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

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Message-ID: <99771f33-8ad8-4ba5-9cf0-f504588d99a0@arm.com>
Date: Wed, 9 Apr 2025 16:50:04 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Matthew Wilcox <willy@...radead.org>, Dave Hansen <dave.hansen@...el.com>
Cc: 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 08/04/2025 19:40, Matthew Wilcox wrote:
> 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;

Correct, that's why I added this patch early in the series (the next
patch adds the ctor call in pte_alloc_one_kernel()).

The BUG() in v1 was indeed triggered by a page_expected_state() check [1].

> as Kevin says, this is all very twisty and confusing with exceptions and
> exceptions to exceptions.  This series should reduce the confusion.

I hope so!

- Kevin

[1] https://lore.kernel.org/oe-lkp/202503211612.e11bd73f-lkp@intel.com/