On Mon, Apr 6, 2020 at 8:47 PM Arvind Raghavan <raghavan.arvind@xxxxxxxxx> wrote: > > Hi! > > I am Arvind Raghavan, an undergraduate student at the Unversity > of Texas at Austin, working with Prof. Vijay Chidambaram. I've > been working on the Crashmonkey [1] project, which is a test > harness for file system crash consistency checks. Specifically, > we've been working on making a fuzzer to find new bugs, and we > discovered some weird behavior. Given the following workload: > > mkdir A > mkdir B > mkdir A/C > creat B/foo > sync (1) > link B/foo A/C/foo > fsync A (2) > <crash> > > Running on Linux 5.5.2, upon recovering the filesystem, the hard > linked file A/C/foo is not present. Expected on btrfs at least. Nothing changed in directory A after the 'sync', so the fsync is a noop. Only directory C and the inode of file B/foo had changes. We don't go recursively checking if any sub-directories changed. That would be very expensive, plus I don't think anyone expects that in real use cases. If you want A/C/foo persisted then fsync A/C or fsync AC/foo or fsync B/foo (at least on btrfs this should work). > However, if we replace (1) > with "fsync B/foo", then upon recovery the link persists. This Replacing the 'sync' with 'fsync B/foo' is different. When that fsync is done the inode is logged in btrfs, and then the "link B/foo A/C/foo" causes the log to be updated with the new dentry because the inode was logged before. It's the link creation that persist the new dentry, not the "fsync A". > behavior seems strange, as intuitively it seems that sync should > have at least as strong effect as fsync. In addition, it seems > that Chris Mason's definition of fsync guarantees here [2] might > require this workload to pass. > > It is important to note that even if we skip the final fsync (2), > the result is the same. Thus, the behavior is coming only from > the syncing operation at line (1). However, we were also curious > to know whether an fsync of the directory A here (2) should > persist the file A/C/foo. Chris mentions [2] that an fsync > of a directory means that "all the files/subdirs will exist". I don't think his definition includes recursion... nor that people usually expect that. I think he meant the sub-directory C and all other dentries of A will exist, not that dentries of C or any other descendants will be persisted. Thanks. > Should this apply to files created in subdirectories? > > > Thanks! > Arvind > > [1] https://www.github.com/utsaslab/crashmonkey > [2] https://www.spinics.net/lists/linux-btrfs/msg77330.html -- Filipe David Manana, “Whether you think you can, or you think you can't — you're right.”
