On Fri, Aug 11, 2017 at 4:27 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Thu, Aug 10, 2017 at 11:39:28PM -0700, Dan Williams wrote: >> >From falloc.h: >> >> FALLOC_FL_SEAL_BLOCK_MAP is used to seal (make immutable) all of the >> file logical-to-physical extent offset mappings in the file. The >> purpose is to allow an application to assume that there are no holes >> or shared extents in the file and that the metadata needed to find >> all the physical extents of the file is stable and can never be >> dirtied. >> >> For now this patch only permits setting the in-memory state of >> S_IOMAP_IMMMUTABLE. Support for clearing and persisting the state is >> saved for later patches. >> >> The implementation is careful to not allow the immutable state to change >> while any process might have any established mappings. It reuses the >> existing xfs_reflink_unshare() and xfs_alloc_file_space() to unshare >> extents and fill all holes in the file. It then holds XFS_ILOCK_EXCL >> while it validates the file is in the proper state and sets >> S_IOMAP_IMMUTABLE. > > SO I've been thinking about this - I'm thinking that we need to > separate the changes to the extent map from the action of sealing > the extent map. > > That is, I have a need to freeze an extent map without any > modification to it at all and breaking all the sharing and filling > all the holes completely screws up the file layout I need to > preserve. i.e. I want to be able to freeze the maps of a pair of > reflinked files so I can use FIEMAP to query only the changed blocks > and then read out the data in the private/changed blocks in the > reflinked file. I only need a temporary seal (if we crash it goes > away), so maybe there's another command modifier flag needed here, > too. > > The DAX allocation requirements can be handled by a preallocation > call to filll holes with zeros, followed by an unshare call to break > all the COW mappings, and then the extent map can be sealed. If you > need to check for holes after sealing, SEEK_HOLE will tell you what > you need to know... > > My preference really is for each fallocate command to do just one > thing - having the seal operation also modify the extent map > means it's not useful for the use cases where we need the extent map > to remain unmodified.... > > Thoughts? That does seem to better follow the principle of least surprise and make the interface more composable. However, for the DAX case do we now end up needing a SEEK_SHARED, or something similar to check that we sealed the file without shared extents? -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
