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

[Gao Xiang]

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.

Thanks,
Gao Xiang

> 
> > 
> > >
> > > We need to find the dentry whose parent directory is still exists, i.e. the
> > > parent directory that is counting towards 'inode->i_nlink == 1'.
> > 
> > directory counting towards 'inode->i_nlink == 1', what's happening?
> 
> The non-deleted link is the one counted in i_nlink.
> 
> > 
> > >
> > > I think d_find_alias() is what we're looking for.
> > 
> > It may be simply dentry->d_parent (stable/positive as you said before, and it's
> > not empty). why need to d_find_alias()?
> 
> Because we need to get the dentry that hasn't been deleted yet, which isn't
> necessarily the one associated with the file descriptor being fsync()'ed.
> 
> > And what is the original problem? I could not get some clue from the original
> > patch description (I only saw some extra igrab/iput because of some unknown
> > reasons), it there some backtrace related to the problem?
> 
> The problem is that i_pino gets set incorrectly.  I just noticed this while
> reviewing the code.  It's not hard to reproduce, e.g.:
> 
> #include <unistd.h>
> #include <fcntl.h>
> #include <sys/stat.h>
> 
> int main()
> {
>         int fd;
> 
>         mkdir("dir1", 0700);
>         mkdir("dir2", 0700);
>         mknod("dir1/file", S_IFREG|0600, 0);
>         link("dir1/file", "dir2/file");
>         fd = open("dir2/file", O_WRONLY);
>         unlink("dir2/file");
>         write(fd, "X", 1);
>         fsync(fd);
> }
> 
> Then:
> 
> sync
> echo N | dump.f2fs -i $(stat -c %i dir1/file) /dev/vdb | grep 'i_pino'
> echo "dir1 (correct): $(stat -c %i dir1)"
> echo "dir2 (wrong): $(stat -c %i dir2)"
> 
> i_pino will point to dir2 rather than dir1 as expected.
> 
> - Eric
> 
> 
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