linux-kernel - Re: [PATCH 1/4] mm: Export flush_vm_area() to sync the PTEs upon construction

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Date:   Fri, 21 Aug 2020 12:22:04 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-mm@...ck.org, Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Airlie <airlied@...hat.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Vrabel <david.vrabel@...rix.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] mm: Export flush_vm_area() to sync the PTEs upon
 construction

On Fri, Aug 21, 2020 at 10:54:22AM +0100, Chris Wilson wrote:
> Ok. I thought it had to be after assigning the *ptep. If we apply the
> sync first, do not have to worry about PGTBL_PTE_MODIFIED from the
> *ptep?

Hmm, if I understand the code correctly, you are re-implementing some
generic ioremap/vmalloc mapping logic in the i915 driver. I don't know
the reason, but if it is valid you need to manually call
arch_sync_kernel_mappings() from your driver like this to be correct:

	if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PTE_MODIFIED)
		arch_sync_kernel_mappings();

In practice this is a no-op, because nobody sets PGTBL_PTE_MODIFIED in
ARCH_PAGE_TABLE_SYNC_MASK, so the above code would be optimized away.

But what you really care about is the tracking in apply_to_page_range(),
as that allocates the !pte levels of your page-table, which needs
synchronization on x86-32.

Btw, what are the reasons you can't use generic vmalloc/ioremap
interfaces to map the range?

Regards,

	Joerg