linux-kernel - Re: [PATCH v5 00/38] New page table range API

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Date:   Tue, 11 Jul 2023 17:24:40 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v5 00/38] New page table range API

On Tue, 11 Jul 2023 13:36:27 +0100
Matthew Wilcox <willy@...radead.org> wrote:

> On Tue, Jul 11, 2023 at 11:07:06AM +0200, Christian Borntraeger wrote:
> > Am 10.07.23 um 22:43 schrieb Matthew Wilcox (Oracle):  
> > > This patchset changes the API used by the MM to set up page table entries.
> > > The four APIs are:
> > >      set_ptes(mm, addr, ptep, pte, nr)
> > >      update_mmu_cache_range(vma, addr, ptep, nr)
> > >      flush_dcache_folio(folio)
> > >      flush_icache_pages(vma, page, nr)
> > > 
> > > flush_dcache_folio() isn't technically new, but no architecture
> > > implemented it, so I've done that for them.  The old APIs remain around
> > > but are mostly implemented by calling the new interfaces.
> > > 
> > > The new APIs are based around setting up N page table entries at once.
> > > The N entries belong to the same PMD, the same folio and the same VMA,
> > > so ptep++ is a legitimate operation, and locking is taken care of for
> > > you.  Some architectures can do a better job of it than just a loop,
> > > but I have hesitated to make too deep a change to architectures I don't
> > > understand well.
> > > 
> > > One thing I have changed in every architecture is that PG_arch_1 is now a
> > > per-folio bit instead of a per-page bit.  This was something that would
> > > have to happen eventually, and it makes sense to do it now rather than
> > > iterate over every page involved in a cache flush and figure out if it
> > > needs to happen.  
> > 
> > I think we do use PG_arch_1 on s390 for our secure page handling and
> > making this perf folio instead of physical page really seems wrong
> > and it probably breaks this code.  
> 
> Per-page flags are going away in the next few years, so you're going to

For each 4k physical page frame, we need to keep track whether it is
secure or not.

A bit in struct page seems the most logical choice. If that's not
possible anymore, how would you propose we should do?

> need a new design.  s390 seems to do a lot of unusual things.  I wish

s390 is an unusual architecture. we are working on un-weirding our
code, but it takes time

> you'd talk to the rest of us more.