791.20.camel@localhost.localdomain> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed 28-05-08 11:18:59, Mingming Cao wrote: > On Sun, 2008-05-25 at 00:44 +0200, Jan Kara wrote: > > > On Wed, 2008-05-21 at 01:53 +0200, Jan Kara wrote: > > > > > fs/jbd/transaction.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++-- > > > > > mm/filemap.c | 3 -- > > > > > 2 files changed, 54 insertions(+), 4 deletions(-) > > > > > > > > > > Index: linux-2.6.26-rc2/fs/jbd/transaction.c > > > > > =================================================================== > > > > > --- linux-2.6.26-rc2.orig/fs/jbd/transaction.c 2008-05-11 17:09:41.000000000 -0700 > > > > > +++ linux-2.6.26-rc2/fs/jbd/transaction.c 2008-05-19 16:16:41.000000000 -0700 > > > > > @@ -1648,12 +1648,39 @@ out: > > > > > return; > > > > > } > > > > > > > > > > +/* > > > > > + * journal_try_to_free_buffers() could race with journal_commit_transaction() > > > > > + * The later might still hold the reference count to the buffers when inspecting > > > > > + * them on t_syncdata_list or t_locked_list. > > > > > + * > > > > > + * Journal_try_to_free_buffers() will call this function to > > > > > + * wait for the current transaction to finish syncing data buffers, before > > > > > + * try to free that buffer. > > > > > + * > > > > > + * Called with journal->j_state_lock hold. > > > > > + */ > > > > > +static void journal_wait_for_transaction_sync_data(journal_t *journal) > > > > > +{ > > > > > + transaction_t *transaction = NULL; > > > > > + tid_t tid; > > > > > + > > > > > + transaction = journal->j_committing_transaction; > > > > > + > > > > > + if (!transaction) > > > > > + return; > > > > > + > > > > > + tid = transaction->t_tid; > > > > > + spin_unlock(&journal->j_state_lock); > > > > > + log_wait_commit(journal, tid); > > > > > + spin_lock(&journal->j_state_lock); > > > > > +} > > > > What is actually the point of entering the function with j_state_lock > > > > held and also keeping it after return? It should be enough to take it > > > > and release it just inside this function, shouldn't it? > > > > > > > > > > I was worried about the case when we call try_to_free_buffers() again, > > > it races with the current transaction commit again. Is it possible? I > > > guess the question is whether it is possible to have buffers on the same > > > page attached to different transaction. If so, I think we need to keep > > > the journal state lock while retry try_to_free_buffers(), so that the > > > retry won't race with the commit transaction again... > > Well, but by the time log_wait_commit() finishes, it may well > > happen that a new transaction is already started so your lock doesn't > > help you much. And the page you are called on is actually locked, so > > noone can really mess with it until you unlock it... So I think you can > > just use the lock for obtaining tid and then drop it. > > > > You are right that the page was locked during the process we are trying > to free the buffer. so I agree it's safe to drop the lock. > > > Honza > > > > PS: For JBD2 you'd need to be a bit more careful because you cannot call > > log_wait_commit() while holding page lock (we have reversed locking > > order for ext4) - but ordered-mode rewrite patch actually fixes this > > problem and I'm going to submit the splitted patches on Monday or > > Tuesday (I only need to test them that I didn't do something stupid > > while porting them to ext4)... > > > Thanks for pointing this out. I think when we put back the reversed > locking order and new ordered mode the jbd2 patch could go away... > > Updated patch for JBD (take 4) below. > Mingming > > JBD: fix race between journal_try_to_free_buffers() and jbd commit transaction > > From: Mingming Cao > > journal_try_to_free_buffers() could race with jbd commit transaction when > the later is holding the buffer reference while waiting for the data buffer > to flush to disk. If the caller of journal_try_to_free_buffers() request > tries hard to release the buffers, it will treat the failure as error and return > back to the caller. We have seen the directo IO failed due to this race. > Some of the caller of releasepage() also expecting the buffer to be dropped > when passed with GFP_KERNEL mask to the releasepage()->journal_try_to_free_buffers(). > > With this patch, if the caller is passing the GFP_KERNEL to indicating this > call could wait, in case of try_to_free_buffers() failed, let's waiting for > journal_commit_transaction() to finish commit the current committing transaction > , then try to free those buffers again with journal locked. > > Signed-off-by: Mingming Cao > Reviewed-by: Badari Pulavarty > --- > fs/jbd/transaction.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++-- > mm/filemap.c | 3 -- > 2 files changed, 56 insertions(+), 4 deletions(-) > > Index: linux-2.6.26-rc3/fs/jbd/transaction.c > =================================================================== > --- linux-2.6.26-rc3.orig/fs/jbd/transaction.c 2008-05-28 10:55:37.000000000 -0700 > +++ linux-2.6.26-rc3/fs/jbd/transaction.c 2008-05-28 10:57:32.000000000 -0700 > @@ -1648,12 +1648,42 @@ out: > return; > } > > +/* > + * journal_try_to_free_buffers() could race with journal_commit_transaction() > + * The later might still hold the reference count to the buffers when inspecting > + * them on t_syncdata_list or t_locked_list. > + * > + * Journal_try_to_free_buffers() will call this function to > + * wait for the current transaction to finish syncing data buffers, before > + * try to free that buffer. > + * > + * Called with journal->j_state_lock hold. > + */ > +static void journal_wait_for_transaction_sync_data(journal_t *journal) > +{ > + transaction_t *transaction = NULL; > + tid_t tid; > + > + spin_lock(&journal->j_state_lock); > + transaction = journal->j_committing_transaction; > + > + if (!transaction) { > + spin_unlock(&journal->j_state_lock); > + return; > + } > + > + tid = transaction->t_tid; > + spin_unlock(&journal->j_state_lock); > + log_wait_commit(journal, tid); > +} > > /** > * int journal_try_to_free_buffers() - try to free page buffers. > * @journal: journal for operation > * @page: to try and free > - * @unused_gfp_mask: unused > + * @gfp_mask: we use the mask to detect how hard should we try to release > + * buffers. If __GFP_WAIT and __GFP_FS is set, we wait for commit code to > + * release the buffers. > * > * > * For all the buffers on this page, > @@ -1682,9 +1712,11 @@ out: > * journal_try_to_free_buffer() is changing its state. But that > * cannot happen because we never reallocate freed data as metadata > * while the data is part of a transaction. Yes? > + * > + * Return 0 on failure, 1 on success > */ > int journal_try_to_free_buffers(journal_t *journal, > - struct page *page, gfp_t unused_gfp_mask) > + struct page *page, gfp_t gfp_mask) > { > struct buffer_head *head; > struct buffer_head *bh; > @@ -1713,7 +1745,28 @@ int journal_try_to_free_buffers(journal_ > if (buffer_jbd(bh)) > goto busy; > } while ((bh = bh->b_this_page) != head); > + > ret = try_to_free_buffers(page); > + > + /* > + * There are a number of places where journal_try_to_free_buffers() > + * could race with journal_commit_transaction(), the later still > + * holds the reference to the buffers to free while processing them. > + * try_to_free_buffers() failed to free those buffers. Some of the > + * caller of releasepage() request page buffers to be dropped, otherwise > + * treat the fail-to-free as errors (such as generic_file_direct_IO()) > + * > + * So, if the caller of try_to_release_page() wants the synchronous > + * behaviour(i.e make sure buffers are dropped upon return), > + * let's wait for the current transaction to finish flush of > + * dirty data buffers, then try to free those buffers again, > + * with the journal locked. > + */ > + if (ret == 0 && (gfp_mask & GFP_KERNEL == GFP_KERNEL)) { I think Andrew prefered this test to be expanded but otherwise the patch is fine now. You can add: Acked-by: Jan Kara Thanks for solving this :) > + journal_wait_for_transaction_sync_data(journal); > + ret = try_to_free_buffers(page); > + } > + > busy: > return ret; > } > Index: linux-2.6.26-rc3/mm/filemap.c > =================================================================== > --- linux-2.6.26-rc3.orig/mm/filemap.c 2008-05-28 10:55:38.000000000 -0700 > +++ linux-2.6.26-rc3/mm/filemap.c 2008-05-28 10:55:43.000000000 -0700 > @@ -2581,9 +2581,8 @@ out: > * Otherwise return zero. > * > * The @gfp_mask argument specifies whether I/O may be performed to release > - * this page (__GFP_IO), and whether the call may block (__GFP_WAIT). > + * this page (__GFP_IO), and whether the call may block (__GFP_WAIT & __GFP_FS). > * > - * NOTE: @gfp_mask may go away, and this function may become non-blocking. > */ > int try_to_release_page(struct page *page, gfp_t gfp_mask) > { > Honza -- Jan Kara SUSE Labs, CR