linux-ext4 - [PATCH 1/7] ext4: Update stale comment about write constraints

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Message-Id: <20230228051319.4085470-2-tytso@mit.edu>
Date:   Tue, 28 Feb 2023 00:13:13 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>
Subject: [PATCH 1/7] ext4: Update stale comment about write constraints

From: Jan Kara <jack@...e.cz>

The comment above do_journal_get_write_access() is very stale. Most of
it just does not refer to what the function does today or how jbd2
works. The bit about transaction handling during write(2) is still
correct so just update the function names in that part and move the
comment to a more appropriate place.

Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Theodore Ts'o <tytso@....edu>
---
 fs/ext4/inode.c | 31 +++++++------------------------
 1 file changed, 7 insertions(+), 24 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d251d705c276..178978484acc 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1004,30 +1004,6 @@ int ext4_walk_page_buffers(handle_t *handle, struct inode *inode,
 	return ret;
 }
 
-/*
- * To preserve ordering, it is essential that the hole instantiation and
- * the data write be encapsulated in a single transaction.  We cannot
- * close off a transaction and start a new one between the ext4_get_block()
- * and the commit_write().  So doing the jbd2_journal_start at the start of
- * prepare_write() is the right place.
- *
- * Also, this function can nest inside ext4_writepage().  In that case, we
- * *know* that ext4_writepage() has generated enough buffer credits to do the
- * whole page.  So we won't block on the journal in that case, which is good,
- * because the caller may be PF_MEMALLOC.
- *
- * By accident, ext4 can be reentered when a transaction is open via
- * quota file writes.  If we were to commit the transaction while thus
- * reentered, there can be a deadlock - we would be holding a quota
- * lock, and the commit would never complete if another thread had a
- * transaction open and was blocking on the quota lock - a ranking
- * violation.
- *
- * So what we do is to rely on the fact that jbd2_journal_stop/journal_start
- * will _not_ run commit under these circumstances because handle->h_ref
- * is elevated.  We'll still have enough credits for the tiny quotafile
- * write.
- */
 int do_journal_get_write_access(handle_t *handle, struct inode *inode,
 				struct buffer_head *bh)
 {
@@ -1149,6 +1125,13 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
 }
 #endif
 
+/*
+ * To preserve ordering, it is essential that the hole instantiation and
+ * the data write be encapsulated in a single transaction.  We cannot
+ * close off a transaction and start a new one between the ext4_get_block()
+ * and the ext4_write_end().  So doing the jbd2_journal_start at the start of
+ * ext4_write_begin() is the right place.
+ */
 static int ext4_write_begin(struct file *file, struct address_space *mapping,
 			    loff_t pos, unsigned len,
 			    struct page **pagep, void **fsdata)
-- 
2.31.0