[PATCH] jbd2: preserve original nofs flag during journal restart

[PATCH] jbd2: preserve original nofs flag during journal restart

[Tahsin Erdogan]

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 <tahsin@google.com>
---
 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;
 }
-- 
2.13.0.303.g4ebf302169-goog

Re: [PATCH] jbd2: preserve original nofs flag during journal restart

[Jan Kara]

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 <tahsin@google.com>

Thanks for catching this! The patch looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  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;
>  }
> -- 
> 2.13.0.303.g4ebf302169-goog
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] jbd2: preserve original nofs flag during journal restart

[Michal Hocko]

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 <tahsin@google.com>
> ---
>  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

Re: [PATCH] jbd2: preserve original nofs flag during journal restart

[Jan Kara]

On Wed 17-05-17 16:13:57, Michal Hocko wrote:
> 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 <tahsin@google.com>
> > ---
> >  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.

I was thinking about his as well but the fact is jbd2__journal_restart()
actually does equivalent of jbd2_journal_stop() for the handle above the
place where memalloc_nofs_restore() was added so in this sense we really
miss memalloc_nofs_restore() there... So what Tahsin did makes sense.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] jbd2: preserve original nofs flag during journal restart

[Michal Hocko]

On Wed 17-05-17 16:56:27, Jan Kara wrote:
> On Wed 17-05-17 16:13:57, Michal Hocko wrote:
> > 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 <tahsin@google.com>
> > > ---
> > >  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.
> 
> I was thinking about his as well but the fact is jbd2__journal_restart()
> actually does equivalent of jbd2_journal_stop() for the handle above the
> place where memalloc_nofs_restore() was added so in this sense we really
> miss memalloc_nofs_restore() there... So what Tahsin did makes sense.

OK, I will trust you ;). This deserves a comment then, I believe.

Thanks!
-- 
Michal Hocko
SUSE Labs

[PATCH v2] jbd2: preserve original nofs flag during journal restart

[Tahsin Erdogan]

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 <tahsin@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
v2: added comment to jbd2__journal_restart()

 fs/jbd2/transaction.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 9ee4832b6f8b..05c0323493fb 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -680,6 +680,8 @@ 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;
+	/* Restore original nofs flag as jbd2_journal_stop() would do. */
+	memalloc_nofs_restore(handle->saved_alloc_context);
 	ret = start_this_handle(journal, handle, gfp_mask);
 	return ret;
 }
-- 
2.13.0.303.g4ebf302169-goog

Re: [PATCH v2] jbd2: preserve original nofs flag during journal restart

[Michal Hocko]

On Wed 17-05-17 09:16:29, Tahsin Erdogan wrote:
[...]
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 9ee4832b6f8b..05c0323493fb 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -680,6 +680,8 @@ 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;
> +	/* Restore original nofs flag as jbd2_journal_stop() would do. */

I was thinking something more specific like
	/*
	 * Restore the original nofs context because the journal restart
	 * is basically the same thing as journal stop and start.
	 * start_this_handle will start a new nofs context.
	 */
> +	memalloc_nofs_restore(handle->saved_alloc_context);
>  	ret = start_this_handle(journal, handle, gfp_mask);
>  	return ret;
>  }
> -- 
> 2.13.0.303.g4ebf302169-goog
> 

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] jbd2: preserve original nofs flag during journal restart

[Jan Kara]

On Thu 18-05-17 11:12:53, Michal Hocko wrote:
> On Wed 17-05-17 09:16:29, Tahsin Erdogan wrote:
> [...]
> > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > index 9ee4832b6f8b..05c0323493fb 100644
> > --- a/fs/jbd2/transaction.c
> > +++ b/fs/jbd2/transaction.c
> > @@ -680,6 +680,8 @@ 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;
> > +	/* Restore original nofs flag as jbd2_journal_stop() would do. */
> 
> I was thinking something more specific like
> 	/*
> 	 * Restore the original nofs context because the journal restart
> 	 * is basically the same thing as journal stop and start.
> 	 * start_this_handle will start a new nofs context.
> 	 */

Yeah, this is much better. Tahsin, please use Michal's version. Thanks!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

[PATCH v3] jbd2: preserve original nofs flag during journal restart

[Tahsin Erdogan]

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 <tahsin@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
v3: Use Michal's suggested comment in jbd2__journal_restart()
v2: added comment to jbd2__journal_restart()

 fs/jbd2/transaction.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 9ee4832b6f8b..2d30a6da7013 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -680,6 +680,12 @@ 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;
+	/*
+	 * Restore the original nofs context because the journal restart
+	 * is basically the same thing as journal stop and start.
+	 * start_this_handle will start a new nofs context.
+	 */
+	memalloc_nofs_restore(handle->saved_alloc_context);
 	ret = start_this_handle(journal, handle, gfp_mask);
 	return ret;
 }
-- 
2.13.0.303.g4ebf302169-goog

Re: [PATCH v3] jbd2: preserve original nofs flag during journal restart

[Theodore Ts'o]

On Thu, May 18, 2017 at 09:28:50AM -0700, 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 <tahsin@google.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

Thanks, applied.

						- Ted