Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <3259ad30-c129-84fc-9643-0aeaeeb3c806@redhat.com>
Date:   Thu, 3 Nov 2022 18:36:09 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.0
Subject: Re: mm: delay rmap removal until after TLB flush
Content-Language: en-US
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
        Nick Piggin <npiggin@gmail.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Jann Horn <jannh@google.com>,
        John Hubbard <jhubbard@nvidia.com>, X86 ML <x86@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Joerg Roedel <jroedel@suse.de>,
        Uros Bizjak <ubizjak@gmail.com>,
        Alistair Popple <apopple@nvidia.com>,
        linux-arch <linux-arch@vger.kernel.org>
References: <B88D3073-440A-41C7-95F4-895D3F657EF2@gmail.com>
 <47678198-C502-47E1-B7C8-8A12352CDA95@gmail.com>
 <CAHk-=wjzngbbwHw4nAsqo_RpyOtUDk5G+Wus=O0w0A6goHvBWA@mail.gmail.com>
 <CAHk-=wijU_YHSZq5N7vYK+qHPX0aPkaePaGOyWk4aqMvvSXxJA@mail.gmail.com>
 <140B437E-B994-45B7-8DAC-E9B66885BEEF@gmail.com>
 <CAHk-=wjX_P78xoNcGDTjhkgffs-Bhzcwp-mdsE1maeF57Sh0MA@mail.gmail.com>
 <CAHk-=wio=UKK9fX4z+0CnyuZG7L+U9OB7t7Dcrg4FuFHpdSsfw@mail.gmail.com>
 <CAHk-=wgz0QQd6KaRYQ8viwkZBt4xDGuZTFiTB8ifg7E3F2FxHg@mail.gmail.com>
 <CAHk-=wiwt4LC-VmqvYrphraF0=yQV=CQimDCb0XhtXwk8oKCCA@mail.gmail.com>
 <Y1+XCALog8bW7Hgl@hirez.programming.kicks-ass.net>
 <CAHk-=wjnvPA7mi-E3jVEfCWXCNJNZEUjm6XODbbzGOh9c8mhgw@mail.gmail.com>
 <CAHk-=wjjXQP7PTEXO4R76WPy1zfQad_DLKw1GKU_4yWW1N4n7w@mail.gmail.com>
 <4f6d8fb5-6be5-a7a8-de8e-644da66b5a3d@redhat.com>
 <CAHk-=wiDg_1up8K4PhK4+kzPN7xJG297=nw+tvgrGn7aVgZdqw@mail.gmail.com>
 <CAHk-=wgReY6koZTKT97NsCczzr4uYAA66iePv=S_RL-_D-9mmQ@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <CAHk-=wgReY6koZTKT97NsCczzr4uYAA66iePv=S_RL-_D-9mmQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 03.11.22 18:09, Linus Torvalds wrote:
> On Thu, Nov 3, 2022 at 9:54 AM Linus Torvalds
> <torvalds@linux-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