Messages in this thread | | | Subject | Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino | From | Chao Yu <> | Date | Thu, 7 May 2020 14:38:39 +0800 |
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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
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 > > > _______________________________________________ > Linux-f2fs-devel mailing list > Linux-f2fs-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel > . >
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