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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="btak/NjA"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EEB6E606A326 for ; Sat, 7 Nov 2026 19:29:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=btak/NjA Received: by smtp.kernel.org (Postfix) with ESMTPSA id 507901F008A2; Sat, 7 Nov 2026 19:29:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794079746; bh=7Ov7GeLhcYhxCP17ziUm+7IIU7rjkhp7m+eZjNGqqtY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=btak/NjA5HXiNm9eB53iO2P0kVP6gcust1hNujA8Xhe6SIGL7ZPNyeyKDTGunctTR +wKgGgu/BEey5ixlamyuuMx7Bcdih5YAlVE86Ut8vPi14t2nTSknpdYOCBbtRyD31Z 43q3G1u4wi8qbH6BgQk89ak9+8YtQ47eqIDkb9DJFZ5WcgDxml8QILGXk4mJORmmio TMvcFjn52TIiV5wVNRNgUOElBw5Qrl22KgPhv0ObjtodP+I++g087oaMQpVvp+sica 4lColLpj+W/eH7yV/gc5tMuHVdVLNehJWSNNGUbPy1VCMfCagDWMrq8ZN3ehE7BBlz vbreZ447FvvKA== Message-ID: Date: Sat, 7 Nov 2026 20:28:56 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: clear pmd in pmdp_invalidate() To: Guilherme Giacomo Simoes Cc: akpm@linux-foundation.org, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org References: <20261104111254.320059-1-trintaeoitogc@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/4/26 12:12, Guilherme Giacomo Simoes wrote: > David, I haved immersed inside this problem on last days, and how sashiko-bot > very well apointed, I'm haved doing a wrong duplicated clear on pmd's. I not > notice that the pmd_establish() already called the page_table_check_pmd_set() > wich in turn call __page_table_check_pmd_clear(), my bad. > > At first time, I thinked the problem is a duplicated > page_table_check_pmd_set(), but with a LLM helpful I ruled out this hypothesis, > because I not found any obvious call to page_table_check_pmd_set() before > remove_migration_pte(). At second time, I suspicious that maybe we are haved a > race condition, that make no sense... > > After a lot of debug and tests, I found a thing.. And I attempt to test this. > If you see inside arch/riscv/include/asm/pgtable.h, the pmd_mkinvalid() do: > ``` > static inline pmd_t pmd_mkinvalid(pmd_t pmd) > { > return __pmd(pmd_val(pmd) & ~(_PAGE_PRESENT|_PAGE_PROT_NONE)); > } > ``` > > but , the pmd_user_accessible_page() check `pmd_leaf(pmd) && pmd_user(pmd);` > and pmd_leaf(): > ``` > static inline bool pmd_leaf(pmd_t pmd) > { > return pmd_present(pmd) && (pmd_val(pmd) & _PAGE_LEAF); > } > ``` > > and the pmd_present(): > ``` > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > static inline int pmd_present(pmd_t pmd) > { > /* > * Checking for _PAGE_LEAF is needed too because: > * When splitting a THP, split_huge_page() will temporarily clear > * the present bit, in this situation, pmd_present() and > * pmd_trans_huge() still needs to return true. > */ > return (pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROT_NONE | _PAGE_LEAF)); > } > #else > static inline int pmd_present(pmd_t pmd) > { > return (pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROT_NONE)); > } > #endif > ``` > > And the TRANSPARENT HUGE PAGE , is exactly a page that I was allocated with > mmap() (see my previous emails with my repro.c). > > So, I change the pmd_mkinvalid() adding PAGE_LEAF flag: > ``` > static inline pmd_t pmd_mkinvalid(pmd_t pmd) > { > return __pmd(pmd_val(pmd) & ~(_PAGE_PRESENT|_PAGE_PROT_NONE|_PAGE_LEAF)); > } > ``` > > And this work for my repro.c. > The problem here is what the documentation of pmd_present() say: > ``` > /* > * Checking for _PAGE_LEAF is needed too because: > * When splitting a THP, split_huge_page() will temporarily clear > * the present bit, in this situation, pmd_present() and > * pmd_trans_huge() still needs to return true. > */ > ``` > > So the documentation say that if a THP is splitting, this should return true > event though PAGE_PRESENT and PAGE_PROT_NONE is cleared. > If you think about this, maybe make sense. > > And after some searchs I saw that was already fixed by Yunhui Cui > > https://lore.kernel.org/all/20260523042052.35476-1-cuiyunhui@bytedance.com/ I remember that Zi is currently also fixing some pmd_mkinvalid() implementations. Is there anything to fix from your side, or does the above fix already resolve your problem? -- Cheers, David