[PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

[Linus Torvalds]

On Mon, Dec 28, 2020 at 10:47 AM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> I personally think it's wrong to update vmf->pte at all. We should
> just have a local 'ptep' pointer that we update as we walk along. But
> that requires another change to the calling convention, namely to
> "do_set_pte()".

Actually, I think we should not use do_set_pte() at all.

About half of do_set_pte() is about the FAULT_FLAG_WRITE case, which
the fault-around code never has set (and would be wrong if it did).

So I think do_set_pte() should be made local to mm/memory.c, and the
filemap_map_pages() code should do it's own simplified version that
just does the non-writable case, and that just gets passed the address
and the pte pointer.

At that point, there would no longer be any need to update the
address/pte fields in the vmf struct, and in fact I think it could be
made a "const" pointer in this cal chain.

This is all just from looking at the code, I haven't tried to write a
patch to do this, so I might be missing some case.

             Linus