[PATCH v3 03/34] s390: Use pt_frag_refcount for pagetables

[Vishal Moola]

On Thu, Jun 1, 2023 at 6:19 AM Gerald Schaefer
<gerald.schaefer at linux.ibm.com> wrote:
>
>  On Wed, 31 May 2023 14:30:01 -0700
> "Vishal Moola (Oracle)" <vishal.moola at gmail.com> wrote:
>
> > s390 currently uses _refcount to identify fragmented page tables.
> > The page table struct already has a member pt_frag_refcount used by
> > powerpc, so have s390 use that instead of the _refcount field as well.
> > This improves the safety for _refcount and the page table tracking.
> >
> > This also allows us to simplify the tracking since we can once again use
> > the lower byte of pt_frag_refcount instead of the upper byte of _refcount.
>
> This would conflict with s390 impact of pte_free_defer() work from Hugh Dickins
> https://lore.kernel.org/lkml/35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com/
> https://lore.kernel.org/lkml/6dd63b39-e71f-2e8b-7e0-83e02f3bcb39@google.com/
>
> There he uses pt_frag_refcount, or rather pt_mm in the same union, to save
> the mm_struct for deferred pte_free().
>
> I still need to look closer into both of your patch series, but so far it
> seems that you have no hard functional requirement to switch from _refcount
> to pt_frag_refcount here, for s390.
>
> If this is correct, and you do not e.g. need this to make some other use
> of _refcount, I would suggest to drop this patch.

The goal of this preparation patch is to consolidate s390's usage of
struct page fields so that struct ptdesc can be smaller. Its not particularly
mandatory; leaving _refcount in ptdesc only increases the struct by
8 bytes and can always be changed later.

However it is a little annoying since s390 is the only architecture
that egregiously uses space throughout struct page for their page
tables, rather than just the page table struct. For example, s390
gmap uses page->index which also aliases with pt_mm and
pt_frag_refcount. I'm not sure if/how gmap page tables interact
with s390 process page tables at all, but if it does that could
potentially cause problems with Hugh's patch as well :(

I can add _refcount to ptdesc if we would like, but I still
prefer if s390 could be simplified instead.