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

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Date:   Thu, 3 Nov 2022 18:36:09 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 03.11.22 18:09, Linus Torvalds wrote:
> 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.

Thanks for the explanation. I brought it up because the generic name 
somehow felt weird in include/asm-generic/tlb.h. Skimming over the code 
I'd have expected something like TLB_ENCODE_PAGE_BITS, so making the 
"very generic" things "very specific" as long as it lives in tlb.h :)

-- 
Thanks,

David / dhildenb