Re: [PATCH V4] f2fs: Avoid double lock for cp_rwsem during checkpoint

Chao Yu
2020-05-11T01:28:53Z

On 2020/5/10 3:03, Jaegeuk Kim wrote:
> On 05/09, Chao Yu wrote:
>> On 2020/5/9 0:10, Jaegeuk Kim wrote:
>>> 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@kernel.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@codeaurora.org>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.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)) {
>>
>> IIRC, this flow was designed to avoid running out of free space issue
>> during checkpoint:
>>
>> 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
>>
>> The sceanrio is:
>> 1. create fully node blocks
>> 2. flush node blocks
>> 3. write inline_data for all the node blocks again
>> 4. flush node blocks redundantly
>>
>> I guess this may cause failing one case of fstest.
>
> Yeah, actually I was hitting 204 failure, and thus, revised like this.
> Now, I don't see any regression in fstest.
>
>>From 8f1882acfb0a5fc43e5a2bbd576a8f3c681a7d2c Mon Sep 17 00:00:00 2001
> From: Sayali Lokhande <sayalil@codeaurora.org>
> Date: Thu, 30 Apr 2020 16:28:29 +0530
> 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@codeaurora.org>
> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> ---
> fs/f2fs/checkpoint.c | 9 ++++++++-
> fs/f2fs/f2fs.h | 4 ++--
> fs/f2fs/node.c | 10 +++++-----
> 3 files changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index d49f7a01d8a26..928aea4ff663d 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1168,6 +1168,12 @@ static int block_operations(struct f2fs_sb_info *sbi)
> };
> int err = 0, cnt = 0;
>
> + /*
> + * Let's flush node pages first to flush inline_data.
> + * We'll actually guarantee everything below under f2fs_lock_all.
> + */
> + f2fs_sync_node_pages(sbi, &wbc, false, false, FS_CP_NODE_IO);

It is possible that user write a large number of inline data in between
f2fs_sync_node_pages() and f2fs_lock_all(), it will cause the no-space issue in
race condition.

Also, if there is huge number of F2FS_DIRTY_IMETA, after this change, we will
flush inode page twice which is unneeded.

f2fs_sync_node_pages() --- flush dirty inode page
f2fs_lock_all()
...
f2fs_sync_inode_meta() --- update dirty inode page
f2fs_sync_node_pages() --- flush dirty inode page again.

Thanks,

> +
> retry_flush_quotas:
> f2fs_lock_all(sbi);
> if (__need_flush_quota(sbi)) {
> @@ -1222,7 +1228,8 @@ static int block_operations(struct f2fs_sb_info *sbi)
> if (get_pages(sbi, F2FS_DIRTY_NODES)) {
> up_write(&sbi->node_write);
> atomic_inc(&sbi->wb_sync_req[NODE]);
> - err = f2fs_sync_node_pages(sbi, &wbc, false, FS_CP_NODE_IO);
> + err = f2fs_sync_node_pages(sbi, &wbc, false,
> + true, FS_CP_NODE_IO);
> atomic_dec(&sbi->wb_sync_req[NODE]);
> if (err) {
> up_write(&sbi->node_change);
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index d916540f12813..ac6ae42b9dd4e 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -3286,8 +3286,8 @@ int f2fs_fsync_node_pages(struct f2fs_sb_info *sbi, struct inode *inode,
> struct writeback_control *wbc, bool atomic,
> unsigned int *seq_id);
> int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> - struct writeback_control *wbc,
> - bool do_balance, enum iostat_type io_type);
> + struct writeback_control *wbc,
> + bool do_balance, bool sync, enum iostat_type io_type);
> int f2fs_build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount);
> bool f2fs_alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid);
> void f2fs_alloc_nid_done(struct f2fs_sb_info *sbi, nid_t nid);
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 1db8cabf727ef..fd00a8c119088 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -1808,8 +1808,8 @@ static bool flush_dirty_inode(struct page *page)
> }
>
> int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> - struct writeback_control *wbc,
> - bool do_balance, enum iostat_type io_type)
> + struct writeback_control *wbc,
> + bool do_balance, bool sync, enum iostat_type io_type)
> {
> pgoff_t index;
> struct pagevec pvec;
> @@ -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 async context. */
> + if (!sync && is_inline_node(page)) {
> clear_inline_node(page);
> unlock_page(page);
> flush_inline_data(sbi, ino_of_node(page));
> @@ -1999,7 +1999,7 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>
> diff = nr_pages_to_write(sbi, NODE, wbc);
> blk_start_plug(&plug);
> - f2fs_sync_node_pages(sbi, wbc, true, FS_NODE_IO);
> + f2fs_sync_node_pages(sbi, wbc, true, false, FS_NODE_IO);
> blk_finish_plug(&plug);
> wbc->nr_to_write = max((long)0, wbc->nr_to_write - diff);
>
>