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

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Message-ID: <716f5a42-6828-4f0e-9002-d3bd5c4ad363@intel.com>
Date: Thu, 6 Mar 2025 08:09:33 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>
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

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?

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.