linux-kernel - Re: [PATCH/RFC] mm: add and use batched version of __tlb_remove

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Date:   Sat, 18 Dec 2021 16:35:48 +0300
From:   Nikita Yushchenko <nikita.yushchenko@...tuozzo.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
        kernel@...nvz.org
Subject: Re: [PATCH/RFC] mm: add and use batched version of
 __tlb_remove_table()

> Oh gawd, that's terrible. Never, ever duplicate code like that.

What the patch does is:
- formally shift the loop one level down in the call graph, adding instances of __tmp_remove_tables() 
exactly to locations where instances of __tmp_remove_table() already exist,
- on architectures where __tmp_remove_tables() resulted into calling free_page_and_swap_cache() in loop, 
call batched free_page_and_swap_cache_nolru() instead,
- on other places, keep the loop as is - perhaps as a possible target for future optimizations.

The extra duplication added by this patch just highlights already existing duplication of 
__tlb_remove_table() implementations.

Ok let's follow your suggestion instead. AFAIU, that is:
- remove the free_page_and_swap_cache() based implementation from archs,
- instead, add it into mm/mmu_gather.c, ifdef-ed by a new Kconfig key, and define that Kconfig key into 
the archs that use it,
- then, keep the optimization inside mm/mmu_gather.c.

Indeed, the overall change will become smaller then. Thanks for the idea. Will post patches doing that soon.

Nikita