From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D3EB11CDFCA for ; Wed, 17 Jun 2026 02:32:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781663549; cv=none; b=GeeEqsuy+KHqtzoG8eyOe9BZS/tez3Zk8nXemTE2jxNtewg1yx4POKLES1pjV/6b+51RmbadzXE8NijxS/4CJ1WnkjDb/X00euznwFxlXZzP/Hvc+nWtUaU+4A63NfuJvQENAeLwF+PH0aPGY2TRW2/Zz0rgtfAexYIO691bIe0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781663549; c=relaxed/simple; bh=8Jkg4hWaGSgxim555NVdr/IGldICyHHu0eKJiB7qjp0=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=OTlJU3C3dgLfm5AxiSVfWCEPAL4nFy3QlJeAlg+b2VrfHlOwmzzZDRJiNn/A39dWPDauWxfrENRmEXdVPdRuzePtrKY2wAoqqpySSQx/tYXp9orDlONjmZ78ewf+W6Yj/GwbjA1iXiTqfVFszVGjsa8fBz/4uH2CQWiYlDONMVM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=EOPnEcRo; arc=none smtp.client-ip=95.215.58.174 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) header.d=linux.dev header.i=@linux.dev header.b="EOPnEcRo" 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=1781663545; 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=KWSwI0JSp8IpusvL17b7isqNuxgesNcqyUkzefJ+C/g=; b=EOPnEcRoyEvrckUiMuG+/W56AB0fjfv6Ssjj7zOAWHhVtqtmbkqSe23THUbNf4yk623rCB KKdaBgxBn6yPULoRDg3R5eb4fJrpdPUYPyraPTes0TevoJqpC4OX2BKnX0fcH8iJrheN1q 4tQ8m/GCPRbWd9hOjxwAnPooOewsAx0= From: Lance Yang To: richard.weiyang@gmail.com Cc: lance.yang@linux.dev, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, balbirs@nvidia.com, ziy@nvidia.com, sj@kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, stable@vger.kernel.org Subject: Re: [Patch v2] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd Date: Wed, 17 Jun 2026 10:32:11 +0800 Message-Id: <20260617023211.80409-1-lance.yang@linux.dev> In-Reply-To: <20260616235022.iesy2jeb2p7zof2l@master> References: <20260616235022.iesy2jeb2p7zof2l@master> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Tue, Jun 16, 2026 at 11:50:22PM +0000, Wei Yang wrote: >On Tue, Jun 16, 2026 at 08:30:01PM +0800, Lance Yang wrote: >> >>On Tue, Jun 16, 2026 at 06:34:36AM +0000, Wei Yang wrote: >>[...] >>>diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >>>index 2ccbabfb2cc1..21635fab209c 100644 >>>--- a/mm/page_vma_mapped.c >>>+++ b/mm/page_vma_mapped.c >>>@@ -243,40 +243,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >>> */ >>> pmde = pmdp_get_lockless(pvmw->pmd); >>> >>>- if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) { >>>- pvmw->ptl = pmd_lock(mm, pvmw->pmd); >>>- pmde = *pvmw->pmd; >>>- if (!pmd_present(pmde)) { >>>- softleaf_t entry; >>>- >>>- if (!thp_migration_supported() || >>>- !(pvmw->flags & PVMW_MIGRATION)) >>>- return not_found(pvmw); >>>- entry = softleaf_from_pmd(pmde); >>>- >>>- if (!softleaf_is_migration(entry) || >>>- !check_pmd(softleaf_to_pfn(entry), pvmw)) >>>- return not_found(pvmw); >>>- return true; >>>- } >>>- if (likely(pmd_trans_huge(pmde))) { >>>- if (pvmw->flags & PVMW_MIGRATION) >>>- return not_found(pvmw); >>>- if (!check_pmd(pmd_pfn(pmde), pvmw)) >>>- return not_found(pvmw); >>>- return true; >>>- } >>>- /* THP pmd was split under us: handle on pte level */ >>>- spin_unlock(pvmw->ptl); >>>- pvmw->ptl = NULL; >>>- } else if (!pmd_present(pmde)) { >>>- const softleaf_t entry = softleaf_from_pmd(pmde); >>>- >>>- if (softleaf_is_device_private(entry)) { >>>- pvmw->ptl = pmd_lock(mm, pvmw->pmd); >>>- return true; >>>- } >>>+ if (pmd_present(pmde)) { >>>+ if (!pmd_leaf(pmde)) >>>+ goto pte_table; >>>+ if (pvmw->flags & PVMW_MIGRATION) >>>+ return not_found(pvmw); >>>+ if (!check_pmd(pmd_pfn(pmde), pvmw)) >>>+ return not_found(pvmw); >>>+ } else if (pmd_is_migration_entry(pmde)) { >>>+ softleaf_t entry = softleaf_from_pmd(pmde); >>>+ >>>+ if (!(pvmw->flags & PVMW_MIGRATION)) >>>+ return not_found(pvmw); >> >>Looked at history a bit, and I wonder if this changed something old >>here ... >> >>Since 616b8371539a ("mm: thp: enable thp migration in generic path"), PMD >>migration handling took PTL before doing PVMW_MIGRATION/PFN checks, >>including not_found() cases. So lockless PMD read was just a filter ... >> >>With this fix, true case gets final pmd_same() check, but this >>not_found() case happens before taking PTL. >> >>So a !PVMW_MIGRATION walker could race with someone, e.g. >>remove_migration_pmd(): we make the not_found() decision from old PMD >>value that still says "migration", while real *pvmw->pmd may already be >>present again. We return without ever taking PTL :) >> > >Hi, Lance > >Thanks for take a look. > >I am trying to understand the scenario you mentioned. Let's say A migrate a >pmd and B want to unmap the pmd. > > A B > > try to migrate a pmd > pmd is set to migration entry > unmap the pmd ... > managed to finish migration > ...still see migration entry, > so skipped and unmap fail > >Would this be a timing case? Even B grab the PTL, it still could see migration >entry if B visit pmd before A finish migration. > >Maybe I miss something, look forward your insight. Right, seeing migration entry while migration is still ongoing is fine. What I meant was this ordering: CPU 0: pmde = pmdp_get_lockless(...); /* migration */ CPU 1: remove_migration_pmd() restores PMD to present CPU 0: returns not_found() from old pmde, without ever taking PTL and rechecking *pvmw->pmd So issue is not seeing migration entry itself, but making final not_found() decision from stale lockless PMD value ... Before this patch, PMD migration case took PTL before making that decision ... >>Not sure about practical fallout, but should these PMD-level not_found() >>cases also take PTL and restart if PMD changed? >> > >-- >Wei Yang >Help you, Help me >