On Mon, Mar 08, 2021 at 09:13:55AM +0000, David Howells wrote: > Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > (0a) As (0) but using SEEK_DATA/SEEK_HOLE instead of bmap and opening the > > > file for every whole operation (which may combine reads and writes). > > > > I read that NFSv4 supports hole punching, so when using ->bmap() or SEEK_DATA > > to keep track of present data, it's hard to distinguish between an > > invalid cached range and a valid "cached hole". > > I wasn't exactly intending to permit caching over NFS. That leads to fun > making sure that the superblock you're caching isn't the one that has the > cache in it. > > However, we will need to handle hole-punching being done on a cached netfs, > even if that's just to completely invalidate the cache for that file. > > > With ->fiemap() you can at least make the distinction between a non existing > > and an UNWRITTEN extent. > > I can't use that for XFS, Ext4 or btrfs, I suspect. Christoph and Dave's > assertion is that the cache can't rely on the backing filesystem's metadata > because these can arbitrarily insert or remove blocks of zeros to bridge or > split extents. Well, that's not the big problem. The issue that makes FIEMAP unusable for determining if there is user data present in a file is that on-disk extent maps aren't exactly coherent with in-memory user data state. That is, we can have a hole on disk with delalloc user data in memory. There's user data in the file, just not on disk. Same goes for unwritten extents - there can be dirty data in memory over an unwritten extent, and it won't get converted to written until the data is written back and the filesystem runs a conversion transaction. So, yeah, if you use FIEMAP to determine where data lies in a file that is being actively modified, you're going get corrupt data sooner rather than later. SEEK_HOLE/DATA are coherent with in memory user data, so don't have this problem. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
