From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-176.mta1.migadu.com (out-176.mta1.migadu.com [95.215.58.176]) (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 D58003016FC for ; Wed, 17 Jun 2026 03:15:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.176 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781666111; cv=none; b=MAZr390liAx33f9w++/ItN7aDj9pQsJm7Z2JePHPS3GOg+NtazX0FjZWfg2v7AftUPE93xpEF0JnWBMhS6BB4bVt7biIR2tfN76lUZaxTBw40uV0A8Yaq67DRXCI/MiS/PwcAvlEsppDHwyIQpFEtaISxrBINOOyHIOC5T0V61g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781666111; c=relaxed/simple; bh=Qk/DM4744K7XBYGNbGM3g85QtFLrnbrVNMpF5F9IuE8=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=oMpbTxc5TMCR3xXwIIed4IldBASCWVyhWgd3dezf2JOvOOnlwloeejvlahYw6x1Na5l84+JT2+hNMLlryqNlMAZAAUFpqy8treMMPXJxE8BbeQrQC2wEzFRP/ainnrvzqxo6jLpMxVoeOUewGoF/gWwGMl46Y3O7CZDX5OO2Ark= 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=RyvrvsqH; arc=none smtp.client-ip=95.215.58.176 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="RyvrvsqH" 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=1781666108; 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=FYheglZoN7ZQINy1Nkai5yf7WMqEaDWpcm1200AttNM=; b=RyvrvsqHUQr2/2WVE1SSai7a83FHouvM1ASy/wdoWScK1x0Ub9zIsGQpopfn4pf36Mxb25 DnpJnpNqv65Zb0KEYBbk16vQR2SLelhUs2cgnPVUw9x+PvqztbxbbfbOsDMaomrwInQBIw TbjerRr9NnM+ci/z211zqm2KqH7jzZY= From: Lance Yang To: balbirs@nvidia.com Cc: david@kernel.org, lance.yang@linux.dev, richard.weiyang@gmail.com, akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.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 11:14:54 +0800 Message-Id: <20260617031454.29210-1-lance.yang@linux.dev> In-Reply-To: References: 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 Wed, Jun 17, 2026 at 12:11:14PM +1000, Balbir Singh wrote: >On Tue, Jun 16, 2026 at 03:07:53PM +0200, David Hildenbrand (Arm) wrote: >> On 6/16/26 14:30, 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 :) >> > >> > Not sure about practical fallout, but should these PMD-level not_found() >> > cases also take PTL and restart if PMD changed? >> I was hoping that we could so something similar to the PTE case. >> >> In map_pte(), we check whether the PMD changed, which is slightly different. >> >> The actual check happens in check_pte() after grabbing the PTL. >> >> For the case you describe, map_pte() would find !pte_none(ptent) ... >> !pte_present(ptent) and !is_migration, and effectively grab the lock and proceed >> to check_pte(). >> >> In check_pte() we re-check under lock indeed. >> > >Thinking of the practical fallout, not finding the PMD for a non >migration worker should be OK. Is there a case where it's not OK to >report the old state. I was thinking the lockless value should only be used as a first filter, to see whether entry looks worth checking. PTE side is roughly lockless filter + PTL/check_pte(). PMD true case now gets PTL/pmd_same(), but some PMD not_found() cases still come straight from lockless "pmde". That mismatch felt odd to me ... Cheers, Lance