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

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Date:   Thu, 13 Jul 2023 22:27:21 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
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



Am 13.07.23 um 15:42 schrieb Matthew Wilcox:
> On Thu, Jul 13, 2023 at 12:42:44PM +0200, Christian Borntraeger wrote:
>>
>>
>> Am 11.07.23 um 14:36 schrieb Matthew Wilcox:
>>> 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.
>>
>> I understand you point from a logical point of view, but a 4k page frame
>> is also a hardware defined memory region. And I think not only for us.
>> How do you want to implement hardware poisoning for example?
>> Marking the whole folio with PG_hwpoison seems wrong.
> 
> For hardware poison, we can't use the page for any other purpose any more.
> So one of the 16 types of pointer is for hardware poison.  That doesn't
> seem like it's a solution that could work for secure/insecure pages?
> 
> But what I'm really wondering is why you need to transition pages
> between secure/insecure on a 4kB boundary.  What's the downside to doing
> it on a 16kB or 64kB boundary, or whatever size has been allocated?

The export and import for more pages will be more expensive, but I assume that
we would then also use the larger chunks (e.g. for paging). The more interesting
problem is that the guest can make a page shared/non-shared on a 4kb granularity.

Stupid question: can folios be split into folio,single page,folio when needed?