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To:     Matthew Wilcox <willy@infradead.org>
Cc:     Khalid Aziz <khalid.aziz@oracle.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
Message-ID: <e6a31927-8f93-22af-2d5a-9d80578e9317@redhat.com>
Date:   Mon, 16 Aug 2021 17:01:44 +0200
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On 16.08.21 16:40, Matthew Wilcox wrote:
> On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote:
>>>> I did not follow why we have to play games with MAP_PRIVATE, and having
>>>> private anonymous pages shared between processes that don't COW, introducing
>>>> new syscalls etc.
>>>
>>> It's not about SHMEM, it's about file-backed pages on regular
>>> filesystems.  I don't want to have XFS, ext4 and btrfs all with their
>>> own implementations of ARCH_WANT_HUGE_PMD_SHARE.
>>
>> Let me ask this way: why do we have to play such games with MAP_PRIVATE?
> 
> Are you referring to this?

Yes

> 
> : Mappings within this address range behave as if they were shared
> : between threads, so a write to a MAP_PRIVATE mapping will create a
> : page which is shared between all the sharers.
> 
> If so, that's a misunderstanding, because there are no games being played.
> What Khalid's saying there is that because the page tables are already
> shared for that range of address space, the COW of a MAP_PRIVATE will
> create a new page, but that page will be shared between all the sharers.
> The second write to a MAP_PRIVATE page (by any of the sharers) will not
> create a COW situation.  Just like if all the sharers were threads of
> the same process.
> 

It actually seems to be just like I understood it. We'll have multiple 
processes share anonymous pages writable, even though they are not using 
shared memory.

IMHO, sharing page tables to optimize for something kernel-internal 
(page table consumption) should be completely transparent to user space. 
Just like ARCH_WANT_HUGE_PMD_SHARE currently is unless I am missing 
something important.

The VM_MAYSHARE check in want_pmd_share()->vma_shareable() makes me 
assume that we really only optimize for MAP_SHARED right now, never for 
MAP_PRIVATE.

-- 
Thanks,

David / dhildenb