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b=MZKhV/EtmFdCWSgx8/5k65xynUZO3G1bEjmVHu9QPtxdA4JTY9baaFnA/PUDsa0rF DihnGNjXrdDJ/Ypmp13hSRN5hMlwYgg0V4gw3BWkenzNq+4ubqX5FXn4JUv00WLD3O eJ0/c4+pBIEbAIS8gF9coMrZe0vVryYPRiTc8zYpNQavWQMzX+GQvtvyZy8Cw1KQrJ y5r1LpsQgrRLguy/+VNmBIoVLeT1lB7ha2DbOUWeoXS9Y5wNuv4A/+BznGJsY6wzxJ cIOXhfz1GXAQyExcSUKCHMHekOl6qeJ90AXDpLsVgH4+VeT0Fb3cHfMEpQVwlQJE7C FiHTHcDeUcyjw== Message-ID: <3fc697d4-9193-4350-8264-f38ebc4836cc@kernel.org> Date: Wed, 29 Apr 2026 07:50:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/mm: fix freeing of PMD-sized vmemmap pages To: Lance Yang Cc: dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, rppt@kernel.org, jgg@ziepe.ca, baolu.lu@linux.intel.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org References: <20260428-vmemmap-v1-1-b2aa1e6db2c0@kernel.org> <20260429021224.39916-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C143DC000D X-Stat-Signature: z599eh5dk7h4f37pksndngmx77ozknf6 X-HE-Tag: 1777441847-268968 X-HE-Meta: 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 bony3aCM ertPluwN/z1cchgRYxHUPAU6T3EwNtjplEohCh8xhAMHqI3ZKvL8qhQBbcneAPlm3wplWg2pHjcOMExUnOZw2b0xdR6+foXAkbFOafsPwEFuhLRZLBGcfrxYL2RDL3qJUJZV3pxmnXj6pY/fjwkcOtzihZ6SDeXD7kuwJytlvtfiWalLcU1s1aqRG8LtWp01NAmNnYL/QxLx1Chmo0gsOgcPLAKFNqUh4FiC37FOyVO6+iaOsK6anXhXzDdf4yWmuW6As Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/29/26 04:12, Lance Yang wrote: > > On Tue, Apr 28, 2026 at 12:29:36PM +0200, David Hildenbrand (Arm) wrote: >> In commit bf9e4e30f353 ("x86/mm: use pagetable_free()"), we switched >>from freeing non-boot page tables through __free_pages() to >> pagetable_free(). >> >> However, the function is also called to free vmemmap pages. >> >> Given that vmemmap pages are not page tables, already the page_ptdesc(page) >> is wrong. But worse, pagetable_free() calls >> >> __free_pages(page, compound_order(page)); >> >> As vmemmap pages are not compound pages (see vmemmap_alloc_block()) -- >> except for HVO, which doesn't apply here -- we will only free the first >> page when freeing a PMD-sized vmemmap page, leaking the other ones. >> >> Fix it by properly decoupling pagetable and vmemmap freeing. >> free_pagetable() no longer has to mess with SECTION_INFO, as only the >> vmemmap is marked like that in register_page_bootmem_memmap(). >> >> While at it, just wire up the altmap parameter for remove_pte_table(). >> Also, the indentation in remove_pmd_table() is messed up, let's fix that >> while touching it. > > One thing I'm not sure about is passing altmap down into > remove_pte_table(). > > Do we actually know that a non-NULL altmap means that the vmemmap > backing page came from that altmap? I thought with an altmap we'd never get into the situation of allocating outside the altmap. But you're right that sub-section hotplug might actually trigger this. > > On x86 we still have in vmemmap_populate(): > > if (end - start < PAGES_PER_SECTION * sizeof(struct page)) > err = vmemmap_populate_basepages(start, end, node, NULL); > > So for smaller-than-section vmemmap ranges, even if the caller has an > altmap, the backing pages are allocated from normal memory. But with > this fix the PTE removal path would now call vmem_altmap_free() just > because altmap is non-NULL, and would not free the actual backing page, > IIUC :) > > Maybe free_vmemmap_pages() should first check that the backing page is > really inside the altmap range before using vmem_altmap_free()? Probably I'll just leave it as is for now, and simply pass "NULL" for the PTE case like we effectively did before. Thanks! -- Cheers, David