linux-kernel - Re: [PATCH v2 02/12] x86: pgtable: Always use pte_free

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Message-ID: <Z_VQxyqkU8DV7QGy@casper.infradead.org>
Date: Tue, 8 Apr 2025 17:37:27 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
	Andreas Larsson <andreas@...sler.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"David S. Miller" <davem@...emloft.net>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Mark Rutland <mark.rutland@....com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Ryan Roberts <ryan.roberts@....com>, Will Deacon <will@...nel.org>,
	Yang Shi <yang@...amperecomputing.com>, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org, linux-openrisc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH v2 02/12] x86: pgtable: Always use pte_free_kernel()

On Tue, Apr 08, 2025 at 08:22:47AM -0700, Dave Hansen wrote:
> Are there any tests for folio_test_pgtable() at free_page() time? If we
> had that, it would make it less likely that another free_page() user
> could sneak in without calling the destructor.

It's hidden, but yes:

static inline bool page_expected_state(struct page *page,
                                        unsigned long check_flags)
{
        if (unlikely(atomic_read(&page->_mapcount) != -1))
                return false;

PageTable uses page_type which aliases with mapcount, so this check
covers "PageTable is still set when the last refcount to it is put".

I don't think we really use the page refcount when allocating/freeing
page tables.  Anyone want to try switching it over to using
alloc_frozen_pages() / free_frozen_pages()?  Might need to move that API
out of mm/internal.h ...