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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=D9AKER0x; 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 784D4606A31A; Wed, 21 Oct 2026 08:21:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=D9AKER0x Received: by smtp.kernel.org (Postfix) with ESMTPSA id BC1341F000CA; Wed, 21 Oct 2026 08:21:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792570918; bh=eKJ8A+o8IpNbD03Ii9Xprch/mNMzpL/05Y7zRrPcQVo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=D9AKER0xD7Kbyst88Kckbv8qUgnelLQ/7HJT4N37dMWUUEBKjd8IfOAKvbgDVtQnQ HS6nsDejSBSCuybh4RkOE9kS+JmsRRXkO0GgaG/ZI8MGwUKymloYnPlRNyJXQCwpyU PW7XANt6UuYKYogYoNqZo/8+AEHNzkjofFHAJwqTatjz42f68N3Qn/dCNFuKNshK0K ystYSMrzWQCdOszrv+wYOWPi3ffkfSsWL0TateeyYAVB6O5A0agzwvMizMEtoCy4/h Kv8374hTKYtgtrMXUfJIlW6j4DMPYc760OSnCmx4Iv+dKK61gcc7dUoOvLBoxeIe/g Ty/oKIAx1avQw== Message-ID: <68f17ee7-d272-4475-8a59-645442e2c210@kernel.org> Date: Wed, 21 Oct 2026 10:21:53 +0200 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] mm/ksm: keep soft-dirty and uffd-wp when merging a page To: Lance Yang Cc: samarth.colleges@gmail.com, akpm@linux-foundation.org, xu.xin@linux.dev, chengming.zhou@linux.dev, peterx@redhat.com, kas@kernel.org, rppt@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261020055518.36087-1-lance.yang@linux.dev> <3da45e6d-2b46-4fff-9ff3-241f70d68296@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/20/26 15:29, Lance Yang wrote: > > > On 2026/10/20 21:03, David Hildenbrand (Arm) wrote: >> On 10/20/26 07:55, Lance Yang wrote: >>> >>> >>> Right. >>> >>> >>> Haha, yeah, I stared at the code a bit. Say we arm RWP on an existing KSM >>> mapping, then do MADV_UNMERGEABLE. We're just replacing the shared page >>> with a private copy. That shouldn't count as an access to the mapping ... >>> no? >>> >>> But the protnone check sends us to do_uffd_rwp() before the >>> FAULT_FLAG_UNSHARE handling. No FAULT_FLAG_ALLOW_RETRY here, so sync RWP >>> gives us VM_FAULT_SIGBUS. Then break_ksm() does this: >>> >>> static int break_ksm(struct vm_area_struct *vma, unsigned long addr, >>>         unsigned long end, bool lock_vma) >>> { >>>     vm_fault_t ret = 0; >>> ... >>>     do { >>> ... >>>         ret = handle_mm_fault(vma, addr, >>>                       FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE, >>>                       NULL); >>>     } while (!(ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM))); >>> ... >>>     return (ret & VM_FAULT_OOM) ? -ENOMEM : 0; >>> } >>> >>> So we report success with the page still merged. Async RWP does unshare >>> it, but only after clearing the uffd bit. >>> >>> I'd let FAULT_FLAG_UNSHARE skip do_uffd_rwp(). We'd still need to carry >>> PAGE_NONE over in wp_page_copy(), though. That PTE is rebuilt from >>> vma->vm_page_prot, so the uffd bit alone won't stop reads. >>> >>> >>> Tested, this is what I had in mind: >>> >>> ---8<--- >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 976cd03954e3..d99f5ef1dd8c 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -4119,8 +4119,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) >>>           if (unlikely(unshare)) { >>>               if (pte_soft_dirty(vmf->orig_pte)) >>>                   entry = pte_mksoft_dirty(entry); >>> -            if (pte_uffd(vmf->orig_pte)) >>> +            if (pte_uffd(vmf->orig_pte)) { >>>                   entry = pte_mkuffd(entry); >>> +                if (userfaultfd_rwp(vma)) >>> +                    entry = pte_modify(entry, PAGE_NONE); >>> +            } >>>           } else { >>>               entry = maybe_mkwrite(pte_mkdirty(entry), vma); >>>           } >>> @@ -6656,9 +6659,12 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf) >>>            * VM_UFFD_RWP VMA, a protnone PTE without the uffd bit is >>>            * NUMA hinting and must still fall through to do_numa_page(). >>>            */ >>> -        if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) >>> -            return do_uffd_rwp(vmf); >>> -        return do_numa_page(vmf); >>> +        if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) { >>> +            if (!(vmf->flags & FAULT_FLAG_UNSHARE)) >>> +                return do_uffd_rwp(vmf); >>> +        } else { >>> +            return do_numa_page(vmf); >>> +        } >>>       } >> >> Looks like a reasonable direction (without looking into all the details). >> >> I've been also wondering whether KSM should simply skip these uffd regions, they >> it never worked properly ... but I'm sure we'd run into issue with leftover uffd > > +1. Did it ever actually work there :) Why not just skip them and > call it a day :D I think QEMU just always sets MADV_MERGABLE and later activates uffd. So that needs some care to not disable them. So let's rather try to get it fixed properly. Can you send out proper patches? Thanks! -- Cheers, David