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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=H6LTKG6Z; 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 BC7F4606A31C; Mon, 9 Nov 2026 14:25:46 +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=H6LTKG6Z Received: by smtp.kernel.org (Postfix) with ESMTPSA id 069781F0089F; Mon, 9 Nov 2026 14:25:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794234346; bh=pyAb7uutdcjK3DrEF9ZJ1ufyFQKGxeOU/iSNrYyd/3U=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=H6LTKG6ZydDAo1DzAKoJVuPxQqp1iKvDEHkMDVo7XCBdavDijZS0f+ueP/EiRea4E Kw0m9lNhSSueaupP17xzBmtY7gAkZwyErXyTzAriXK9zKzawDojgdl2cNX1tKIdkeL JFG4iFBdswMzi8UaiZXIGO87Z8eIWs6H9NH08nxKRZHbHXulE00GyOd06DQO6vXL83 U5aif9Ubr8/0uHGIxvyBbXwyxcpI+oKqzGwfmu3vQ50C39dkkI89NJqklDqff5siEC SaAYSXPVdlHhQkyT9H4feaDhmTIift58eP5er4xPbqxjZbXWUf2p29M9DthoebZ//K 6R7DdazKEnDTA== Message-ID: <2e3287d7-19d5-4441-8750-bd351df5574a@kernel.org> Date: Mon, 9 Nov 2026 15:25:40 +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 v2 1/2] mm: free unused page table allocated by do_set_pmd() To: Lance Yang Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, kas@kernel.org, qi.zheng@linux.dev, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261106-khugepaged_set_pmd-v2-1-d2457b5e377a@kernel.org> <20261108091455.88759-1-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 On 11/8/26 10:14, Lance Yang wrote: > > On Fri, Nov 06, 2026 at 01:36:12PM +0100, David Hildenbrand (Arm) wrote: >> do_set_pmd() can allocate vmf->prealloc_pte on architectures that need a >> deposited page table, such as PowerPC with the hash MMU. If the PMD is >> no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK >> without consuming that table. >> >> Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent >> userfaultfd operation can install a PTE table before taking the folio >> lock. If that races with set_huge_pmd(), the unused preallocated table >> is lost when its local vm_fault goes out of scope. >> >> Track whether do_set_pmd() allocated the table and free it after dropping >> the PMD lock if it was not consumed. Preserve any preallocated table >> supplied by the caller for PTE fallback. Successful PMD installation >> consumes the table and clears the pointer. >> >> Returning without a preallocated table matches the existing behavior on >> architectures that do not need a deposited table. All callers >> already handle that case. >> >> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()") >> Cc: stable@vger.kernel.org >> Reviewed-by: Kiryl Shutsemau (Meta) >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/memory.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index becf53a2a0944..421064aef3cf0 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -5687,6 +5687,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa >> struct vm_area_struct *vma = vmf->vma; >> bool write = vmf->flags & FAULT_FLAG_WRITE; >> unsigned long haddr = vmf->address & HPAGE_PMD_MASK; >> + bool pgtable_allocated = false; >> pmd_t entry; >> vm_fault_t ret = VM_FAULT_FALLBACK; >> >> @@ -5725,6 +5726,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa >> vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); >> if (!vmf->prealloc_pte) >> return VM_FAULT_OOM; >> + pgtable_allocated = true; >> } >> >> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd); >> @@ -5755,6 +5757,10 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa >> count_vm_event(THP_FILE_MAPPED); >> out: >> spin_unlock(vmf->ptl); >> + if (pgtable_allocated && vmf->prealloc_pte) { > > Took me a sec ... to see why we can't just check vmf->prealloc_pte here > ... > > Emm ... as it turns out, the caller might have supplied it for fallback :D > Yeah, it's all ... complicated :) -- Cheers, David