Re: [PATCH v2] xfs: use current->journal_info for detecting transaction recursion

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 23, 2021 at 05:08:40PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Because the iomap code using PF_MEMALLOC_NOFS to detect transaction
> recursion in XFS is just wrong. Remove it from the iomap code and
> replace it with XFS specific internal checks using
> current->journal_info instead.
> 
> Fixes: 9070733b4efa ("xfs: abstract PF_FSTRANS to PF_MEMALLOC_NOFS")
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Looks ok, though I'm going to add a note about the NOFS flag changes
being bound to the @tp lifetime.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> Version 2:
> - fix use-after-free in xfs_btree_split_worker() and leave comment
>   in function documenting the landmine I stepped on.
> 
>  fs/iomap/buffered-io.c    |  7 -------
>  fs/xfs/libxfs/xfs_btree.c | 12 ++++++++++--
>  fs/xfs/xfs_aops.c         | 17 +++++++++++++++--
>  fs/xfs/xfs_trans.c        | 20 +++++---------------
>  fs/xfs/xfs_trans.h        | 30 ++++++++++++++++++++++++++++++
>  5 files changed, 60 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 16a1e82e3aeb..fcd4a0d71fc1 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1458,13 +1458,6 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
>  			PF_MEMALLOC))
>  		goto redirty;
>  
> -	/*
> -	 * Given that we do not allow direct reclaim to call us, we should
> -	 * never be called in a recursive filesystem reclaim context.
> -	 */
> -	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> -		goto redirty;
> -
>  	/*
>  	 * Is this page beyond the end of the file?
>  	 *
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index b56ff451adce..5b6fcb9b44e2 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
> @@ -2805,7 +2805,7 @@ xfs_btree_split_worker(
>  	struct xfs_btree_split_args	*args = container_of(work,
>  						struct xfs_btree_split_args, work);
>  	unsigned long		pflags;
> -	unsigned long		new_pflags = PF_MEMALLOC_NOFS;
> +	unsigned long		new_pflags = 0;
>  
>  	/*
>  	 * we are in a transaction context here, but may also be doing work
> @@ -2817,12 +2817,20 @@ xfs_btree_split_worker(
>  		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
>  
>  	current_set_flags_nested(&pflags, new_pflags);
> +	xfs_trans_set_context(args->cur->bc_tp);
>  
>  	args->result = __xfs_btree_split(args->cur, args->level, args->ptrp,
>  					 args->key, args->curp, args->stat);
> -	complete(args->done);
>  
> +	xfs_trans_clear_context(args->cur->bc_tp);
>  	current_restore_flags_nested(&pflags, new_pflags);
> +
> +	/*
> +	 * Do not access args after complete() has run here. We don't own args
> +	 * and the owner may run and free args before we return here.
> +	 */
> +	complete(args->done);
> +
>  }
>  
>  /*
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 4304c6416fbb..b4186d666157 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -62,7 +62,7 @@ xfs_setfilesize_trans_alloc(
>  	 * We hand off the transaction to the completion thread now, so
>  	 * clear the flag here.
>  	 */
> -	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> +	xfs_trans_clear_context(tp);
>  	return 0;
>  }
>  
> @@ -125,7 +125,7 @@ xfs_setfilesize_ioend(
>  	 * thus we need to mark ourselves as being in a transaction manually.
>  	 * Similarly for freeze protection.
>  	 */
> -	current_set_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> +	xfs_trans_set_context(tp);
>  	__sb_writers_acquired(VFS_I(ip)->i_sb, SB_FREEZE_FS);
>  
>  	/* we abort the update if there was an IO error */
> @@ -568,6 +568,12 @@ xfs_vm_writepage(
>  {
>  	struct xfs_writepage_ctx wpc = { };
>  
> +	if (WARN_ON_ONCE(current->journal_info)) {
> +		redirty_page_for_writepage(wbc, page);
> +		unlock_page(page);
> +		return 0;
> +	}
> +
>  	return iomap_writepage(page, wbc, &wpc.ctx, &xfs_writeback_ops);
>  }
>  
> @@ -578,6 +584,13 @@ xfs_vm_writepages(
>  {
>  	struct xfs_writepage_ctx wpc = { };
>  
> +	/*
> +	 * Writing back data in a transaction context can result in recursive
> +	 * transactions. This is bad, so issue a warning and get out of here.
> +	 */
> +	if (WARN_ON_ONCE(current->journal_info))
> +		return 0;
> +
>  	xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
>  	return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
>  }
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 44f72c09c203..e2a922f061c7 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -72,6 +72,7 @@ xfs_trans_free(
>  	xfs_extent_busy_clear(tp->t_mountp, &tp->t_busy, false);
>  
>  	trace_xfs_trans_free(tp, _RET_IP_);
> +	xfs_trans_clear_context(tp);
>  	if (!(tp->t_flags & XFS_TRANS_NO_WRITECOUNT))
>  		sb_end_intwrite(tp->t_mountp->m_super);
>  	xfs_trans_free_dqinfo(tp);
> @@ -123,7 +124,8 @@ xfs_trans_dup(
>  
>  	ntp->t_rtx_res = tp->t_rtx_res - tp->t_rtx_res_used;
>  	tp->t_rtx_res = tp->t_rtx_res_used;
> -	ntp->t_pflags = tp->t_pflags;
> +
> +	xfs_trans_switch_context(tp, ntp);
>  
>  	/* move deferred ops over to the new tp */
>  	xfs_defer_move(ntp, tp);
> @@ -157,9 +159,6 @@ xfs_trans_reserve(
>  	int			error = 0;
>  	bool			rsvd = (tp->t_flags & XFS_TRANS_RESERVE) != 0;
>  
> -	/* Mark this thread as being in a transaction */
> -	current_set_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> -
>  	/*
>  	 * Attempt to reserve the needed disk blocks by decrementing
>  	 * the number needed from the number available.  This will
> @@ -167,10 +166,8 @@ xfs_trans_reserve(
>  	 */
>  	if (blocks > 0) {
>  		error = xfs_mod_fdblocks(mp, -((int64_t)blocks), rsvd);
> -		if (error != 0) {
> -			current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> +		if (error != 0)
>  			return -ENOSPC;
> -		}
>  		tp->t_blk_res += blocks;
>  	}
>  
> @@ -244,9 +241,6 @@ xfs_trans_reserve(
>  		xfs_mod_fdblocks(mp, (int64_t)blocks, rsvd);
>  		tp->t_blk_res = 0;
>  	}
> -
> -	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> -
>  	return error;
>  }
>  
> @@ -270,6 +264,7 @@ xfs_trans_alloc(
>  	tp = kmem_cache_zalloc(xfs_trans_zone, GFP_KERNEL | __GFP_NOFAIL);
>  	if (!(flags & XFS_TRANS_NO_WRITECOUNT))
>  		sb_start_intwrite(mp->m_super);
> +	xfs_trans_set_context(tp);
>  
>  	/*
>  	 * Zero-reservation ("empty") transactions can't modify anything, so
> @@ -893,7 +888,6 @@ __xfs_trans_commit(
>  
>  	xfs_log_commit_cil(mp, tp, &commit_lsn, regrant);
>  
> -	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
>  	xfs_trans_free(tp);
>  
>  	/*
> @@ -925,7 +919,6 @@ __xfs_trans_commit(
>  			xfs_log_ticket_ungrant(mp->m_log, tp->t_ticket);
>  		tp->t_ticket = NULL;
>  	}
> -	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
>  	xfs_trans_free_items(tp, !!error);
>  	xfs_trans_free(tp);
>  
> @@ -985,9 +978,6 @@ xfs_trans_cancel(
>  		tp->t_ticket = NULL;
>  	}
>  
> -	/* mark this thread as no longer being in a transaction */
> -	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
> -
>  	xfs_trans_free_items(tp, dirty);
>  	xfs_trans_free(tp);
>  }
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 8b03fbfe9a1b..9dd745cf77c9 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -281,4 +281,34 @@ int xfs_trans_alloc_ichange(struct xfs_inode *ip, struct xfs_dquot *udqp,
>  		struct xfs_dquot *gdqp, struct xfs_dquot *pdqp, bool force,
>  		struct xfs_trans **tpp);
>  
> +static inline void
> +xfs_trans_set_context(
> +	struct xfs_trans	*tp)
> +{
> +	ASSERT(current->journal_info == NULL);
> +	tp->t_pflags = memalloc_nofs_save();
> +	current->journal_info = tp;
> +}
> +
> +static inline void
> +xfs_trans_clear_context(
> +	struct xfs_trans	*tp)
> +{
> +	if (current->journal_info == tp) {
> +		memalloc_nofs_restore(tp->t_pflags);
> +		current->journal_info = NULL;
> +	}
> +}
> +
> +static inline void
> +xfs_trans_switch_context(
> +	struct xfs_trans	*old_tp,
> +	struct xfs_trans	*new_tp)
> +{
> +	ASSERT(current->journal_info == old_tp);
> +	new_tp->t_pflags = old_tp->t_pflags;
> +	old_tp->t_pflags = 0;
> +	current->journal_info = new_tp;
> +}
> +
>  #endif	/* __XFS_TRANS_H__ */