linux-kernel - Re: [PATCH] mm: Update mask post pxd_clear

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Message-ID: <acba6a71-4ee8-445a-aae9-822f88079cb3@arm.com>
Date: Fri, 14 Mar 2025 18:27:43 +0530
From: Dev Jain <dev.jain@....com>
To: Matthew Wilcox <willy@...radead.org>
Cc: jroedel@...e.de, akpm@...ux-foundation.org, ryan.roberts@....com,
 david@...hat.com, hch@....de, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm: Update mask post pxd_clear_bad()

On 14/03/25 2:14 am, Matthew Wilcox wrote:
> On Thu, Mar 13, 2025 at 11:44:14PM +0530, Dev Jain wrote:
>> Since pxd_clear_bad() is an operation changing the state of the page tables,
>> we should call arch_sync_kernel_mappings() post this.
> 
> Could you explain why?  What effect does not calling
> arch_sync_kernel_mappings() have in this case?

Apologies, I again forgot to explain the userspace effect.
I just found this by code inspection, using the logic the fixes commit 
uses: we should sync when we change the pxd.

The question I have been pondering on is, what is the use of the 
pxd_bad() macros, when do we actually hit a bad state, and why don't we 
just trigger a BUG when we hit pxd_bad()?