linux-kernel - Re: [PATCH v2 09/14] mm/vmalloc: Gracefully unmap huge ptes

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Message-ID: <Z7xgWtgxWg3BvaYz@arm.com>
Date: Mon, 24 Feb 2025 12:04:42 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Kevin Brodsky <kevin.brodsky@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 09/14] mm/vmalloc: Gracefully unmap huge ptes

On Mon, Feb 17, 2025 at 02:08:01PM +0000, Ryan Roberts wrote:
> Commit f7ee1f13d606 ("mm/vmalloc: enable mapping of huge pages at pte
> level in vmap") added its support by reusing the set_huge_pte_at() API,
> which is otherwise only used for user mappings. But when unmapping those
> huge ptes, it continued to call ptep_get_and_clear(), which is a
> layering violation. To date, the only arch to implement this support is
> powerpc and it all happens to work ok for it.
> 
> But arm64's implementation of ptep_get_and_clear() can not be safely
> used to clear a previous set_huge_pte_at(). So let's introduce a new
> arch opt-in function, arch_vmap_pte_range_unmap_size(), which can
> provide the size of a (present) pte. Then we can call
> huge_ptep_get_and_clear() to tear it down properly.
> 
> Note that if vunmap_range() is called with a range that starts in the
> middle of a huge pte-mapped page, we must unmap the entire huge page so
> the behaviour is consistent with pmd and pud block mappings. In this
> case emit a warning just like we do for pmd/pud mappings.
> 
> Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>