linux-kernel - Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <ZpA7gvIvxNRkd6hp@zx2c4.com>
Date: Thu, 11 Jul 2024 22:07:30 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev, tglx@...utronix.de,
	linux-crypto@...r.kernel.org, linux-api@...r.kernel.org,
	x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
	Carlos O'Donell <carlos@...hat.com>,
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Jann Horn <jannh@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always
 lazily freeable mappings

Hi Linus,

On Thu, Jul 11, 2024 at 10:57:17AM -0700, Linus Torvalds wrote:
> May I suggest a slightly different approach: do what we did for "pte_mkwrite()".
> 
> It needed the vma too, for not too dissimilar reasons: special dirty
> bit handling for the shadow stack. See

Thanks for the suggestion. That seems pretty clean.

It still needs to avoid setting swapbacked in the first place, but
ensuring that it's never dirty means it won't get turned back on.

The first patch renames pte_dirty() to pte_dirty_novma(). The second
patch adds an inline function, pte_dirty(pte, vma) that just forwards
the pte to pte_dirty_novma(), and then converts callers that have a vma
available to pass to call pte_dirty(). And then the VM_DROPPABLE patch
simply adds the `&& !(vma->vm_flags & VM_DROPPABLE)` condition to
pte_dirty().

I put these in https://git.zx2c4.com/linux-rng/log/ per usual, and I'll
post a new version to the list not before long (unless objections).

Jason