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 462BAFF8864 for ; Wed, 29 Apr 2026 05:55:15 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6E5B76B0005; Wed, 29 Apr 2026 01:55:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 6BD326B008A; Wed, 29 Apr 2026 01:55:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5D2D06B008C; Wed, 29 Apr 2026 01:55:14 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 4CB266B0005 for ; Wed, 29 Apr 2026 01:55:14 -0400 (EDT) Received: from smtpin20.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id CB1261604EA for ; Wed, 29 Apr 2026 05:55:13 +0000 (UTC) X-FDA: 84710530506.20.4449936 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) by imf22.hostedemail.com (Postfix) with ESMTP id E07BBC0008 for ; Wed, 29 Apr 2026 05:55:11 +0000 (UTC) Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=hJQf34fO; spf=pass (imf22.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.171 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1777442112; 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=Pq9ltivsZppH22kMe9SkiYKYPDVBbnNZTvNi+j5XUZA=; b=gvA/kO2SC1oV05aldxqL6grY4JPdLnfZMbALXHlqaCj2SCBkM0fIMhMMVWkrdErjTnSpX3 BBl/nqsASWG6tRgwm4+kyoRj8vF8uqkHfIauPwxPc8EdyJHI4tqnvx5HrEHIDDdxxzkOhP zfPsw0nbSMfzN61nZ4HeocNvIBJo600= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=hJQf34fO; spf=pass (imf22.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.171 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1777442112; a=rsa-sha256; cv=none; b=b6bmFxlgzlLU94NrcbLSMYkasnPxRkfkm/rIwxvATdlTkaDBR7Ru3innbt9hboqkYuCzhp 8ltc5NU5BZxOprn/PBKhMjnRSwmNLJv+ibyUcihlZbIFRRBf+Pavmh5+33ntVLAEXT8kI6 /bRAl6czBhV7qmaL16CWiTz0+RnKcjI= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1777442109; 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=Pq9ltivsZppH22kMe9SkiYKYPDVBbnNZTvNi+j5XUZA=; b=hJQf34fORZxJi3Ay6BjtJJuIHZ2tkCBmo8bGB0RTOW+SvOrD4gg3JTk4Nivgvt0j5p9EZ8 uXdxg6fXiW2clQPuY6YrHN3rP/XDefUDPL1yc2DC/uPODWvPA5bOQ5wkcAdY7orkjBEHUe 7xLPJ9o6UagHW8OpCenQh4RSJU7Mflc= From: Lance Yang To: hughd@google.com Cc: akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, baohua@kernel.org, david@kernel.org, dev.jain@arm.com, lance.yang@linux.dev, liam.howlett@oracle.com, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, npache@redhat.com, zhengqi.arch@bytedance.com, ryan.roberts@arm.com, surenb@google.com, ziy@nvidia.com, linux-mm@kvack.org Subject: Re: [PATCH hotfix] mm: fix pmd_special() fallback to observe huge_zero Date: Wed, 29 Apr 2026 13:54:47 +0800 Message-Id: <20260429055447.9220-1-lance.yang@linux.dev> In-Reply-To: <74a75b59-2e13-3985-ee99-d5521f39df2a@google.com> References: <74a75b59-2e13-3985-ee99-d5521f39df2a@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: E07BBC0008 X-Rspamd-Server: rspam06 X-Stat-Signature: npde46yw5r3wfhfxzbrr3zd3kwhapy5z X-HE-Tag: 1777442111-890873 X-HE-Meta: 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 Ah1/r70Z ufUg02dtPEHuyQcYalGSGGbOmulgb4g4JyTgJrBhRc/OuixomwTVBgN3HeYcJRcfVGzZtNqoozpuI55UMuXqUqFHVfwHEXQKRfAnqRhoaahhIbhWcVv62anlo+dFdtHnFEbV1lmaU4FAKiqYbO7EFBnK7DCziUNJPUTsEglpBUUqy/mJpoRatXE+QXyS6Fm7X9IZRAy6D6uQ2UOL43PtajVC1dFQdjap57j+Cb4dDdKpZx7u7nu3a2MrG7hlTy3rwSO+kmMkumhKN9eIzNiRtIdipHbsDfvFzqjfrewJBt1mGFaDacgdrat1UGQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Apr 28, 2026 at 10:08:37PM -0700, Hugh Dickins wrote: >On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a >"WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the >VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed >by "BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s >when reclaim gets to call shrink_huge_zero_folio_scan(). Good catch! >It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd: >and indeed, whereas pte_special() and pte_mkspecial() are subject to a >dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial() >are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled >on any 32-bit architecture. > >Add CONFIG_ARCH_HAS_PMD_SPECIAL? Perhaps; but I think it's better just >to observe the huge_zero pmd in the fallback version of pmd_special(). > >Fixes: d80a9cb1a64a ("mm/huge_memory: add and use normal_or_softleaf_folio_pmd()") >Signed-off-by: Hugh Dickins >--- > include/linux/mm.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > >diff --git a/include/linux/mm.h b/include/linux/mm.h >index 0b776907152e..3b02ac43bcb7 100644 >--- a/include/linux/mm.h >+++ b/include/linux/mm.h >@@ -3422,7 +3422,7 @@ static inline pte_t pte_mkspecial(pte_t pte) > #ifndef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP > static inline bool pmd_special(pmd_t pmd) > { >- return false; >+ return is_huge_zero_pmd(pmd); > } Emm ... feels a bit odd to me ... On these configs pmd_mkspecial() is still a no-op, so pmd_special() would no longer really mean that the PMD was made special :) Could we handle the huge zero PMD in vm_normal_page_pmd() instead? ---8<--- diff --git a/mm/memory.c b/mm/memory.c index 199214f8de08..3d9ed41a46b8 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -793,6 +793,9 @@ struct folio *vm_normal_folio(struct vm_area_struct *vma, unsigned long addr, struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, pmd_t pmd) { + if (is_huge_zero_pmd(pmd)) + return NULL; + return __vm_normal_page(vma, addr, pmd_pfn(pmd), pmd_special(pmd), pmd_val(pmd), PGTABLE_LEVEL_PMD); } -- zap_huge_pmd() -> normal_or_softleaf_folio_pmd() -> vm_normal_folio_pmd() -> vm_normal_page_pmd() -> is_huge_zero_pmd(pmd) -> return NULL -> return NULL That's where we ask whether the PMD maps a normal page, and for the huge zero PMD the answer is simply "no". -> has_deposited_pgtable() -> is_huge_zero_pmd(pmd) -> true -> zap_deposited_table() -> skip zap_huge_pmd_folio() Then zap_huge_pmd() would still withdraw the deposited pgtable (except for DAX huge zero PMDs), but skip zap_huge_pmd_folio() - no normal folio rmap/RSS accounting. Just a thought :) Thanks, Lance