linux-kernel - Re: [PATCH v4 0/3] Cleanup for PAT

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Date: Tue, 20 Feb 2024 00:37:41 -0800
From: Xin Li <xin@...or.com>
To: Wupeng Ma <mawupeng1@...wei.com>, akpm@...ux-foundation.org,
        dave.hansen@...ux.intel.com, luto@...nel.org, tglx@...utronix.de,
        peterz@...radead.org, hpa@...or.com
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, bp@...e.de, mingo@...hat.com,
        rdunlap@...radead.org, bhelgaas@...gle.com, linux-mm@...ck.org
Subject: Re: [PATCH v4 0/3] Cleanup for PAT

On 2/19/2024 7:48 PM, Wupeng Ma wrote:
> From: Ma Wupeng <mawupeng1@...wei.com>
> 

This patch set is all about follow_phys() cleanups, so "Cleanup for PAT"
seems too generic.

> Patch #1 move follow_phys to memtype.c since only pat use this.
> Patch #2 cleanup parameter in follow_phys.
> Patch #3 drop the unnecessary WARN_ON_ONCE if follow_phys fails.

I'm more curious why follow_phys() ended up this way?

follow_phys() was introduced in commit 28b2ee20c7cba ("access_process_vm
device memory infrastructure") in 2008 for getting a physical page address
for a virtual address, and used in generic_access_phys(). And later it's
used in x86 PAT code.

Commit 03668a4debf4f ("mm: use generic follow_pte() in follow_phys()") made
follow_phys() more of a wrapper of follow_pte(), and commit 96667f8a4382d
("mm: Close race in generic_access_phys") replaced follow_phys() with
follow_pte() in generic_access_phys(). And the end result is that
follow_phys() is used in x86 PAT code only.

As follow_phys() in untrack_pfn() can be replaced with follow_pfn(), then
maybe we don't have to keep follow_phys(), and just use follow_pte() in
track_pfn_copy()?

Thanks!
     Xin

> 
> Changelog since v3:
> - rebase to latest linux
> - fix compile warnings
> 
> Changelog since v2:
> - rebase to latest linux
> 
> Changelog since v1:
> - split patch #1 into two patches based on Boris's advise
> 
> Ma Wupeng (3):
>    x86/mm/pat: Move follow_phys to pat-related file
>    x86/mm/pat: Cleanup unused parameter in follow_phys
>    x86/mm/pat: Remove WARN_ON_ONCE if follow_phys fails
> 
>   arch/x86/mm/pat/memtype.c | 32 ++++++++++++++++++++++++++------
>   include/linux/mm.h        |  2 --
>   mm/memory.c               | 28 ----------------------------
>   3 files changed, 26 insertions(+), 36 deletions(-)
>