linux-kernel - Re: [PATCH V2 0/7] mm: Use pxdp_get() for accessing page table entries

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Message-ID: <f40ea8bf-0862-41a7-af19-70bfbd838568@csgroup.eu>
Date: Wed, 25 Sep 2024 12:05:33 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 x86@...nel.org, linux-m68k@...ts.linux-m68k.org,
 linux-fsdevel@...r.kernel.org, kasan-dev@...glegroups.com,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V2 0/7] mm: Use pxdp_get() for accessing page table
 entries



Le 17/09/2024 à 09:31, Anshuman Khandual a écrit :
> This series converts all generic page table entries direct derefences via
> pxdp_get() based helpers extending the changes brought in via the commit
> c33c794828f2 ("mm: ptep_get() conversion"). First it does some platform
> specific changes for m68k and x86 architecture.
> 
> This series has been build tested on multiple architecture such as x86,
> arm64, powerpc, powerpc64le, riscv, and m68k etc.

Seems like this series imply sub-optimal code with unnecessary reads.

Lets take a simple exemple : function mm_find_pmd() in mm/rmap.c

On a PPC32 platform (2 level pagetables):

Before the patch:

00001b54 <mm_find_pmd>:
     1b54:	80 63 00 18 	lwz     r3,24(r3)
     1b58:	54 84 65 3a 	rlwinm  r4,r4,12,20,29
     1b5c:	7c 63 22 14 	add     r3,r3,r4
     1b60:	4e 80 00 20 	blr

Here, the function reads mm->pgd, then calculates and returns a pointer 
to the PMD entry corresponding to the address.

After the patch:

00001b54 <mm_find_pmd>:
     1b54:	81 23 00 18 	lwz     r9,24(r3)
     1b58:	54 84 65 3a 	rlwinm  r4,r4,12,20,29
     1b5c:	7d 49 20 2e 	lwzx    r10,r9,r4	<= useless read
     1b60:	7c 69 22 14 	add     r3,r9,r4
     1b64:	7d 49 20 2e 	lwzx    r10,r9,r4	<= useless read
     1b68:	7d 29 20 2e 	lwzx    r9,r9,r4	<= useless read
     1b6c:	4e 80 00 20 	blr

Here, the function also reads mm->pgd and still calculates and returns a 
pointer to the PMD entry corresponding to the address. But in addition 
to that it reads three times that entry while doing nothing at all with 
the value read.

On PPC32, PMD/PUD/P4D are single entry tables folded into the 
corresponding PGD entry, it is therefore pointless to read the 
intermediate entries.

Christophe