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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="X/8LMTQP"; 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 4BB59606D808; Mon, 9 Nov 2026 12:53:52 +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=X/8LMTQP Received: by smtp.kernel.org (Postfix) with ESMTPSA id 543AC1F008A2; Mon, 9 Nov 2026 12:53:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794228831; bh=FjlQv+Rt9JvejDpQlYsKV4mpra0NOSmViN81+SW+4Cc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=X/8LMTQPUOjpKrh0CenW3mCNmqe+hLLccgz0vKCIrWnTOLz0yV81FVyo1gMy7sSkJ qeIUK3J8n2bfwQWZkB1iDHL54NcpMQlaqrZfD+NNsIps8TrihyIkdgleYhtMCDPoTU DvY/BwfwWFeqkq37u14OYgNfrEdW1eY1WztddIAfSRvV2HZmcfCpTtMKmanwGI5KtH S8SaJ35jlN+y1uJa3u58lD6k1eiePGkiV6XuojPUVy19MBu5dQaop7DuYWoKUuXuVz 0xU5Ki9ebtL1YiTsZbc5q3MPZZDUCo7+YjOjtFl7pe2NtRhK73X6zmMzME5HTWPkgk waK2LR6WNaAyQ== Message-ID: Date: Mon, 9 Nov 2026 13:53:45 +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/mprotect: don't skip uffd marker install when racing with PTE table reclaim To: Lance Yang Cc: akpm@linux-foundation.org, rppt@kernel.org, peterx@redhat.com, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, qi.zheng@linux.dev, tz2294@columbia.edu, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org> <20261108084035.85404-1-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 09:40, Lance Yang wrote: > > On Sat, Nov 07, 2026 at 12:28:55AM +0100, David Hildenbrand (Arm) wrote: >> change_pmd_range() populates PTE tables before installing uffd-wp >> markers. An empty PTE table can be removed after change_pmd_prepare() >> returns but before the pmd_none() check. >> >> That check then skips the range, allowing UFFDIO_WRITEPROTECT to report >> success without installing the required markers. Subsequent writes by >> userspace to these areas will not trigger uffd events. >> >> PTE table reclaim during MADV_DONTNEED and page table retraction during >> khugepaged can trigger this. >> >> Retry page-table allocation in that case. Simplify the code, getting >> rid of change_pmd_prepare() by routing all code through the "again" label. > > Cool, nice work1 > >> It's a small race though, so likely this is not very urgent. >> >> Fixes: fe2567eb5521 ("mm/shmem: allow uffd wr-protect none pte for file-backed mem") >> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) >> --- >> I expect Andrew to pick this up. >> --- >> mm/mprotect.c | 48 +++++++++++------------------------------------- >> 1 file changed, 11 insertions(+), 37 deletions(-) >> >> diff --git a/mm/mprotect.c b/mm/mprotect.c >> index e18d4e12fc341..ef4e7188e24da 100644 >> --- a/mm/mprotect.c >> +++ b/mm/mprotect.c >> @@ -470,25 +470,8 @@ pgtable_populate_needed(struct vm_area_struct *vma, unsigned long cp_flags) >> } >> >> /* >> - * Populate the pgtable underneath for whatever reason if requested. >> - * When {pte|pmd|...}_alloc() failed we treat it the same way as pgtable >> - * allocation failures during page faults by kicking OOM and returning >> - * error. >> - */ >> -#define change_pmd_prepare(vma, pmd, cp_flags) \ >> - ({ \ >> - long err = 0; \ >> - if (unlikely(pgtable_populate_needed(vma, cp_flags))) { \ >> - if (pte_alloc(vma->vm_mm, pmd)) \ >> - err = -ENOMEM; \ >> - } \ >> - err; \ >> - }) >> - >> -/* >> - * This is the general pud/p4d/pgd version of change_pmd_prepare(). We need to >> - * have separate change_pmd_prepare() because pte_alloc() returns 0 on success, >> - * while {pmd|pud|p4d}_alloc() returns the valid pointer on success. >> + * Populate the lower-level page table if marker installation requires it. >> + * Return -ENOMEM on allocation failure. >> */ >> #define change_prepare(vma, high, low, addr, cp_flags) \ >> ({ \ >> @@ -517,30 +500,21 @@ static inline long change_pmd_range(struct mmu_gather *tlb, >> again: >> next = pmd_addr_end(addr, end); >> >> - ret = change_pmd_prepare(vma, pmd, cp_flags); >> - if (ret) { >> - pages = ret; >> - break; >> - } >> - >> - if (pmd_none(*pmd)) >> + if (unlikely(pgtable_populate_needed(vma, cp_flags))) { >> + if (pte_alloc(vma->vm_mm, pmd)) { >> + pages = -ENOMEM; >> + break; >> + } >> + } else if (pmd_none(*pmd)) { >> goto next; >> + } > > Previously, reclaim could clear the PMD after allocation and we'd just > skip the range without installing the markers ... > > Now it's change_pte_range() -> -EAGAIN -> goto again (retry the same range) > > And yeah, that should do the trick ;) > >> >> _pmd = pmdp_get_lockless(pmd); >> if (pmd_is_huge(_pmd)) { >> if ((next - addr != HPAGE_PMD_SIZE) || >> pgtable_split_needed(vma, cp_flags)) { >> __split_huge_pmd(vma, pmd, addr, false); >> - /* >> - * For file-backed, the pmd could have been >> - * cleared; make sure pmd populated if >> - * necessary, then fall-through to pte level. >> - */ >> - ret = change_pmd_prepare(vma, pmd, cp_flags); >> - if (ret) { >> - pages = ret; >> - break; >> - } >> + goto again; > > And, after the split, we just go back to again -> allocate a PTE table > if needed -> recheck the PMD. > > So, no need for that second change_pmd_prepare() ANYMORE :D > > LGTM! > > Reviewed-by: Lance Yang > > Cheers, Lance Thanks for the review! Let's merge this through Andrew's tree :) -- Cheers, David