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

Gao Xiang
2020-05-07T07:24:42Z

Hi Chao,

On Thu, May 07, 2020 at 02:38:39PM +0800, Chao Yu wrote:
> On 2020/5/7 6:36, Gao Xiang wrote:
> > On Wed, May 06, 2020 at 12:16:13PM -0700, Eric Biggers wrote:
> >> On Wed, May 06, 2020 at 02:47:19PM +0800, Gao Xiang wrote:
> >>> 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...
>
> Another race condition could be:
>
> Thread 1 (fsync) Thread 2 (rename)
> - f2fs_sync_fs
> - try_to_fix_pino
> - f2fs_rename
> - down_write
> - file_lost_pino
> - up_write
> - down_write
> - file_got_pino
> - up_write

Yes, IMHO, I think it could be not proper to
take dir lock in fsync path anyway...

I would suggest as before (if it needs to be fixed).
And it seems no significant performance difference.

Thanks,
Gao Xiang

>
> Thanks,
>
> >>>
> >>
> >> Right, good observation. My patch makes it better, but it's still broken.
> >>
> >> I don't know how to fix it. If we see i_nlink == 1 and multiple hashed
> >> dentries, there doesn't appear to be a way to distingush which one corresponds
> >> to the remaining link on-disk (if any; it may not even be in the dcache), and
> >> which correspond to links that vfs_unlink() has deleted from disk but hasn't yet
> >> done d_delete() on.
> >>
> >> One idea would be choose one, then take inode_lock_shared(dir) and do
> >> __f2fs_find_entry() to check if the dentry is really still on-disk. That's
> >> heavyweight and error-prone though, and the locking could cause problems.
> >>
> >> I'm wondering though, does f2fs really need try_to_fix_pino() at all, and did it
> >> ever really work? It never actually updates the f2fs_inode::i_name to match the
> >> new directory. So independently of this bug with deleted links, I don't see how
> >> it can possibly work as intended.
> >
> > Part of my humble opinion would be "update pino in rename/unlink/link... such ops
> > instead of in fsync" (maybe it makes better sense of locking)... But actually I'm
> > not a f2fs folk now, just curious about what the original patch resolved with
> > these new extra igrab/iput (as I said before, I could not find some clue previously).
> >
> > Thanks,
> > Gao Xiang
> >
> >>
> >> - Eric
> >
> >
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