[PATCH v4 04/34] pgtable: Create struct ptdesc

[Matthew Wilcox]

On Thu, Jun 15, 2023 at 12:57:19AM -0700, Hugh Dickins wrote:
> Probably just trivial collisions in most architectures, which either
> of us can easily adjust to the other; powerpc likely to be more awkward,
> but fairly easily resolved; s390 quite a problem.
> 
> I've so far been unable to post a v2 of my series (and powerpc and s390
> were stupidly wrong in the v1), because a good s390 patch is not yet
> decided - Gerald Schaefer and I are currently working on that, on the
> s390 list (I took off most Ccs until we are settled and I can post v2).
> 
> As you have no doubt found yourself, s390 has sophisticated handling of
> free half-pages already, and I need to add rcu_head usage in there too:
> it's tricky to squeeze it all in, and ptdesc does not appear to help us
> in any way (though mostly it's just changing some field names, okay).
> 
> If ptdesc were actually allowing a flexible structure which architectures
> could add into, that would (in some future) be nice; but of course at
> present it's still fitting it all into one struct page, and mandating
> new restrictions which just make an architecture's job harder.

The intent is to get ptdescs to be dynamically allocated at some point
in the ~2-3 years out future when we have finished the folio project ...
which is not a terribly helpful thing for me to say.

I have three suggestions, probably all dreadful:

1. s390 could change its behaviour to always allocate page tables in
pairs.  That is, it fills in two pmd_t entries any time it takes a fault
in either of them.

2. We could allocate two or four pages at a time for s390 to allocate
2kB pages from.  That gives us a lot more space to store RCU heads.

3. We could use s390 as a guinea-pig for dynamic ptdesc allocation.
Every time we allocate a struct page, we have a slab cache for an
s390-special definition of struct ptdesc, we allocate a ptdesc and store
a pointer to that in compound_head.

We could sweeten #3 by doing that not just for s390 but also for every
configuration which has ALLOC_SPLIT_PTLOCKS today.  That would get rid
of the ambiguity between "is ptl a pointer or a lock".

> But I've no desire to undo powerpc's use of pt_frag_refcount:
> just warning that we may want to undo any use of it in s390.

I would dearly love ppc & s390 to use the _same_ scheme to solve the
same problem.