On Wed, Jun 1, 2022 at 8:10 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: > > So we can fix this by: > > 1. Using a relative pointer value other than 0 to represent a null > > pointer. Andres suggested (Size) -1. > > 2. Not storing the free page manager for the DSM in the main shared > > memory segment at byte offset 0. > > 3. Dropping the assertion while loudly singing "la la la la la la". > > I'm definitely down on #3, because that just leaves the ambiguity > in place to bite somewhere else in future. #1 would work as long > as nobody expects memset-to-zero to produce null relptrs, but that > doesn't seem very nice either. > > On the whole, wasting MAXALIGN worth of memory seems like the least bad > alternative, but I wonder if we ought to do it right here as opposed > to somewhere in the DSM code proper. Why is this DSM space not like > other DSM spaces in starting with a TOC?
This FPM isn't in a DSM. (It happens to have DSMs *inside it*, because I'm using it as a separate DSM allocator: instead of making them with dsm_impl.c mechanisms, this one recycles space from the main shmem area). I view FPM as a reusable 4kb page-based memory allocator that could have many potential uses, not as a thing that must live inside another thing with a TOC. The fact that it uses the relptr thing makes it possible to use FPM inside DSMs too, but that doesn't mean it has to be used inside a DSM. I vote for #1.
