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? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
