linux-kernel - Re: [PATCH 4/8] asm-generic: pgalloc: provide generic pmd_alloc_one() and pmd_free

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Date:   Sat, 27 Jun 2020 20:03:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Joerg Roedel <joro@...tes.org>,
        Max Filippov <jcmvbkbc@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
        Stafford Horne <shorne@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-alpha@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-parisc@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-um@...ts.infradead.org, linux-xtensa@...ux-xtensa.org,
        linuxppc-dev@...ts.ozlabs.org, openrisc@...ts.librecores.org,
        sparclinux@...r.kernel.org
Subject: Re: [PATCH 4/8] asm-generic: pgalloc: provide generic
 pmd_alloc_one() and pmd_free_one()

On Sat, Jun 27, 2020 at 05:34:49PM +0300, Mike Rapoport wrote:
> More elaborate versions on arm64 and x86 account memory for the user page
> tables and call to pgtable_pmd_page_ctor() as the part of PMD page
> initialization.
> 
> Move the arm64 version to include/asm-generic/pgalloc.h and use the generic
> version on several architectures.
> 
> The pgtable_pmd_page_ctor() is a NOP when ARCH_ENABLE_SPLIT_PMD_PTLOCK is
> not enabled, so there is no functional change for most architectures except
> of the addition of __GFP_ACCOUNT for allocation of user page tables.

Thanks for including this line; it reminded me that we're not setting
the PageTable flag on the page, nor accounting it to the zone page stats.
Hope you don't mind me tagging a patch to do that on as 9/8.

We could also do with a pud_page_[cd]tor and maybe even p4d/pgd versions.
But that brings me to the next question -- could/should some of this
be moved over to asm-generic/pgalloc.h?  The ctor/dtor aren't called
from anywhere else, and there's value to reducing the total amount of
code in mm.h, but then there's also value to keeping all the ifdef
ARCH_ENABLE_SPLIT_PMD_PTLOCK code together too.  So I'm a bit torn.
What do you think?