On Tue, Sep 01, 2020 at 03:18:54PM -0400, J. Bruce Fields wrote: > On Tue, Sep 01, 2020 at 01:40:16PM -0400, Anna Schumaker wrote: > > On Tue, Sep 1, 2020 at 12:49 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > > > On Mon, Aug 31, 2020 at 02:16:26PM -0400, Anna Schumaker wrote: > > > > On Fri, Aug 28, 2020 at 5:56 PM J. Bruce Fields <bfields@xxxxxxxxxx> wrote: > > > > > We really don't want to bother encoding small holes. I doubt > > > > > filesystems want to bother with them either. Do they give us any > > > > > guarantees as to the minimum size of a hole? > > > > > > > > The minimum size seems to be PAGE_SIZE from everything I've seen. > > > > > > OK, can we make that assumption explicit? It'd simplify stuff like > > > this. > > > > I'm okay with that, but it's technically up to the underlying filesystem. > > Maybe we should ask on linux-fsdevel. > > Maybe minimum hole length isn't the right question: suppose at time 1 a > file has a single hole at bytes 100-200, then it's modified so at time 2 > it has a hole at bytes 50-150. If you lseek(fd, 0, SEEK_HOLE) at time > 1, you'll get 100. Then if you lseek(fd, 100, SEEK_DATA) at time 2, > you'll get 150. So you'll encode a 50-byte hole in the READ_PLUS reply > even though the file never had a hole smaller than 100 bytes. > > Minimum hole alignment might be the right idea. > > If we can't get that: maybe just teach encode_read to stop when it > *either* returns maxcount worth of file data (and holes) *or* maxcount > of encoded xdr data, just to prevent a weird filesystem from triggering > a bug. Alternatively, if it's easier, we could enforce a minimum alignment by rounding up the result of SEEK_HOLE to the nearest multiple of (say) 512 bytes, and rounding down the result of SEEK_DATA. --b.
