Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino

Gao Xiang
2020-05-06T06:47:46Z

On Wed, May 06, 2020 at 09:58:22AM +0800, Gao Xiang wrote:
> On Tue, May 05, 2020 at 06:24:28PM -0700, Eric Biggers wrote:
> > On Wed, May 06, 2020 at 08:14:07AM +0800, Gao Xiang wrote:
> > > >
> > > > Actually, I think this is wrong because the fsync can be done via a file
> > > > descriptor that was opened to a now-deleted link to the file.
> > >
> > > I'm still confused about this...
> > >
> > > I don't know what's wrong with this version from my limited knowledge?
> > > inode itself is locked when fsyncing, so
> > >
> > > if the fsync inode->i_nlink == 1, this inode has only one hard link
> > > (not deleted yet) and should belong to a single directory; and
> > >
> > > the only one parent directory would not go away (not deleted as well)
> > > since there are some dirents in it (not empty).
> > >
> > > Could kindly explain more so I would learn more about this scenario?
> > > Thanks a lot!
> >
> > i_nlink == 1 just means that there is one non-deleted link. There can be links
> > that have since been deleted, and file descriptors can still be open to them.
>
> Thanks for your inspiration. You are right, thanks.
>
> Correct my words... I didn't check f2fs code just now, it seems f2fs doesn't
> take inode_lock as some other fs like __generic_file_fsync or ubifs_fsync.
>
> And i_sem locks nlink / try_to_fix_pino similarly in some extent. It seems
> no race by using d_find_alias here. Thanks again.
>

(think more little bit just now...)

Thread 1: Thread 2 (fsync):
vfs_unlink try_to_fix_pino
f2fs_unlink
f2fs_delete_entry
f2fs_drop_nlink (i_sem, inode->i_nlink = 1)

(... but this dentry still hashed) i_sem, check inode->i_nlink = 1
i_sem d_find_alias

d_delete

I'm not sure if fsync could still use some wrong alias by chance..
completely untested, maybe just noise...

Thanks,
Gao Xiang