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

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Date:   Tue, 11 Jul 2023 13:36:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        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, 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
need a new design.  s390 seems to do a lot of unusual things.  I wish
you'd talk to the rest of us more.