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

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Message-ID: <967d8ca8-b945-d4ec-dce7-a4180830384f@linux.ibm.com>
Date:   Wed, 2 Nov 2022 10:23:14 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     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


Am 02.11.22 um 10:14 schrieb Christian Borntraeger:
> Am 31.10.22 um 19:43 schrieb Linus Torvalds:
>> Updated subject line, and here's the link to the original discussion
>> for new people:
>>
>>      https://lore.kernel.org/all/B88D3073-440A-41C7-95F4-895D3F657EF2@gmail.com/
>>
>> On Mon, Oct 31, 2022 at 10:28 AM Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>>>
>>> Ok. At that point we no longer have the pte or the virtual address, so
>>> it's not going to be exactly the same debug output.
>>>
>>> But I think it ends up being fairly natural to do
>>>
>>>          VM_WARN_ON_ONCE_PAGE(page_mapcount(page) < 0, page);
>>>
>>> instead, and I've fixed that last patch up to do that.
>>
>> Ok, so I've got a fixed set of patches based on the feedback from
>> PeterZ, and also tried to do the s390 updates for this blindly, and
>> pushed them out into a git branch:
>>
>>      https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?h=mmu_gather-race-fix
>>
>> If people really want to see the patches in email again, I can do
>> that, but most of you already have, and the changes are either trivial
>> fixes or the s390 updates.
>>
>> For the s390 people that I've now added to the participant list maybe
>> the git tree is fine - and the fundamental explanation of the problem
>> is in that top-most commit (with the three preceding commits being
>> prep-work). Or that link to the thread about this all.
> 
> Adding Gerald.

now the correct Gerald....
> 
>>
>> That top-most commit is also where I tried to fix things up for s390
>> that uses its own non-gathering TLB flush due to
>> CONFIG_MMU_GATHER_NO_GATHER.
>>
>> NOTE NOTE NOTE! Unlike my regular git branch, this one may end up
>> rebased etc for further comments and fixes. So don't consider that
>> stable, it's still more of an RFC branch.
>>
>> At a minimum I'll update it with Ack's etc, assuming I get those, and
>> my s390 changes are entirely untested and probably won't work.
>>
>> As far as I can tell, s390 doesn't actually *have* the problem that
>> causes this change, because of its synchronous TLB flush, but it
>> obviously needs to deal with the change of rmap zapping logic.
>>
>> Also added a few people who are explicitly listed as being mmu_gather
>> maintainers. Maybe people saw the discussion on the linux-mm list, but
>> let's make it explicit.
>>
>> Do people have any objections to this approach, or other suggestions?
>>
>> I do *not* consider this critical, so it's a "queue for 6.2" issue for me.
>>
>> It probably makes most sense to queue in the -MM tree (after the thing
>> is acked and people agree), but I can keep that branch alive too and
>> just deal with it all myself as well.
>>
>> Anybody?
>>
>>                       Linus
> 
> It certainly needs a build fix for s390:
> 
> 
> In file included from kernel/sched/core.c:78:
> ./arch/s390/include/asm/tlb.h: In function '__tlb_remove_page_size':
> ./arch/s390/include/asm/tlb.h:50:17: error: implicit declaration of function 'page_zap_pte_rmap' [-Werror=implicit-function-declaration]
>     50 |                 page_zap_pte_rmap(page);
>        |                 ^~~~~~~~~~~~~~~~~