linux-kernel - Re: [PATCH 2/2] mm/madvise: allow guard page install/remove under VMA lock

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Message-ID: <a65dcd48-d3aa-4372-9c58-2278fc161b68@kernel.org>
Date: Mon, 10 Nov 2025 16:44:45 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/madvise: allow guard page install/remove under VMA
 lock

On 09.11.25 12:16, Lorenzo Stoakes wrote:
> We only need to keep the page table stable so we can perform this operation
> under the VMA lock. PTE installation is stabilised via the PTE lock.
> 
> One caveat is that, if we prepare vma->anon_vma we must hold the mmap read
> lock. We can account for this by adapting the VMA locking logic to
> explicitly check for this case and prevent a VMA lock from being acquired
> should it be the case.
> 
> This check is safe, as while we might be raced on anon_vma installation,
> this would simply make the check conservative, there's no way for us to see
> an anon_vma and then for it to be cleared, as doing so requires the
> mmap/VMA write lock.
> 
> We abstract the VMA lock validity logic to is_vma_lock_valid() for this
> purpose, and add prepares_anon_vma() to abstract the anon_vma logic.
> 
> In order to do this we need to have a way of installing page tables
> explicitly for an identified VMA, so we export walk_page_range_vma() in an
> unsafe variant - walk_page_range_vma_unsafe() and use this should the VMA
> read lock be taken.
> 
> We additionally update the comments in madvise_guard_install() to more
> accurately reflect the cases in which the logic may be reattempted,
> specifically THP huge pages being present.
> 
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---

[...]

> 
> +/* Does this operation invoke anon_vma_prepare()? */
> +static bool prepares_anon_vma(int behavior)
> +{
> +	switch (behavior) {
> +	case MADV_GUARD_INSTALL:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +
> +/*
> + * We have acquired a VMA read lock, is the VMA valid to be madvise'd under VMA
> + * read lock only now we have a VMA to examine?
> + */
> +static bool is_vma_lock_valid(struct vm_area_struct *vma,
> +		struct madvise_behavior *madv_behavior)


Not sure about the "valid" terminology here.

Would "is_vma_lock_sufficient" be a better name, that would imply when 
"false" that another lock is required, because the VMA lock is insufficient?


-- 
Cheers

David