linux-kernel - Re: mm: delay rmap removal until after TLB flush

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Date:   Thu, 3 Nov 2022 10:09:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Nick Piggin <npiggin@...il.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Joerg Roedel <jroedel@...e.de>,
        Uros Bizjak <ubizjak@...il.com>,
        Alistair Popple <apopple@...dia.com>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: mm: delay rmap removal until after TLB flush

On Thu, Nov 3, 2022 at 9:54 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But again, those changes would have made the patch bigger, which I
> didn't want at this point (and 'release_pages()' would need that
> clean-in-place anyway, unless we changed *that* too and made the whole
> page encoding be something widely available).

And just to clarify: this is not just me trying to expand the reach of my patch.

I'd suggest people look at mlock_pagevec(), and realize that LRU_PAGE
and NEW_PAGE are both *exactly* the same kind of "encoded_page" bits
that TLB_ZAP_RMAP is.

Except the mlock code does *not* show that in the type system, and
instead just passes a "struct page **" array around in pvec->pages,
and then you'd just better know that "oh, it's not *really* just a
page pointer".

So I really think that the "array of encoded page pointers" thing is a
generic notion that we *already* have.

It's just that we've done it disgustingly in the past, and I didn't
want to do that disgusting thing again.

So I would hope that the nasty things that the mlock code would some
day use the same page pointer encoding logic to actually make the
whole "this is not a page pointer that you can use directly, it has
low bits set for flags" very explicit.

I am *not* sure if then the actual encoded bits would be unified.
Probably not - you might have very different and distinct uses of the
encode_page() thing where the bits mean different things in different
contexts.

Anyway, this is me just explaining the thinking behind it all. The
page bit encoding is a very generic thing (well, "very generic" in
this case means "has at least one other independent user"), explaining
the very generic naming.

But at the same time, the particular _patch_ was meant to be very targeted.

So slightly schizophrenic name choices as a result.

             Linus