Re: [f2fs-dev] [PATCH] f2fs: synchronize atomic write aborts

From: Chao Yu <chao@kernel.org>
To: Daeho Jeong <daeho43@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
	Daeho Jeong <daehojeong@google.com>,
	syzbot+823000d23b3400619f7c@syzkaller.appspotmail.com
Subject: Re: [f2fs-dev] [PATCH] f2fs: synchronize atomic write aborts
Date: Tue, 31 Jan 2023 19:37:49 +0800	[thread overview]
Message-ID: <8f1c15a3-d056-7709-af45-fe7cba56463f@kernel.org> (raw)
In-Reply-To: <CACOAw_zhVgS84gOXpfZuvptMgsZDhP3QX2EFm=5CoKibB+3V1A@mail.gmail.com>

Hi Daeho,

On 2023/1/31 0:34, Daeho Jeong wrote:
> Hi Chao,
> 
> I read your patch series now and I like it.

Thank you for checking the patches. :)

> However, how about a race condition between start_atomic_write and
> abort_atomic_write?

Yup, I noticed that issue, I guess we can avoid this race condition by
covering these two flows w/ i_atomic_sem.

> abort_atomic_write is called without inode_lock in closing filp scenarios.
> What do you think about this?

I'm fine w/ your change as it's more clean, but it's better to drop cow_inode's
page cache if atomic_write is committed or aborted to avoid caching obsolete page?

Thanks,

> 
> Thanks,
> 
> 
> On Fri, Jan 27, 2023 at 6:07 PM Chao Yu <chao@kernel.org> wrote:
>>
>> Hi Daeho, Jaegeuk,
>>
>> Please take a look at patchset in below link:
>>
>> https://lore.kernel.org/linux-f2fs-devel/20230109034453.490176-1-chao@kernel.org/T/#t
>>
>> In PATCH 4/5, I'm trying to fix the same issue w/ alternative way, let me
>> know your preference. :)
>>
>> One comment as below.
>>
>> On 2023/1/13 8:49, Daeho Jeong wrote:
>>> From: Daeho Jeong <daehojeong@google.com>
>>>
>>> To fix a race condition between atomic write aborts, I use the inode
>>> lock and make COW inode to be re-usable thoroughout the whole
>>> atomic file inode lifetime.
>>>
>>> Reported-by: syzbot+823000d23b3400619f7c@syzkaller.appspotmail.com
>>> Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
>>> Signed-off-by: Daeho Jeong <daehojeong@google.com>
>>> ---
>>>    fs/f2fs/file.c    | 43 ++++++++++++++++++++++++++++---------------
>>>    fs/f2fs/inode.c   | 11 +++++++++--
>>>    fs/f2fs/segment.c |  3 ---
>>>    fs/f2fs/super.c   |  2 --
>>>    4 files changed, 37 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>> index ecbc8c135b49..ff072a9ed258 100644
>>> --- a/fs/f2fs/file.c
>>> +++ b/fs/f2fs/file.c
>>> @@ -1866,7 +1866,10 @@ static int f2fs_release_file(struct inode *inode, struct file *filp)
>>>                        atomic_read(&inode->i_writecount) != 1)
>>>                return 0;
>>>
>>> +     inode_lock(inode);
>>>        f2fs_abort_atomic_write(inode, true);
>>> +     inode_unlock(inode);
>>> +
>>>        return 0;
>>>    }
>>>
>>> @@ -1880,8 +1883,11 @@ static int f2fs_file_flush(struct file *file, fl_owner_t id)
>>>         * until all the writers close its file. Since this should be done
>>>         * before dropping file lock, it needs to do in ->flush.
>>>         */
>>> -     if (F2FS_I(inode)->atomic_write_task == current)
>>> +     if (F2FS_I(inode)->atomic_write_task == current) {
>>> +             inode_lock(inode);
>>>                f2fs_abort_atomic_write(inode, true);
>>> +             inode_unlock(inode);
>>> +     }
>>>        return 0;
>>>    }
>>>
>>> @@ -2087,19 +2093,28 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
>>>                goto out;
>>>        }
>>>
>>> -     /* Create a COW inode for atomic write */
>>> -     pinode = f2fs_iget(inode->i_sb, fi->i_pino);
>>> -     if (IS_ERR(pinode)) {
>>> -             f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
>>> -             ret = PTR_ERR(pinode);
>>> -             goto out;
>>> -     }
>>> +     /* Check if the inode already has a COW inode */
>>> +     if (fi->cow_inode == NULL) {
>>> +             /* Create a COW inode for atomic write */
>>> +             pinode = f2fs_iget(inode->i_sb, fi->i_pino);
>>> +             if (IS_ERR(pinode)) {
>>> +                     f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
>>> +                     ret = PTR_ERR(pinode);
>>> +                     goto out;
>>> +             }
>>>
>>> -     ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
>>> -     iput(pinode);
>>> -     if (ret) {
>>> -             f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
>>> -             goto out;
>>> +             ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
>>> +             iput(pinode);
>>> +             if (ret) {
>>> +                     f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
>>> +                     goto out;
>>> +             }
>>> +
>>> +             set_inode_flag(fi->cow_inode, FI_COW_FILE);
>>> +             clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
>>> +     } else {
>>> +             /* Reuse the already created COW inode */
>>> +             f2fs_do_truncate_blocks(fi->cow_inode, 0, true);
>>>        }
>>>
>>>        f2fs_write_inode(inode, NULL);
>>> @@ -2107,8 +2122,6 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
>>>        stat_inc_atomic_inode(inode);
>>>
>>>        set_inode_flag(inode, FI_ATOMIC_FILE);
>>> -     set_inode_flag(fi->cow_inode, FI_COW_FILE);
>>> -     clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
>>>
>>>        isize = i_size_read(inode);
>>>        fi->original_i_size = isize;
>>> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
>>> index ff6cf66ed46b..4921f7209e28 100644
>>> --- a/fs/f2fs/inode.c
>>> +++ b/fs/f2fs/inode.c
>>> @@ -766,11 +766,18 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
>>>    void f2fs_evict_inode(struct inode *inode)
>>>    {
>>>        struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>> -     nid_t xnid = F2FS_I(inode)->i_xattr_nid;
>>> +     struct f2fs_inode_info *fi = F2FS_I(inode);
>>> +     nid_t xnid = fi->i_xattr_nid;
>>>        int err = 0;
>>>
>>>        f2fs_abort_atomic_write(inode, true);
>>>
>>> +     if (fi->cow_inode) {
>>> +             clear_inode_flag(fi->cow_inode, FI_COW_FILE);
>>> +             iput(fi->cow_inode);
>>> +             fi->cow_inode = NULL;
>>> +     }
>>> +
>>>        trace_f2fs_evict_inode(inode);
>>>        truncate_inode_pages_final(&inode->i_data);
>>>
>>> @@ -857,7 +864,7 @@ void f2fs_evict_inode(struct inode *inode)
>>>        stat_dec_inline_inode(inode);
>>>        stat_dec_compr_inode(inode);
>>>        stat_sub_compr_blocks(inode,
>>> -                     atomic_read(&F2FS_I(inode)->i_compr_blocks));
>>> +                     atomic_read(&fi->i_compr_blocks));
>>>
>>>        if (likely(!f2fs_cp_error(sbi) &&
>>>                                !is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index ae3c4e5474ef..536d7c674b04 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -192,9 +192,6 @@ void f2fs_abort_atomic_write(struct inode *inode, bool clean)
>>>        if (!f2fs_is_atomic_file(inode))
>>>                return;
>>>
>>> -     clear_inode_flag(fi->cow_inode, FI_COW_FILE);
>>> -     iput(fi->cow_inode);
>>> -     fi->cow_inode = NULL;
>>>        release_atomic_write_cnt(inode);
>>>        clear_inode_flag(inode, FI_ATOMIC_COMMITTED);
>>>        clear_inode_flag(inode, FI_ATOMIC_REPLACE);
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 1f812b9ce985..10463f084d30 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -1430,8 +1430,6 @@ static int f2fs_drop_inode(struct inode *inode)
>>>                        atomic_inc(&inode->i_count);
>>>                        spin_unlock(&inode->i_lock);
>>>
>>> -                     f2fs_abort_atomic_write(inode, true);
>>
>> In order to avoid caching obsolete page of cow_inode, how about truncating
>> them here?
>>
>> if (f2fs_is_atomic_file() && cow_inode)
>>          truncate_inode_pages_final(&cow_inode->i_data);
>>
>> Thanks,
>>
>>> -
>>>                        /* should remain fi->extent_tree for writepage */
>>>                        f2fs_destroy_extent_node(inode);
>>>