[PATCH v4 2/2] xfs: avoid transaction reservation recursion

[Yafang Shao <laoar.shao@gmail.com>]

From: Yafang Shao <shaoyafang@didiglobal.com>

PF_FSTRANS which is used to avoid transaction reservation recursion, is
dropped since commit 9070733b4efa ("xfs: abstract PF_FSTRANS to
PF_MEMALLOC_NOFS") and commit 7dea19f9ee63 ("mm: introduce
memalloc_nofs_{save,restore} API") and replaced by PF_MEMALLOC_NOFS which
means to avoid filesystem reclaim recursion. That change is subtle.
Let's take the exmple of the check of WARN_ON_ONCE(current->flags &
PF_MEMALLOC_NOFS)) to explain why this abstraction from PF_FSTRANS to
PF_MEMALLOC_NOFS is not proper.
Below comment is quoted from Dave,
> It wasn't for memory allocation recursion protection in XFS - it was for
> transaction reservation recursion protection by something trying to flush
> data pages while holding a transaction reservation. Doing
> this could deadlock the journal because the existing reservation
> could prevent the nested reservation for being able to reserve space
> in the journal and that is a self-deadlock vector.
> IOWs, this check is not protecting against memory reclaim recursion
> bugs at all (that's the previous check [1]). This check is
> protecting against the filesystem calling writepages directly from a
> context where it can self-deadlock.
> So what we are seeing here is that the PF_FSTRANS ->
> PF_MEMALLOC_NOFS abstraction lost all the actual useful information
> about what type of error this check was protecting against.

As a result, we should reintroduce PF_FSTRANS. As current->journal_info
isn't used in XFS, we can reuse it to indicate whehter the task is in
fstrans or not.

[1]. Below check is to avoid memory reclaim recursion.
if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
        PF_MEMALLOC))
        goto redirty;

Signed-off-by: Yafang Shao <shaoyafang@didiglobal.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
---
 fs/iomap/buffered-io.c    |  4 ++--
 fs/xfs/libxfs/xfs_btree.c |  2 ++
 fs/xfs/xfs_aops.c         |  3 +++
 fs/xfs/xfs_linux.h        | 19 +++++++++++++++++++
 fs/xfs/xfs_trans.c        |  8 +++++++-
 5 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index bcfc288dba3f..b3f66b6b5116 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1500,9 +1500,9 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
 
 	/*
 	 * Given that we do not allow direct reclaim to call us, we should
-	 * never be called in a recursive filesystem reclaim context.
+	 * never be called while in a filesystem transaction.
 	 */
-	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
+	if (WARN_ON_ONCE(current->journal_info))
 		goto redirty;
 
 	/*
diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
index 2d25bab68764..0795511f9e6a 100644
--- a/fs/xfs/libxfs/xfs_btree.c
+++ b/fs/xfs/libxfs/xfs_btree.c
@@ -2825,6 +2825,7 @@ xfs_btree_split_worker(
 	if (args->kswapd)
 		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
 
+	xfs_trans_context_start();
 	current_set_flags_nested(&pflags, new_pflags);
 
 	args->result = __xfs_btree_split(args->cur, args->level, args->ptrp,
@@ -2832,6 +2833,7 @@ xfs_btree_split_worker(
 	complete(args->done);
 
 	current_restore_flags_nested(&pflags, new_pflags);
+	xfs_trans_context_end();
 }
 
 /*
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index b35611882ff9..39ef95acdd8e 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -63,6 +63,8 @@ xfs_setfilesize_trans_alloc(
 	 * clear the flag here.
 	 */
 	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
+	xfs_trans_context_end();
+
 	return 0;
 }
 
@@ -125,6 +127,7 @@ xfs_setfilesize_ioend(
 	 * thus we need to mark ourselves as being in a transaction manually.
 	 * Similarly for freeze protection.
 	 */
+	xfs_trans_context_start();
 	current_set_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
 	__sb_writers_acquired(VFS_I(ip)->i_sb, SB_FREEZE_FS);
 
diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index 9f70d2f68e05..1192b660a968 100644
--- a/fs/xfs/xfs_linux.h
+++ b/fs/xfs/xfs_linux.h
@@ -111,6 +111,25 @@ typedef __u32			xfs_nlink_t;
 #define current_restore_flags_nested(sp, f)	\
 		(current->flags = ((current->flags & ~(f)) | (*(sp) & (f))))
 
+static inline void xfs_trans_context_start(void)
+{
+	long flags = (long)current->journal_info;
+
+	/*
+	 * Reuse journal_info to indicate whehter the current is in fstrans
+	 * or not.
+	 */
+	current->journal_info = (void *)(flags + 1);
+}
+
+static inline void xfs_trans_context_end(void)
+{
+	long flags = (long)current->journal_info;
+
+	WARN_ON_ONCE(flags <= 0);
+	current->journal_info = ((void *)(flags - 1));
+}
+
 #define NBBY		8		/* number of bits per byte */
 
 /*
diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index 9ff41970d0c7..38d94679ad41 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -153,6 +153,7 @@ xfs_trans_reserve(
 	bool			rsvd = (tp->t_flags & XFS_TRANS_RESERVE) != 0;
 
 	/* Mark this thread as being in a transaction */
+	xfs_trans_context_start();
 	current_set_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
 
 	/*
@@ -859,6 +860,7 @@ __xfs_trans_commit(
 	xfs_log_commit_cil(mp, tp, &commit_lsn, regrant);
 
 	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
+	xfs_trans_context_end();
 	xfs_trans_free(tp);
 
 	/*
@@ -891,6 +893,7 @@ __xfs_trans_commit(
 		tp->t_ticket = NULL;
 	}
 	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
+	xfs_trans_context_end();
 	xfs_trans_free_items(tp, !!error);
 	xfs_trans_free(tp);
 
@@ -952,6 +955,7 @@ xfs_trans_cancel(
 
 	/* mark this thread as no longer being in a transaction */
 	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
+	xfs_trans_context_end();
 
 	xfs_trans_free_items(tp, dirty);
 	xfs_trans_free(tp);
@@ -1005,8 +1009,10 @@ xfs_trans_roll(
 	tres.tr_logflags = XFS_TRANS_PERM_LOG_RES;
 	tp = *tpp;
 	error = xfs_trans_reserve(tp, &tres, 0, 0);
-	if (error)
+	if (error) {
 		current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
+		xfs_trans_context_end();
+	}
 
 	return error;
 }
-- 
2.18.1