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Date:   Wed, 4 Aug 2021 15:17:51 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>,
        akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        christian.brauner@ubuntu.com, ebiederm@xmission.com,
        adobriyan@gmail.com, songmuchun@bytedance.com, axboe@kernel.dk,
        vincenzo.frascino@arm.com, catalin.marinas@arm.com,
        peterz@infradead.org, chinwen.chang@mediatek.com,
        linmiaohe@huawei.com, jannh@google.com, apopple@nvidia.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, ivan.teterevkov@nutanix.com,
        florian.schmidt@nutanix.com, carl.waldspurger@nutanix.com,
        jonathan.davies@nutanix.com
Subject: Re: [PATCH 0/1] pagemap: swap location for shared pages
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On Wed, Aug 04, 2021 at 08:49:14PM +0200, David Hildenbrand wrote:
> TBH, I tend to really dislike the PTE marker idea. IMHO, we shouldn't store
> any state information regarding shared memory in per-process page tables: it
> just doesn't make too much sense.
> 
> And this is similar to SOFTDIRTY or UFFD_WP bits: this information actually
> belongs to the shared file ("did *someone* write to this page", "is
> *someone* interested into changes to that page", "is there something"). I
> know, that screams for a completely different design in respect to these
> features.
> 
> I guess we start learning the hard way that shared memory is just different
> and requires different interfaces than per-process page table interfaces we
> have (pagemap, userfaultfd).
> 
> I didn't have time to explore any alternatives yet, but I wonder if tracking
> such stuff per an actual fd/memfd and not via process page tables is
> actually the right and clean approach. There are certainly many issues to
> solve, but conceptually to me it feels more natural to have these shared
> memory features not mangled into process page tables.

Yes, we can explore all the possibilities, I'm totally fine with it.

I just want to say I still don't think when there's page cache then we must put
all the page-relevant things into the page cache.

They're shared by processes, but process can still have its own way to describe
the relationship to that page in the cache, to me it's as simple as "we allow
process A to write to page cache P", while "we don't allow process B to write
to the same page" like the write bit.

Swap information is indeed tricky because it's shared by all the processes, but
so far I still see it as a doable approach as long as it can solve the problem.

I am not against the approach mentioned in this patch either - but I still
share my concerns, as that's not normally what we do with existing interfaces.

Thanks,

-- 
Peter Xu