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

Jaegeuk Kim
2020-05-12T03:24:43Z

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.

>
> 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().

>
> - 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));
> >