linux-kernel - Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings

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Message-ID: <fbfae348-909b-48fa-9083-67696b02f15e@suse.cz>
Date: Tue, 18 Feb 2025 13:12:05 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
 linux-kselftest@...r.kernel.org, linux-api@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>, Juan Yescas <jyescas@...gle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem
 mappings

On 2/13/25 19:16, Lorenzo Stoakes wrote:
> The guard regions feature was initially implemented to support anonymous
> mappings only, excluding shmem.
> 
> This was done such as to introduce the feature carefully and incrementally
> and to be conservative when considering the various caveats and corner
> cases that are applicable to file-backed mappings but not to anonymous
> ones.
> 
> Now this feature has landed in 6.13, it is time to revisit this and to
> extend this functionality to file-backed and shmem mappings.
> 
> In order to make this maximally useful, and since one may map file-backed
> mappings read-only (for instance ELF images), we also remove the
> restriction on read-only mappings and permit the establishment of guard
> regions in any non-hugetlb, non-mlock()'d mapping.

Do you plan to address mlock later too? I guess somebody might find use for
those. Is there some fundamental issue or just that we need to define some
good semantics for corner cases? (i.e. if pages are already populated in the
mlocked area, discarding them by replacing with guard pte's goes against
that, so do we allow it or not?).

Otherwise nice discussion of all the potential issues here!