linux-kernel - Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <3fc75720-8da7-4f6c-bdce-1e1280b8e28f@lucifer.local>
Date: Thu, 7 Aug 2025 20:20:55 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Barry Song <baohua@...nel.org>,
        Dev Jain <dev.jain@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on
 mremap folio pte batch

+cc Ryan for ContPTE stuff.

On Thu, Aug 07, 2025 at 09:10:52PM +0200, David Hildenbrand wrote:
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks!

>
> Wondering whether we could then just use the patch hint instead of going via
> the folio.
>
> IOW,
>
> return pte_batch_hint(ptep, pte);

Wouldn't that break the A/D stuff? Also this doesn't mean that the PTE won't
have some conflicting flags potentially. The check is empirical:

static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
{
	if (!pte_valid_cont(pte))
		return 1;

	return CONT_PTES - (((unsigned long)ptep >> 3) & (CONT_PTES - 1));
}

So it's 'the most number of PTEs that _might_ coalesce'.

(note that a bit grossly we'll call it _again_ in folio_pte_batch_flags()).

I suppose we could not even bother with checking if same folio and _just_ check
if PTEs have consecutive PFNs, which is not very likely if different folio
but... could that break something?

It seems the 'magic' is in set_ptes() on arm64 where it'll know to do the 'right
thing' for a contPTE batch (I may be missing something - please correct me if so
Dev/Ryan).

So actually do we even really care that much about folio?

>
>
> Not sure if that was discussed at some point before we went into the
> direction of using folios. But there really doesn't seem to be anything
> gained for other architectures here (as raised by Jann).

Yup... I wonder about the other instances of this... ruh roh.

>
> --
> Cheers,
>
> David / dhildenb
>
>

Cheers, Lorenzo