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

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Date:   Tue, 5 May 2020 18:24:28 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gao Xiang <hsiangkao@....com>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino

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.

> 
> >
> > 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