Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

[Linus Torvalds]

On Mon, Sep 28, 2020 at 12:36 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> So I'll do the pte wrprotect/restore removal. Anybody willing to do
> and test the sequence count approach?

So the wrprotect removal is trivial, with most of it being about the comments.

However, when I look at this, I am - once again - tempted to just add a

        if (__page_mapcount(page) > 1)
                return 1;

there too. Because we know it's a private mapping (shared mappings we
checked for with the "is_cow_mapping()" earlier), and the only case we
really care about is the one where the page is only mapped in the
current mm (because that's what a write pinning will have done, and as
mentioned, a read pinning doesn't do anything wrt fork() right now
anyway).

So if it's mapped in another mm, the COW clearly hasn't been broken by
a pin, and a read pinned page had already gone through a fork.

But the more I look at this code, the more I go "ok, I want somebody
to actually test this with the rdma case".

So I'll attach my suggested patch, but I won't actually commit it. I'd
really like to have this tested, possibly _together_ with the sequence
count addition..

               Linus

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