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

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Date:   Mon, 28 Dec 2020 13:58:40 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

On Mon, Dec 28, 2020 at 10:47 AM Linus Torvalds
<torvalds@...ux-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