From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.160 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794127245; b=eAemxvQBbUEbZIgh1iZ1o3wGpTKtSPOAG07mzGWFugoeixSpuIutAUSMfDz1m0+PJp0o CHWxQT/5+OIbuOL4zra5OV90J/inI1V4lzOJc8O2B0RV6jSPh6T5nuPkO+gDe0HzWsrrE E5avHbD65RuoyD0xnnlEeP1KZldEKW1X2XAaoWCVZqbV+S/50rhQR8Nd+uexhyiYwFhCg qUoqvKorojFiOIP3weKrfGCTl0iGadeAP8ks27E+7nu4sv6lSkvYCH3M9robuSv97J6Cp tJAFwn7y4IwdUUQ1CfNJNZrurY7LWrL3r4wojmVIjG4M8X5cf7XM2I6yv/VDu68pobw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794127245; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:From:To:Cc:Subject:Date:Message-ID: X-Mailer:In-Reply-To:References:MIME-Version:Content-Type: Content-Transfer-Encoding; bh=aYoUmexxPLJigYcrWiTOVyZolQmaaSPk22SDsY8yV2M=; b=J1bqqeU0u83DXP2Wj9DjbmuUXNfZLJ8vc/MUjwMW17ojuL1jX/YFNol3NzfLoQjKBOX5 B9OQkqyrz/7Fb3NZfaNcEWqj8ojh9DgiUyqm1F7NLQ6EI6q7/PTZ9CtS4fhjNrp9afGQX MA3RWbyn3jK1okHsRSXcjBKFHWQg7g2H36TJBJ8UukUluqMST1Q2chTpclw04qoeYx61v pvjnIKRGWefs8u4iersb5WNpuNGBUNqBy6p/jRqwNQZGatfMOZnhDAdJoZcnBdaoUZojk DWJ6PNHt0vxGhwsg8dPatBssiANf4ytPBiD5BHrKfP7jJClkzGQgM9EqmBBfzVaNlKQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=KkGdeozW; arc=none smtp.remote-ip=91.218.175.160 Received: from mta0.migadu.com (out-160.mta0.migadu.com [91.218.175.160]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0FADB606A320 for ; Sun, 8 Nov 2026 08:40:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 0FADB606A320 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=KkGdeozW X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=BYyngODmPBf6+ONXmDqmz1GGGko3fLEGOZKdoUTwChM=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1794127241; v=1; x=1794732041; b=KkGdeozWw6GzIN8/N1hxwiHvUcHndwbzmWMXS7USNunUJ+FeltMBIA61FH1QyV1noHYwTZaL lqQnkwuDy9gYU9aZHZ0BwPRLdCkrMG8Ryi4wOesGVzbn7U+RHJ+Ci/bcrIJGcab1pufAWsnLlyi gr5PHUZgMheMxtfRnWLaiPRI= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 8756b258539f3c21; Sun, 08 Nov 2026 08:40:41 +0000 X-Mizu-Trace-ID: 8756b258539f3c21 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org 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 Subject: Re: [PATCH] mm/mprotect: don't skip uffd marker install when racing with PTE table reclaim Date: Sun, 8 Nov 2026 16:40:35 +0800 Message-ID: <20261108084035.85404-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org> References: <20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org> 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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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