linux-kernel - Re: [PATCH V4] f2fs: Avoid double lock for cp

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Message-ID: <20200508161052.GA49579@google.com>
Date:   Fri, 8 May 2020 09:10:52 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Sayali Lokhande <sayalil@...eaurora.org>
Cc:     yuchao0@...wei.com, linux-f2fs-devel@...ts.sourceforge.net,
        stummala@...eaurora.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4] f2fs: Avoid double lock for cp_rwsem during checkpoint

Hi Sayali,

In order to address the perf regression, how about this?

>From 48418af635884803ffb35972df7958a2e6649322 Mon Sep 17 00:00:00 2001
From: Jaegeuk Kim <jaegeuk@...nel.org>
Date: Fri, 8 May 2020 09:08:37 -0700
Subject: [PATCH] f2fs: avoid double lock for cp_rwsem during checkpoint

There could be a scenario where f2fs_sync_node_pages gets
called during checkpoint, which in turn tries to flush
inline data and calls iput(). This results in deadlock as
iput() tries to hold cp_rwsem, which is already held at the
beginning by checkpoint->block_operations().

Call stack :

Thread A		Thread B
f2fs_write_checkpoint()
- block_operations(sbi)
 - f2fs_lock_all(sbi);
  - down_write(&sbi->cp_rwsem);

                        - open()
                         - igrab()
                        - write() write inline data
                        - unlink()
- f2fs_sync_node_pages()
 - if (is_inline_node(page))
  - flush_inline_data()
   - ilookup()
     page = f2fs_pagecache_get_page()
     if (!page)
      goto iput_out;
     iput_out:
			-close()
			-iput()
       iput(inode);
       - f2fs_evict_inode()
        - f2fs_truncate_blocks()
         - f2fs_lock_op()
           - down_read(&sbi->cp_rwsem);

Fixes: 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
Signed-off-by: Sayali Lokhande <sayalil@...eaurora.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
---
 fs/f2fs/node.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 1db8cabf727ef..626d7daca09de 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -1870,8 +1870,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
 				goto continue_unlock;
 			}
 
-			/* flush inline_data */
-			if (is_inline_node(page)) {
+			/* flush inline_data, if it's not sync path. */
+			if (do_balance && is_inline_node(page)) {
 				clear_inline_node(page);
 				unlock_page(page);
 				flush_inline_data(sbi, ino_of_node(page));
-- 
2.26.2.645.ge9eca65c58-goog