From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] jbd2: preserve original nofs flag during journal restart Date: Wed, 17 May 2017 16:13:57 +0200 Message-ID: <20170517141357.GN18247@dhcp22.suse.cz> References: <20170517123301.24211-1-tahsin@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Theodore Ts'o , Jan Kara , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: Tahsin Erdogan Return-path: Content-Disposition: inline In-Reply-To: <20170517123301.24211-1-tahsin@google.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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 > --- > 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