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

Chao Yu
2020-05-12T06:50:02Z

On 2020/5/12 11:24, Jaegeuk Kim wrote:
> On 05/12, Chao Yu wrote:
>> On 2020/5/12 6:11, Jaegeuk Kim wrote:
>>> On 05/11, Chao Yu wrote:
>>>> 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.
>>>>
>>>
>>> Another version:
>>>
>>> >From 6b430b72af57c65c20dea7b87f7ba7e9df36be98 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 | 5 +++++
>>> fs/f2fs/f2fs.h | 1 +
>>> fs/f2fs/node.c | 51 ++++++++++++++++++++++++++++++++++++++++++--
>>> 3 files changed, 55 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>>> index d49f7a01d8a26..79e605f38f4fa 100644
>>> --- a/fs/f2fs/checkpoint.c
>>> +++ b/fs/f2fs/checkpoint.c
>>> @@ -1168,6 +1168,11 @@ static int block_operations(struct f2fs_sb_info *sbi)
>>> };
>>> int err = 0, cnt = 0;
>>>
>>> + /*
>>> + * Let's flush inline_data in dirty node pages.
>>> + */
>>> + f2fs_flush_inline_data(sbi);
>>
>> Still there is a gap, user can write a large number of inline data here...
>
> I think generic/204 is the case, and I don't hit a panic with this patch.

Yes, may be triggered by a variant of generic/204 which is not exist yet, and
it's a rare corner case.

>
>>
>> Would that be enough? I doubt we can suffer this issue in below pathes
>> as well:
>
> I don't think so, since iput is called after f2fs_unlock_all().

That's correct.

Thanks,

>
>>
>> - block_operations
>> - f2fs_sync_dirty_inodes
>> - iput
>> - f2fs_sync_inode_meta
>> - iput
>>
>> Thanks,
>>
>>> +
>>> retry_flush_quotas:
>>> f2fs_lock_all(sbi);
>>> if (__need_flush_quota(sbi)) {
>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>> index 2a8ea81c52a15..7f3d259e7e376 100644
>>> --- a/fs/f2fs/f2fs.h
>>> +++ b/fs/f2fs/f2fs.h
>>> @@ -3282,6 +3282,7 @@ void f2fs_ra_node_page(struct f2fs_sb_info *sbi, nid_t nid);
>>> struct page *f2fs_get_node_page(struct f2fs_sb_info *sbi, pgoff_t nid);
>>> struct page *f2fs_get_node_page_ra(struct page *parent, int start);
>>> int f2fs_move_node_page(struct page *node_page, int gc_type);
>>> +int f2fs_flush_inline_data(struct f2fs_sb_info *sbi);
>>> int f2fs_fsync_node_pages(struct f2fs_sb_info *sbi, struct inode *inode,
>>> struct writeback_control *wbc, bool atomic,
>>> unsigned int *seq_id);
>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>> index 1db8cabf727ef..e632de10aedab 100644
>>> --- a/fs/f2fs/node.c
>>> +++ b/fs/f2fs/node.c
>>> @@ -1807,6 +1807,53 @@ static bool flush_dirty_inode(struct page *page)
>>> return true;
>>> }
>>>
>>> +int f2fs_flush_inline_data(struct f2fs_sb_info *sbi)
>>> +{
>>> + pgoff_t index = 0;
>>> + struct pagevec pvec;
>>> + int nr_pages;
>>> + int ret = 0;
>>> +
>>> + pagevec_init(&pvec);
>>> +
>>> + while ((nr_pages = pagevec_lookup_tag(&pvec,
>>> + NODE_MAPPING(sbi), &index, PAGECACHE_TAG_DIRTY))) {
>>> + int i;
>>> +
>>> + for (i = 0; i < nr_pages; i++) {
>>> + struct page *page = pvec.pages[i];
>>> +
>>> + if (!IS_DNODE(page))
>>> + continue;
>>> +
>>> + lock_page(page);
>>> +
>>> + if (unlikely(page->mapping != NODE_MAPPING(sbi))) {
>>> +continue_unlock:
>>> + unlock_page(page);
>>> + continue;
>>> + }
>>> +
>>> + if (!PageDirty(page)) {
>>> + /* someone wrote it for us */
>>> + goto continue_unlock;
>>> + }
>>> +
>>> + /* flush inline_data, if it's async context. */
>>> + if (is_inline_node(page)) {
>>> + clear_inline_node(page);
>>> + unlock_page(page);
>>> + flush_inline_data(sbi, ino_of_node(page));
>>> + continue;
>>> + }
>>> + unlock_page(page);
>>> + }
>>> + pagevec_release(&pvec);
>>> + cond_resched();
>>> + }
>>> + return ret;
>>> +}
>>> +
>>> int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
>>> struct writeback_control *wbc,
>>> bool do_balance, enum iostat_type io_type)
>>> @@ -1870,8 +1917,8 @@ 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 (do_balance && is_inline_node(page)) {
>>> clear_inline_node(page);
>>> unlock_page(page);
>>> flush_inline_data(sbi, ino_of_node(page));
>>>
> .
>