Messages in this thread | | | Date | Tue, 5 May 2020 18:24:28 -0700 | From | Eric Biggers <> | Subject | Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino |
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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
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