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

[Daeho Jeong <daeho43@gmail.com>]

On Mon, Feb 13, 2023 at 1:47 AM Chao Yu <chao@kernel.org> wrote:
>
> On 2023/2/10 2:18, 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    | 44 +++++++++++++++++++++++++++++---------------
> >   fs/f2fs/inode.c   | 11 +++++++++--
> >   fs/f2fs/segment.c |  3 ---
> >   fs/f2fs/super.c   |  2 --
> >   4 files changed, 38 insertions(+), 22 deletions(-)
> >
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 300eae8b5415..6436c52e7913 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -1863,7 +1863,10 @@ 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;
> >   }
> >
> > @@ -1878,8 +1881,12 @@ static int f2fs_file_flush(struct file *file, fl_owner_t id)
> >        * before dropping file lock, it needs to do in ->flush.
> >        */
> >       if (F2FS_I(inode)->atomic_write_task == current &&
> > -                             (current->flags & PF_EXITING))
> > +                             (current->flags & PF_EXITING)) {
> > +             inode_lock(inode);
> >               f2fs_abort_atomic_write(inode, true);
> > +             inode_unlock(inode);
> > +     }
> > +
> >       return 0;
> >   }
> >
> > @@ -2085,19 +2092,28 @@ 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);
> > @@ -2105,8 +2121,6 @@ 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 28c9c72dda2a..7bf660d4cad9 100644
> > --- a/fs/f2fs/inode.c
> > +++ b/fs/f2fs/inode.c
> > @@ -777,11 +777,18 @@ 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;
> > +     }
>
> It looks "fi->cow_inode = NULL" here may race w/ cow_inode allocation in
> f2fs_ioc_start_atomic_write due to f2fs_write_inode() has not been covered
> by inode_lock()? IIUC.

Sorry, I couldn't understand it, since I couldn't find any relation
between f2fs_ioc_start_atomic_write and f2fs_write_inode. Could you
elaborate more on this? I thought the code might be safe, since it
happens in the inode eviction phase.

>
> Thanks,
>
> > +
> >       trace_f2fs_evict_inode(inode);
> >       truncate_inode_pages_final(&inode->i_data);
> >
> > @@ -866,7 +873,7 @@ 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 ead3f35f501d..719329c1808c 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 c11a161ba5be..aa55dc12aff2 100644
> > --- a/fs/f2fs/super.c
> > +++ b/fs/f2fs/super.c
> > @@ -1427,8 +1427,6 @@ static int f2fs_drop_inode(struct inode *inode)
> >                       atomic_inc(&inode->i_count);
> >                       spin_unlock(&inode->i_lock);
> >
> > -                     f2fs_abort_atomic_write(inode, true);
> > -
> >                       /* should remain fi->extent_tree for writepage */
> >                       f2fs_destroy_extent_node(inode);
> >


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