linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <914e826e-3fab-4540-d3a1-24ca39b1cf0a@redhat.com>
Date:   Wed, 19 Apr 2023 13:30:57 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
        hca@...ux.ibm.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, will@...nel.org, aneesh.kumar@...ux.ibm.com,
        akpm@...ux-foundation.org, arnd@...db.de, keescook@...omium.org,
        paulmck@...nel.org, jpoimboe@...nel.org, samitolvanen@...gle.com,
        ardb@...nel.org, juerg.haefliger@...onical.com,
        rmk+kernel@...linux.org.uk, geert+renesas@...der.be,
        tony@...mide.com, linus.walleij@...aro.org,
        sebastian.reichel@...labora.com, nick.hawkins@....com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, vschneid@...hat.com, dhildenb@...hat.com,
        alougovs@...hat.com, jannh@...gle.com,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

On 06.04.23 20:27, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 05:51:52PM +0200, David Hildenbrand wrote:
>> On 06.04.23 17:02, Peter Zijlstra wrote:
> 
>>> DavidH, what do you thikn about reviving Jann's patches here:
>>>
>>>     https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1
>>>
>>> Those are far more invasive, but afaict they seem to do the right thing.
>>>
>>
>> I recall seeing those while discussed on security@...nel.org. What we
>> currently have was (IMHO for good reasons) deemed better to fix the issue,
>> especially when caring about backports and getting it right.
> 
> Yes, and I think that was the right call. However, we can now revisit
> without having the pressure of a known defect and backport
> considerations.
> 
>> The alternative that was discussed in that context IIRC was to simply
>> allocate a fresh page table, place the fresh page table into the list
>> instead, and simply free the old page table (then using common machinery).
>>
>> TBH, I'd wish (and recently raised) that we could just stop wasting memory
>> on page tables for THPs that are maybe never going to get PTE-mapped ... and
>> eventually just allocate on demand (with some caching?) and handle the
>> places where we're OOM and cannot PTE-map a THP in some descend way.
>>
>> ... instead of trying to figure out how to deal with these page tables we
>> cannot free but have to special-case simply because of GUP-fast.
> 
> Not keeping them around sounds good to me, but I'm not *that* familiar
> with the THP code, most of that happened after I stopped tracking mm. So
> I'm not sure how feasible is it.
> 
> But it does look entirely feasible to rework this page-table freeing
> along the lines Jann did.

It's most probably more feasible, although the easiest would be to just 
allocate a fresh page table to deposit and free the old one using the 
mmu gatherer.

This way we can avoid the khugepaged of tlb_remove_table_smp_sync(), but 
not the tlb_remove_table_one() usage. I suspect khugepaged isn't really 
relevant in RT kernels (IIRC, most of RT setups disable THP completely).

tlb_remove_table_one() only triggers if __get_free_page(GFP_NOWAIT | 
__GFP_NOWARN); fails. IIUC, that can happen easily under memory pressure 
because it doesn't wait for direct reclaim.

I don't know much about RT workloads (so I'd appreciate some feedback), 
but I guess we can run int memory pressure as well due to some !rt 
housekeeping task on the system?

-- 
Thanks,

David / dhildenb