From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B7712CCD192 for ; Tue, 14 Oct 2025 15:02:27 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DDAC28E013E; Tue, 14 Oct 2025 11:01:42 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D8BCC8E010A; Tue, 14 Oct 2025 11:01:42 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C7AC28E013E; Tue, 14 Oct 2025 11:01:42 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id B4D468E010A for ; Tue, 14 Oct 2025 11:01:42 -0400 (EDT) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 6D00E5BFE4 for ; Tue, 14 Oct 2025 15:01:42 +0000 (UTC) X-FDA: 83997034044.29.1DB443C Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) by imf25.hostedemail.com (Postfix) with ESMTP id 2EAC3A001F for ; Tue, 14 Oct 2025 15:01:33 +0000 (UTC) Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=mc3BGk1l; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf25.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.182 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1760454100; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=AB9j6O3TPji8YNEzc+bgFaMn1t+j1zQSsTJC9AP4fx4=; b=eScYgx4DDOMZDSU47n6VqvmRtc7jIzzkML6oJBYCbfXg+hpRsvH4BkwuTGt/ADEIQM9ssW 7oNwOz0aa4q1tQYA7rMCiv8kidpp6R5DH9EYU5codSDA/uDKbSvdw/3rU8PTwZkPsjUJhb Ge2dTtxLgQXYF+34n2GolzIAFSWJgrA= ARC-Authentication-Results: i=1; imf25.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=mc3BGk1l; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf25.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.182 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1760454100; a=rsa-sha256; cv=none; b=MGAAa2HaydO1z7+IeRqufQiMHGEIoG0dRKxtQVbCsnZtAL3YM0kPNbFScLZOz6yY/uYDun G2Xw0I3b+xCL4eYFdeC56wUtf9lybCEaC1C0pRsWCvLq6CBoHX/L6cLRZApnG/JH4GuY5p e6bGp2EQENafCJNQpfxw0Mz0y7mGZ3Y= Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760454091; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=AB9j6O3TPji8YNEzc+bgFaMn1t+j1zQSsTJC9AP4fx4=; b=mc3BGk1laLOWzAF5w/uZV0flGEZx0baY7aeoTRTSkf+Blbng98lexfcRFo5vWyJFzc98eP 0pkpMSXcdutVbpE9GhGemQE1cLazHEkR0wheL8UQ2RfZeOpZEQSyeMDgmlpQEX2+LUQaC+ WdXJX3Hr31s7/2r7jIDxnIUFzo2+4A4= Date: Tue, 14 Oct 2025 23:01:13 +0800 MIME-Version: 1.0 Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries Content-Language: en-US To: Lorenzo Stoakes Cc: akpm@linux-foundation.org, david@redhat.com, Liam.Howlett@oracle.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, hughd@google.com, ioworker0@gmail.com, kirill@shutemov.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mpenttil@redhat.com, npache@redhat.com, ryan.roberts@arm.com, ziy@nvidia.com, richard.weiyang@gmail.com References: <20251008032657.72406-1-lance.yang@linux.dev> <0bfdbccd-9d4a-409f-ae43-b44bb7347d70@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 2EAC3A001F X-Rspamd-Server: rspam03 X-Stat-Signature: bwurqetiywk4ardp8y45oe6korwp4wxj X-HE-Tag: 1760454093-589079 X-HE-Meta: U2FsdGVkX1/JdlawDFhY2L1w3TaagoD3q/Et/4YywR2mkEnWjt1kOYb86eN0rrbZBXF0uQePKeiSaiQyWN4OC7d6pzcldFVi7YjqIfcYQb9S2n/wp9GGtcjhOhZ1URHb3jXfxO3hk3kwKou9kUXy1vbCmNkXEpCGuIDzZerWei8NJcjMrq79iGNM6tV9aRdzBr8RsskpcOSFUfGxwKmfazRVHz30xJ9NWSmKZeoPoOslyBq8MIMo+kG/MhVbN5Z8vChL4TtcZ1pdgctvMLorcBGqluPaggzHVJObH94Eg8vzh0YmvwPzowksE//YObaE6+IP8JuB7QGUx4P6VYdsbd7rAiqYDSMv/2WNQWyRKoFe8m25vgW9DAGVg+fhsZOCq+OUIzqno4i0mwma+kKEfp/lfhQzjZ99n3R1w6VUsN/Z8/AbUCVfm9sMzYtuAAVrJAN59myRXvncRBVhrfVgPo/SZj5uhIQAEzmUnvKWr8dzWFf7NjHF8xlHDYSNhnKVxqBSYg11sVyz3LEIouZKph0+QdRzfDBh61HdvyA5FIEgd2PECAk88hffY6QvsemnCRpqx8PYsRq817AFAGE3HonL+PLuJGKLZ5Y/rbDfnXD1UVfbD/YlbK7qhFQ6aE+K3XC6Nsa8rECFo7+H0VsxtORL3wrRFyNlNygqGkvNoloKbopwEvuPPlr8l0AAAYcsLCGvc8K8NBcOa5Wsg4uSW/tJLRYaC63kv2yn19wF8lOnV173GxsRC6eOJ/+YTCQDsZzjHYqRMyktKBBZmFlYC76z9vW4+nva5Pn6H4sgs8ijGVmsQp96hL1d1D+lIxZ/GW6YxjVzP+jdUx86EtcUKb1qb1iigvUlvgqUhZYja3pYlqovuEIBUmWCO4LfIuO7JoiUG4azL2Sxx8Rsqolh8KNaoyzrPGDxHMXpSPH5SCaVGkwXb1LUQ/QLwpmCywLcI1Gj1prIqIbLmwlJ5i5 yszSk5o5 wHC5TU+RSY4HweHluS3WJo477C6WNYUJEWdZNUUoFLLQK61kjCrsxNoKLrElpWaMd48q58prVz+yntRCJC7XJsdpJ+WkEU16gk6bQDCL59vCNAM6Jr6PHopZPT38FMyt/JhWiu9GqzhL1E7Fl6A53srZAHVz/1heVUn70ESDcy7elDB11sUWvNJt4SLFbPyw40ip15gDIBl+pYMCYLFgsjcD57UqVdLkg89acMbuWoR1GKUJrKbzH0CsxdwUJE3gsrf7XymBZyHL3Pcd7pe7B3seQ5A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2025/10/14 22:39, Lorenzo Stoakes wrote: > On Tue, Oct 14, 2025 at 10:26:20PM +0800, Lance Yang wrote: >> >> >> On 2025/10/14 19:08, Lorenzo Stoakes wrote: >>> On Wed, Oct 08, 2025 at 11:26:57AM +0800, Lance Yang wrote: >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index abe54f0043c7..bec3e268dc76 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -1020,6 +1020,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm, >>>> if (!is_swap_pte(vmf.orig_pte)) >>>> continue; >>>> >>>> + if (non_swap_entry(pte_to_swp_entry(vmf.orig_pte))) { >>>> + result = SCAN_PTE_NON_PRESENT; >>>> + goto out; >>>> + } >>> >>> OK seems in line with what we were discussing before... >> >> Yep. That's the idea :) >> >>> >>>> + >>>> vmf.pte = pte; >>>> vmf.ptl = ptl; >>>> ret = do_swap_page(&vmf); >>>> @@ -1281,7 +1286,23 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >>>> for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR; >>>> _pte++, addr += PAGE_SIZE) { >>>> pte_t pteval = ptep_get(_pte); >>>> - if (is_swap_pte(pteval)) { >>>> + if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >>>> + ++none_or_zero; >>>> + if (!userfaultfd_armed(vma) && >>>> + (!cc->is_khugepaged || >>>> + none_or_zero <= khugepaged_max_ptes_none)) { >>>> + continue; >>>> + } else { >>>> + result = SCAN_EXCEED_NONE_PTE; >>>> + count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >>>> + goto out_unmap; >>>> + } >>>> + } else if (!pte_present(pteval)) { >>>> + if (non_swap_entry(pte_to_swp_entry(pteval))) { >>> >> >> Thanks for pointing that out! > > You've deleted what I've said here and also not indicated whether you'll do what > I asked :) > > Please be clearer... Oh, I didn't delete your comment at all ... It's just below ... > >>>>> Hm but can't this be pte_protnone() at this stage (or something else)? And then <-- Here! >> >> Yeah. The funny thing is, a protnone pte cannot actually get here, IIUC. >> >> ``` >> static inline int pte_protnone(pte_t pte) >> { >> return (pte_flags(pte) & (_PAGE_PROTNONE | _PAGE_PRESENT)) >> == _PAGE_PROTNONE; >> } >> >> static inline int pte_present(pte_t a) >> { >> return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE); >> } >> ``` >> >> On x86, pte_present() returns true for a protnone pte. And I'd assume >> other archs behave similarly ... > > This was one example, we may make changes in the future that result in entries > that are non-present but also non-swap. > > I don't see the point in eliminating this check based on an implicit, open-coded > assumption that this can never be the case, this is just asking for trouble. > >> >>> we're just assuming pte_to_swp_entry() is operating on a swap entry when it in >>> fact might not be? >>> >>> Couldn't we end up with false positives here? >> >> Emm, I think we're good here and the code is doing the right thing. > > I mean sorry but just - NO - to doing swap operations based on open-coded checks > that you implicitly feel must imply a swap entry. > > This makes the code a lot more confusing, it opens us up to accidentally > breaking things in future and has little to no benefit, I don't see why we're > doing it. > > I don't think every little 'aha X must imply Y so just eliminate Z' idea need be > implemented, this feels like a sort of 'mathematical reduction of code ignoring > all other factors'. Understood. Changing !pte_present() to is_swap_pte() will resolve all your concerns, right? ``` if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { [...] } else if (is_swap_pte(pteval)) { <-- Here if (non_swap_entry(pte_to_swp_entry(pteval))) { [...] } [...]} ``` > >> >>> >>>> + result = SCAN_PTE_NON_PRESENT; >>>> + goto out_unmap; >>>> + } >>>> + >>>> ++unmapped; >>>> if (!cc->is_khugepaged || >>>> unmapped <= khugepaged_max_ptes_swap) { >>>> @@ -1290,7 +1311,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, >>>> * enabled swap entries. Please see >>>> * comment below for pte_uffd_wp(). >>>> */ >>>> - if (pte_swp_uffd_wp_any(pteval)) { >>>> + if (pte_swp_uffd_wp(pteval)) { >>> >>> Again you're assuming it's a swap entry but you're not asserting this is a swap >>> entry in this branch? >> >> As we discussed above, the non_swap_entry() check has already kicked out >> anything that isn't a true swap entry, right? > > This is a different function? > > Actually I'm mistaken here I think - you check in the code above: > > if (is_swap_pte(pteval)) { > ... > } > > So this is fine, please ignore sorry :) No worries at all, thanks for double-checking and clarifying! > >> >>> >>> Also an aside - I hate, hate, hate how this uffd wp stuff has infiltrated all >>> kinds of open-coded stuff. It's so gross (not your fault, just a general >>> comment...) >> >> Haha, tell me about it. No argument from me there ;) > > :) > >> >> Thanks, >> Lance > > Cheers, Lorenzo