Message-ID: <CAOsHCa1t+LeeAG2PDJ0BfYtoh_+CUmLjZcp1+dGZWR5PPfmFSQ@mail.gmail.com>
Date: Tue, 19 Nov 2024 14:46:53 +0800
From: Xiuhong Wang <xiuhong.wang.cn@...il.com>
To: Chao Yu <chao@...nel.org>
Cc: Xiuhong Wang <xiuhong.wang@...soc.com>, jaegeuk@...nel.org, 
	linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org, 
	niuzhiguo84@...il.com, hao_hao.wang@...soc.com, ke.wang@...soc.com
Subject: Re: [PATCH] f2fs: Fix to avoid long time to shrink extent cache

Chao Yu <chao@...nel.org> 于2024年11月19日周二 14:05写道：
>
> On 2024/11/12 19:06, Xiuhong Wang wrote:
> > We encountered a system hang problem based on the following
> > experiment:
> > There are 17 processes, 8 of which do 4k data read, write and
> > compare tests, and 8 do 64k read, write and compare tests. Each
> > thread writes a 256M file, and another thread writes a large file
> > to 80% of the disk, and then keeps doing read operations, all of
> > which are direct operations. This will cause the large file to be
> > filled to 80% of the disk to be severely fragmented. On a 512GB
> > device, this large file may generate a huge zombie extent tree.
> >
> > When system shutting down, the init process needs to wait for the
> > writeback process, and the writeback process may encounter the
> > situation where the READ_EXTENT_CACHE space is insufficient and
> > needs to free the zombie extent tree. The extent tree has a large
> > number of extent nodes, it will a long free time to free, which
> > triggers system hang.
>  > > The stack when the problem occurs is as follows:
> > crash_arm64> bt 1
> > PID: 1        TASK: ffffff80801a9200  CPU: 1    COMMAND: "init"
> >   #0 [ffffffc00806b9a0] __switch_to at ffffffc00810711c
> >   #1 [ffffffc00806ba00] __schedule at ffffffc0097c1c4c
> >   #2 [ffffffc00806ba60] schedule at ffffffc0097c2308
> >   #3 [ffffffc00806bab0] wb_wait_for_completion at ffffffc0086320d4
> >   #4 [ffffffc00806bb20] writeback_inodes_sb at ffffffc00863719c
> >   #5 [ffffffc00806bba0] sync_filesystem at ffffffc00863c98c
> >   #6 [ffffffc00806bbc0] f2fs_quota_off_umount at ffffffc00886fc60
> >   #7 [ffffffc00806bc20] f2fs_put_super at ffffffc0088715b4
> >   #8 [ffffffc00806bc60] generic_shutdown_super at ffffffc0085cd61c
> >   #9 [ffffffc00806bcd0] kill_f2fs_super at ffffffc00886b3dc
> >
> > crash_arm64> bt 14997
> > PID: 14997    TASK: ffffff8119d82400  CPU: 3    COMMAND: "kworker/u16:0"
> >   #0 [ffffffc019f8b760] __detach_extent_node at ffffffc0088d5a58
> >   #1 [ffffffc019f8b790] __release_extent_node at ffffffc0088d5970
> >   #2 [ffffffc019f8b810] f2fs_shrink_extent_tree at ffffffc0088d5c7c
> >   #3 [ffffffc019f8b8a0] f2fs_balance_fs_bg at ffffffc0088c109c
> >   #4 [ffffffc019f8b910] f2fs_write_node_pages at ffffffc0088bd4d8
> >   #5 [ffffffc019f8b990] do_writepages at ffffffc0084a0b5c
> >   #6 [ffffffc019f8b9f0] __writeback_single_inode at ffffffc00862ee28
> >   #7 [ffffffc019f8bb30] writeback_sb_inodes at ffffffc0086358c0
> >   #8 [ffffffc019f8bc10] wb_writeback at ffffffc0086362dc
> >   #9 [ffffffc019f8bcc0] wb_do_writeback at ffffffc008634910
> >
> > Process 14997 ran for too long and caused the system hang.
> >
> > At this time, there are still 1086911 extent nodes in this zombie
> > extent tree that need to be cleaned up.
> >
> > crash_arm64_sprd_v8.0.3++> extent_tree.node_cnt ffffff80896cc500
> >    node_cnt = {
> >      counter = 1086911
> >    },
> >
> > The root cause of this problem is that when the f2fs_balance_fs
> > function is called in the write process, it will determine
> > whether to call f2fs_balance_fs_bg, but it is difficult to
> > meet the condition of excess_cached_nats. When the
> > f2fs_shrink_extent_tree function is called to free during
> > f2fs_write_node_pages, there are too many extent nodes on the
> > extent tree, which causes a loop and causes a system hang.
> >
> > To solve this problem, when calling f2fs_balance_fs, check whether
> > the extent cache is sufficient. If not, release the zombie extent
> > tree.
> >
> > Signed-off-by: Xiuhong Wang <xiuhong.wang@...soc.com>
> > Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
> > ---
> > Test the problem with the temporary versions:
> > patch did not reproduce the problem, the patch is as follows:
> > @@ -415,7 +415,7 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
> >                  f2fs_stop_checkpoint(sbi, false, STOP_CP_REASON_FAULT_INJECT);
> >
> >          /* balance_fs_bg is able to be pending */
> > -       if (need && excess_cached_nats(sbi))
> > +       if (need)
> >                  f2fs_balance_fs_bg(sbi, false);
> >
> > ---
> >   fs/f2fs/segment.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 1766254279d2..390bec177567 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -415,7 +415,9 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
> >               f2fs_stop_checkpoint(sbi, false, STOP_CP_REASON_FAULT_INJECT);
> >
> >       /* balance_fs_bg is able to be pending */
> > -     if (need && excess_cached_nats(sbi))
> > +     if (need && (excess_cached_nats(sbi) ||
> > +                     !f2fs_available_free_memory(sbi, READ_EXTENT_CACHE) ||
> > +                     !f2fs_available_free_memory(sbi, AGE_EXTENT_CACHE)))
>
> Hi,
>
> I doubt if there is no enough memory, we may still run into
> f2fs_shrink_extent_tree() and suffer such long time delay.
>
> So, can we just let __free_extent_tree() break the loop once we have
> released entries w/ target number? something like this:
>
> ---
>   fs/f2fs/extent_cache.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
> index 019c1f7b7fa5..38c71c1c4fb7 100644
> --- a/fs/f2fs/extent_cache.c
> +++ b/fs/f2fs/extent_cache.c
> @@ -379,11 +379,12 @@ static struct extent_tree *__grab_extent_tree(struct inode *inode,
>   }
>
>   static unsigned int __free_extent_tree(struct f2fs_sb_info *sbi,
> -                                       struct extent_tree *et)
> +                               struct extent_tree *et, unsigned int nr_shrink)
>   {
>         struct rb_node *node, *next;
>         struct extent_node *en;
>         unsigned int count = atomic_read(&et->node_cnt);
> +       unsigned int i = 0;
>
>         node = rb_first_cached(&et->root);
>         while (node) {
> @@ -391,6 +392,9 @@ static unsigned int __free_extent_tree(struct f2fs_sb_info *sbi,
>                 en = rb_entry(node, struct extent_node, rb_node);
>                 __release_extent_node(sbi, et, en);
>                 node = next;
> +
> +               if (nr_shrink && ++i >= nr_shrink)
> +                       break;
>         }
>
>         return count - atomic_read(&et->node_cnt);
> @@ -761,7 +765,7 @@ static void __update_extent_tree_range(struct inode *inode,
>         }
>
>         if (is_inode_flag_set(inode, FI_NO_EXTENT))
> -               __free_extent_tree(sbi, et);
> +               __free_extent_tree(sbi, et, 0);
>
>         if (et->largest_updated) {
>                 et->largest_updated = false;
> @@ -942,7 +946,8 @@ static unsigned int __shrink_extent_tree(struct f2fs_sb_info *sbi, int nr_shrink
>         list_for_each_entry_safe(et, next, &eti->zombie_list, list) {
>                 if (atomic_read(&et->node_cnt)) {
>                         write_lock(&et->lock);
> -                       node_cnt += __free_extent_tree(sbi, et);
> +                       node_cnt += __free_extent_tree(sbi, et,
> +                                       nr_shrink - node_cnt - tree_cnt);
>                         write_unlock(&et->lock);
>                 }
>                 f2fs_bug_on(sbi, atomic_read(&et->node_cnt));
> @@ -1095,7 +1100,7 @@ static unsigned int __destroy_extent_node(struct inode *inode,
>                 return 0;
>
>         write_lock(&et->lock);
> -       node_cnt = __free_extent_tree(sbi, et);
> +       node_cnt = __free_extent_tree(sbi, et, 0);
>         write_unlock(&et->lock);
>
>         return node_cnt;
> @@ -1117,7 +1122,7 @@ static void __drop_extent_tree(struct inode *inode, enum extent_type type)
>                 return;
>
>         write_lock(&et->lock);
> -       __free_extent_tree(sbi, et);
> +       __free_extent_tree(sbi, et, 0);
>         if (type == EX_READ) {
>                 set_inode_flag(inode, FI_NO_EXTENT);
>                 if (et->largest.len) {
> --
> 2.40.1
>
> Thanks,
>
> >               f2fs_balance_fs_bg(sbi, false);
> >
> >       if (!f2fs_is_checkpoint_ready(sbi))
>


Hi chao,

We have also considered this approach, but the problem still occurs
after retesting.
1. The problem still occurs in the following call of the unmount data process.
f2fs_put_super -> f2fs_leave_shrinker
2. Writing back the inode in the normal write-back process will
release the extent cache, and the problem still occurs. The stack is
as follows:
[H 103098.974356] c2 [<ffffffc008aee8a4>] (rb_erase+0x204/0x334)
[H 103098.974389] c2 [<ffffffc0088f8fd0>] (__release_extent_node+0xc8/0x168)
[H 103098.974425] c2 [<ffffffc0088fad74>]
(f2fs_update_extent_tree_range+0x4a0/0x724)
[H 103098.974459] c2 [<ffffffc0088fa8c0>] (f2fs_update_extent_cache+0x19c/0x1b0)
[H 103098.974495] c2 [<ffffffc0088edc70>] (f2fs_outplace_write_data+0x74/0xf0)
[H 103098.974525] c2 [<ffffffc0088ca834>] (f2fs_do_write_data_page+0x3e4/0x6c8)
[H 103098.974552] c2 [<ffffffc0088cb150>]
(f2fs_write_single_data_page+0x478/0xab0)
[H 103098.974574] c2 [<ffffffc0088d0bd0>] (f2fs_write_cache_pages+0x454/0xaac)
[H 103098.974596] c2 [<ffffffc0088d0698>] (__f2fs_write_data_pages+0x40c/0x4f0)
[H 103098.974617] c2 [<ffffffc0088cc860>] (f2fs_write_data_pages+0x30/0x40)
[H 103098.974645] c2 [<ffffffc0084c0e00>] (do_writepages+0x18c/0x3e8)
[H 103098.974678] c2 [<ffffffc0086503cc>] (__writeback_single_inode+0x48/0x498)
[H 103098.974720] c2 [<ffffffc0086562c8>] (writeback_sb_inodes+0x454/0x9b0)
[H 103098.974754] c2 [<ffffffc008655de8>] (__writeback_inodes_wb+0x198/0x224)
[H 103098.974788] c2 [<ffffffc008656d0c>] (wb_writeback+0x1c0/0x698)
[H 103098.974819] c2 [<ffffffc008655614>] (wb_do_writeback+0x420/0x54c)
[H 103098.974853] c2 [<ffffffc008654f50>] (wb_workfn+0xe4/0x388)

Powered by blists - more mailing lists