linux-kernel - Re: [PATCH 1/1] x86/vmemmap: Use direct-mapped VA instead of vmemmap-based VA

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Message-ID: <eb8a6e71-601a-435f-a79b-ce95ee012c8f@intel.com>
Date: Fri, 7 Mar 2025 10:56:04 +0200
From: Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, Andrew Morton
	<akpm@...ux-foundation.org>
CC: "Harry Yoo (Oracle)" <42.hyeyoo@...il.com>,
	<linux-kernel@...r.kernel.org>, <osalvador@...e.de>, <byungchul@...com>,
	<dave.hansen@...ux.intel.com>, <luto@...nel.org>, <peterz@...radead.org>,
	<max.byungchul.park@...com>, <max.byungchul.park@...il.com>, <x86@...nel.org>
Subject: Re: [PATCH 1/1] x86/vmemmap: Use direct-mapped VA instead of
 vmemmap-based VA

Sorry for the late reply.

On 3/6/25 6:09 PM, Dave Hansen wrote:
> On 3/5/25 19:46, Andrew Morton wrote:
>> Can we please have review from x86 maintainers?
> 
> I didn't respond here because I thought this was the same problem from
> the same contributor that we addressed in another thread:
> 
>> https://lore.kernel.org/all/d1da214c-53d3-45ac-a8b6-51821c5416e4@intel.com/
> 
> I think this approach is a hack. It basically requires that every bit of
> code that _might_ update (and then use) a PGD in the init_mm know how to
> find the direct map alias and then use that instead. This would further
> specialize the x86 code. I have no reason to believe that this is truly
> an x86-specific problem. Are we really the only arch that has a
> per-process PGD that maps the shared kernel page tables?
> 
currently the scenario that causes this issue only happens on x86, 
therfore I did a hotfix to make sure it would not affect other archtecture.

And I agree, I don't think this is an x86 only issue, and if there is a 
scenario that can cause the situation I reported on other platforms, I 
think it is possible.
> The right solution (like I mentioned in the other thread) is to sync the
> PGDs more aggressively. Syncing them is expensive, of course, but it's a
> pretty darn rare operation.
I'll send a new patch that introdues helper functions suggested by Dave 
above.

Thank you for not forgetting this issue and bringing it back to the surface.

Thanks,

G.G.