linux-kernel - Re: [PATCH v3 05/11] arm64: hugetlb: Use set_ptes_anysz() and ptep_get_and_clear

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Message-ID: <Z-7z4ZFGIZCrNq6h@arm.com>
Date: Thu, 3 Apr 2025 21:47:29 +0100
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 v3 05/11] arm64: hugetlb: Use set_ptes_anysz() and
 ptep_get_and_clear_anysz()

On Tue, Mar 04, 2025 at 03:04:35PM +0000, Ryan Roberts wrote:
> Refactor the huge_pte helpers to use the new common set_ptes_anysz() and
> ptep_get_and_clear_anysz() APIs.

Nitpick: maybe add underscores to this new API to suggest it's private.
Up to you.

> This provides 2 benefits; First, when page_table_check=on, hugetlb is
> now properly/fully checked. Previously only the first page of a hugetlb
> folio was checked. Second, instead of having to call __set_ptes(nr=1)
> for each pte in a loop, the whole contiguous batch can now be set in one
> go, which enables some efficiencies and cleans up the code.
> 
> One detail to note is that huge_ptep_clear_flush() was previously
> calling ptep_clear_flush() for a non-contiguous pte (i.e. a pud or pmd
> block mapping). This has a couple of disadvantages; first
> ptep_clear_flush() calls ptep_get_and_clear() which transparently
> handles contpte. Given we only call for non-contiguous ptes, it would be
> safe, but a waste of effort. It's preferable to go straight to the layer
> below. However, more problematic is that ptep_get_and_clear() is for
> PAGE_SIZE entries so it calls page_table_check_pte_clear() and would not
> clear the whole hugetlb folio. So let's stop special-casing the non-cont
> case and just rely on get_clear_contig_flush() to do the right thing for
> non-cont entries.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

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