[PATCH] btrfs: move ulist allocation out of transaction in quota enable

[PATCH] btrfs: move ulist allocation out of transaction in quota enable

[David Sterba]

The allocation happens with GFP_KERNEL after a transaction has been
started, this can potentially cause deadlock if reclaim tries to get the
memory by flushing filesystem data.

The qgroup_ulist is not used during transaction start as the quotas are
not yet enabled but to provide the same semantics, use a temporary
variable and assign fs_info::qgroup_ulist as before.

Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/qgroup.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index c1cd5558a646..9e49d5e132b8 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -887,6 +887,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	struct btrfs_key found_key;
 	struct btrfs_qgroup *qgroup = NULL;
 	struct btrfs_trans_handle *trans = NULL;
+	struct ulist *tmp_ulist;
 	int ret = 0;
 	int slot;
 
@@ -894,6 +895,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	if (fs_info->quota_root)
 		goto out;
 
+	tmp_ulist = ulist_alloc(GFP_KERNEL);
+	if (!tmp_ulist) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	/*
 	 * 1 for quota root item
 	 * 1 for BTRFS_QGROUP_STATUS item
@@ -909,12 +916,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 		goto out;
 	}
 
-	fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
-	if (!fs_info->qgroup_ulist) {
-		ret = -ENOMEM;
-		btrfs_abort_transaction(trans, ret);
-		goto out;
-	}
+	fs_info->qgroup_ulist = tmp_ulist;
 
 	/*
 	 * initially create the quota tree
-- 
2.20.1

Re: [PATCH] btrfs: move ulist allocation out of transaction in quota enable

[Filipe Manana]

On Mon, Feb 25, 2019 at 6:58 PM David Sterba <dsterba@suse.com> wrote:
>
> The allocation happens with GFP_KERNEL after a transaction has been
> started, this can potentially cause deadlock if reclaim tries to get the
> memory by flushing filesystem data.
>
> The qgroup_ulist is not used during transaction start as the quotas are
> not yet enabled but to provide the same semantics, use a temporary
> variable and assign fs_info::qgroup_ulist as before.
>
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/qgroup.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index c1cd5558a646..9e49d5e132b8 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -887,6 +887,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
>         struct btrfs_key found_key;
>         struct btrfs_qgroup *qgroup = NULL;
>         struct btrfs_trans_handle *trans = NULL;
> +       struct ulist *tmp_ulist;
>         int ret = 0;
>         int slot;
>
> @@ -894,6 +895,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
>         if (fs_info->quota_root)
>                 goto out;
>
> +       tmp_ulist = ulist_alloc(GFP_KERNEL);
> +       if (!tmp_ulist) {
> +               ret = -ENOMEM;
> +               goto out;
> +       }

tmp_ulist needs to be freed if starting the transaction below fails.

Other then that, looks good.

> +
>         /*
>          * 1 for quota root item
>          * 1 for BTRFS_QGROUP_STATUS item
> @@ -909,12 +916,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
>                 goto out;
>         }
>
> -       fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
> -       if (!fs_info->qgroup_ulist) {
> -               ret = -ENOMEM;
> -               btrfs_abort_transaction(trans, ret);
> -               goto out;
> -       }
> +       fs_info->qgroup_ulist = tmp_ulist;
>
>         /*
>          * initially create the quota tree
> --
> 2.20.1
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH] btrfs: move ulist allocation out of transaction in quota enable

[David Sterba]

On Mon, Feb 25, 2019 at 07:09:02PM +0000, Filipe Manana wrote:
> On Mon, Feb 25, 2019 at 6:58 PM David Sterba <dsterba@suse.com> wrote:
> >
> > The allocation happens with GFP_KERNEL after a transaction has been
> > started, this can potentially cause deadlock if reclaim tries to get the
> > memory by flushing filesystem data.
> >
> > The qgroup_ulist is not used during transaction start as the quotas are
> > not yet enabled but to provide the same semantics, use a temporary
> > variable and assign fs_info::qgroup_ulist as before.
> >
> > Signed-off-by: David Sterba <dsterba@suse.com>
> > ---
> >  fs/btrfs/qgroup.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> > index c1cd5558a646..9e49d5e132b8 100644
> > --- a/fs/btrfs/qgroup.c
> > +++ b/fs/btrfs/qgroup.c
> > @@ -887,6 +887,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
> >         struct btrfs_key found_key;
> >         struct btrfs_qgroup *qgroup = NULL;
> >         struct btrfs_trans_handle *trans = NULL;
> > +       struct ulist *tmp_ulist;
> >         int ret = 0;
> >         int slot;
> >
> > @@ -894,6 +895,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
> >         if (fs_info->quota_root)
> >                 goto out;
> >
> > +       tmp_ulist = ulist_alloc(GFP_KERNEL);
> > +       if (!tmp_ulist) {
> > +               ret = -ENOMEM;
> > +               goto out;
> > +       }
> 
> tmp_ulist needs to be freed if starting the transaction below fails.

Right. If I did not try to preserve the semantics (that does not seem to
be necessary), the freeing would happen on qgroup_ulist as before.

Starting transaction uses qgruop_ulist if quotas are enabled, so at this
time of btrfs_quota_enable, this is not yet true. The status bit is set
later.

I'll update the patch with this explanation and drop the temporary
variable.

[PATCH v2] btrfs: move ulist allocation out of transaction in quota enable

[David Sterba]

The allocation happens with GFP_KERNEL after a transaction has been
started, this can potentially cause deadlock if reclaim tries to get the
memory by flushing filesystem data.

The fs_info::qgroup_ulist is not used during transaction start when
quotas are not enabled. The status bit BTRFS_FS_QUOTA_ENABLED is set
later in btrfs_quota_enable so it's safe to move it before the
transaction start.

Signed-off-by: David Sterba <dsterba@suse.com>

v2:
- use qgroup_ulist directly, drop the temporary variable
- update changelog
---
 fs/btrfs/qgroup.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index c1cd5558a646..eb680b715dd6 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -894,6 +894,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	if (fs_info->quota_root)
 		goto out;
 
+	fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
+	if (!fs_info->qgroup_ulist) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	/*
 	 * 1 for quota root item
 	 * 1 for BTRFS_QGROUP_STATUS item
@@ -909,13 +915,6 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 		goto out;
 	}
 
-	fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
-	if (!fs_info->qgroup_ulist) {
-		ret = -ENOMEM;
-		btrfs_abort_transaction(trans, ret);
-		goto out;
-	}
-
 	/*
 	 * initially create the quota tree
 	 */
-- 
2.20.1

Re: [PATCH v2] btrfs: move ulist allocation out of transaction in quota enable

[Filipe Manana]

On Wed, Feb 27, 2019 at 12:12 PM David Sterba <dsterba@suse.com> wrote:
>
> The allocation happens with GFP_KERNEL after a transaction has been
> started, this can potentially cause deadlock if reclaim tries to get the
> memory by flushing filesystem data.
>
> The fs_info::qgroup_ulist is not used during transaction start when
> quotas are not enabled. The status bit BTRFS_FS_QUOTA_ENABLED is set
> later in btrfs_quota_enable so it's safe to move it before the
> transaction start.
>
> Signed-off-by: David Sterba <dsterba@suse.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Looks good now.

>
> v2:
> - use qgroup_ulist directly, drop the temporary variable
> - update changelog
> ---
>  fs/btrfs/qgroup.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index c1cd5558a646..eb680b715dd6 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -894,6 +894,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
>         if (fs_info->quota_root)
>                 goto out;
>
> +       fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
> +       if (!fs_info->qgroup_ulist) {
> +               ret = -ENOMEM;
> +               goto out;
> +       }
> +
>         /*
>          * 1 for quota root item
>          * 1 for BTRFS_QGROUP_STATUS item
> @@ -909,13 +915,6 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
>                 goto out;
>         }
>
> -       fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
> -       if (!fs_info->qgroup_ulist) {
> -               ret = -ENOMEM;
> -               btrfs_abort_transaction(trans, ret);
> -               goto out;
> -       }
> -
>         /*
>          * initially create the quota tree
>          */
> --
> 2.20.1
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”