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

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Message-ID: <Z_VfeFgrj23Oa0fX@casper.infradead.org>
Date: Tue, 8 Apr 2025 18:40:08 +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 09:54:42AM -0700, Dave Hansen wrote:
> On 4/8/25 09:37, Matthew Wilcox wrote:
> > 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".
> 
> Huh, so shouldn't we have ended up in bad_page() for these, other than:
> 
>         pagetable_dtor(virt_to_ptdesc(pmd));
>         free_page((unsigned long)pmd);

I think at this point in Kevin's series, we don't call the ctor for
these pages, so we never set PageTable() on them.  I could be wrong;
as Kevin says, this is all very twisty and confusing with exceptions and
exceptions to exceptions.  This series should reduce the confusion.