linux-kernel - Re: [PATCH 1/5] xfs: use kfree_rcu_mightsleep to free the perag structures

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Message-ID: <20240821161939.GC865349@frogsfrogsfrogs>
Date: Wed, 21 Aug 2024 09:19:39 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Chandan Babu R <chandan.babu@...cle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/5] xfs: use kfree_rcu_mightsleep to free the perag
 structures

On Wed, Aug 21, 2024 at 08:38:28AM +0200, Christoph Hellwig wrote:
> Using the kfree_rcu_mightsleep is simpler and removes the need for a
> rcu_head in the perag structure.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  fs/xfs/libxfs/xfs_ag.c | 12 +-----------
>  fs/xfs/libxfs/xfs_ag.h |  3 ---
>  2 files changed, 1 insertion(+), 14 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> index 7e80732cb54708..4b5a39a83f7aed 100644
> --- a/fs/xfs/libxfs/xfs_ag.c
> +++ b/fs/xfs/libxfs/xfs_ag.c
> @@ -235,16 +235,6 @@ xfs_initialize_perag_data(
>  	return error;
>  }
>  
> -STATIC void
> -__xfs_free_perag(
> -	struct rcu_head	*head)
> -{
> -	struct xfs_perag *pag = container_of(head, struct xfs_perag, rcu_head);
> -
> -	ASSERT(!delayed_work_pending(&pag->pag_blockgc_work));
> -	kfree(pag);
> -}
> -
>  /*
>   * Free up the per-ag resources associated with the mount structure.
>   */
> @@ -270,7 +260,7 @@ xfs_free_perag(
>  		xfs_perag_rele(pag);
>  		XFS_IS_CORRUPT(pag->pag_mount,
>  				atomic_read(&pag->pag_active_ref) != 0);
> -		call_rcu(&pag->rcu_head, __xfs_free_perag);
> +		kfree_rcu_mightsleep(pag);

I started wondering, have you seen any complaints from might_sleep when
freeing pags after a failed growfs?  Then I wondered if growfs_data
could actually take any locks that would prevent sleeping, which led me
to another question: why do growfs_{data,log} hold m_growlock but
growfs_rt doesn't?  Is that actually safe?

I think the kfree_rcu_mightsleep conversion is ok, but I want to see if
anything blows up with the rtgroups variant.

--D

>  	}
>  }
>  
> diff --git a/fs/xfs/libxfs/xfs_ag.h b/fs/xfs/libxfs/xfs_ag.h
> index 35de09a2516c70..d62c266c0b44d5 100644
> --- a/fs/xfs/libxfs/xfs_ag.h
> +++ b/fs/xfs/libxfs/xfs_ag.h
> @@ -63,9 +63,6 @@ struct xfs_perag {
>  	/* Blocks reserved for the reverse mapping btree. */
>  	struct xfs_ag_resv	pag_rmapbt_resv;
>  
> -	/* for rcu-safe freeing */
> -	struct rcu_head	rcu_head;
> -
>  	/* Precalculated geometry info */
>  	xfs_agblock_t		block_count;
>  	xfs_agblock_t		min_block;
> -- 
> 2.43.0
> 
>