linux-kernel - [QUESTION] Plain dereference and READ

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Message-Id: <20250305102159.96420-1-dev.jain@arm.com>
Date: Wed,  5 Mar 2025 15:51:59 +0530
From: Dev Jain <dev.jain@....com>
To: david@...hat.com,
	willy@...radead.org,
	ziy@...dia.com,
	hughd@...gle.com,
	ryan.roberts@....com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [QUESTION] Plain dereference and READ_ONCE() in fault handler

In __handle_mm_fault(),

1. Why is there a barrier() for the PUD logic?
2. For the PMD logic, in the if block, we use *vmf.pmd, and in the else block
   we use pmdp_get_lockless(); what if someone changes the pmd just when we
   have begun processing the conditions in the if block, fail in the if block
   and then the else block operates on a different pmd value. Shouldn't we cache
   the value of the pmd and operate on a single consistent value until we take the
   lock and then finally check using pxd_same() and friends?

Thanks,
Dev