On Tue, 25 Feb 2020, David Hildenbrand wrote: > > > > I notice that this thread has veered off into QEMU ballooning > > territory: which may indeed be important, but there's nothing at all > > that I can contribute on that. I certainly do not want to slow down > > anything important, but remain convinced that the correct filesystem > > implementation for punching a hole is to punch a hole. > > I am not completely sure I follow all the shmem details (sorry!). But > trying to "punch a partial hole punch" into a hugetlbfs page will result > in the very same behavior as with shmem as of now, no? I believe so. > > FALLOC_FL_PUNCH_HOLE: "Within the specified range, partial filesystem > blocks are zeroed, and whole filesystem blocks are removed from the > file." ... After a successful call, subsequent reads from this range > will return zeros." > > So, as long as we are talking about partial blocks the documented > behavior seems to be to only zero the memory. > > Does this patch fix "FALLOC_FL_PUNCH_HOLE does not free blocks if called > in block granularity on shmem" (which would be a valid fix), Yes. The block size of tmpfs is (talking x86_64 for simplicity) 4KiB; but when mounted huge, it transparently takes advantage of 2MiB extents when it can. Rather like a disk-based filesystem always presenting a 4KiB block interface, but stored on disk in multisector extents. Whereas hugetlbfs is a different filesystem, which is and always has been limited to supporting only certain larger block sizes. > or does it > try to implement something that is not documented? (removing partial > blocks when called in sub-block granularity) No. > > I assume the latter, in which case I would interpret "punching a hole is > to punch a hole" as "punching sub-blocks will not free blocks". > > (if somebody could enlighten me which important piece I am missing or > messing up, that would be great :) ) > > -- > Thanks, > > David / dhildenb
