From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 E3DCC2D7D2E for ; Tue, 12 May 2026 18:55:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778612155; cv=none; b=Oad110cVewF3U9MzYt/o6k5ZM4d6Wkxv/4VdINQluL/Jhriz9JFNnZZ2bMMHPvg5pVtiOotgWcOE7eysUJB2rlYBDgFkOx9+OtaDCQFYtAd65l0Uu9vytiMgDZADD8yVt9ZxIP7GVRAUL3ogOH8hit9A4Ei3scp2JXUPloTjTx4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778612155; c=relaxed/simple; bh=YpNkICHwO3jCdKAl0LL5HmWKMFmNgj4OCEDikfq3svo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aOYPay8lk+VnAcQJZvKTYpdhvcqQgkauM91Kt1YrpaTCbdUQrN7yCNqb6kmxstRb+6UDTyhIV40PkC2cc4TmKLHOo9PlO+7vJntMc4PAuQNwLwptvOqSJ/TamTP2nUlmOsel0hXtUgdaC8zIaBF/vS8ona8UflhwTEl7R7bQZWk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q6wQtfYO; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Q6wQtfYO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 19A10C2BCB0; Tue, 12 May 2026 18:55:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778612154; bh=YpNkICHwO3jCdKAl0LL5HmWKMFmNgj4OCEDikfq3svo=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Q6wQtfYON2Lo65wpxSC2KY3N3ETpsdAh0mAs3IpVAIoLlyWrSEoAGOwtAXQy/kQV2 b5kcexlK+Zc/LTP6q608vEkFO9tLtb+LmsFA7M0fqFtsfgghYyLvU0f3yK1GESb67N bs5iwV/1KijpkOlb9Sk7dgOZML1JcNDPTaxdjmph6JRGeB67If/iWuYKST0m1XGc7/ 3i8f88qIW1XWfXworlXF9yKKH1oXVujLBEM8ylGEkUw0jO1SeGhFycTkiTxqqL89ow j6Dwlx3xX6ISfWUQ+1dztTeKihqzLo0/CVuK3QpFdpHlf9F5cKFSn5lLiuuoJ0DFGU rhk+2ZqAMbljA== Message-ID: <113dddc5-27e3-4e9e-a90c-f076a4629f51@kernel.org> Date: Tue, 12 May 2026 20:55:47 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd To: Wei Yang Cc: Balbir Singh , akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, Lorenzo Stoakes , stable@vger.kernel.org References: <20260508013728.21285-1-richard.weiyang@gmail.com> <5e9ee072-b927-41e0-ba98-c9fdf11eccbc@nvidia.com> <0aab59b8-71c5-4059-8281-5dd876946528@kernel.org> <20260512143542.izpp3gu4iqxttw3f@master> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/12/26 16:35, Wei Yang wrote: > On Tue, May 12, 2026 at 02:43:54PM +0200, David Hildenbrand (Arm) wrote: >> On 5/9/26 00:48, Balbir Singh wrote: >>> >>> Could you elaborate a more on the improper situation? >>> >>> >>> Do we need to check softleaf_is_device_private() twice, can't we hold the pmd >>> lock and check once? >> >> I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there. >> >> I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock. >> >> Something a lot cleaner like: >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index a4d52fdb3056..de6a255cc847 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -242,40 +242,28 @@ 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); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> + return not_found(pvmw); >> + } else if (pmd_is_device_private_entry(pmde)) { >> + softleaf_t entry = softleaf_from_pmd(pmde); >> >> + if (pvmw->flags & PVMW_MIGRATION) >> + return not_found(pvmw); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> + return not_found(pvmw); >> + } else { >> if ((pvmw->flags & PVMW_SYNC) && >> thp_vma_suitable_order(vma, pvmw->address, >> PMD_ORDER) && >> @@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> step_forward(pvmw, PMD_SIZE); >> continue; >> } >> + >> + /* Double-check under PTL that the PMD didn't change. */ >> + pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> + if (pmd_same(pmde, pmdp_get(pvmw->pmd))) >> + return true; >> + spin_unlock(pvmw->ptl); >> + pvmw->ptl = NULL; >> + goto restart; >> +pte_table: >> if (!map_pte(pvmw, &pmde, &ptl)) { >> if (!pvmw->pte) >> >> >> >> >> There is likely room to clean this up / compress it further. > > I tried to compress above logic like this, hope it could look cleaner. > > if (pmd_trans_huge(pmde) || pmd_is_valid_softleaf(pmde)) { > unsigned long pfn; > bool is_migration = pmd_is_migration_entry(pmde); > bool for_migration = !!(pvmw->flags & PVMW_MIGRATION); > > if (is_migration != for_migration) > return not_found(pvmw); > > if (pmd_trans_huge(pmde)) > pfn = pmd_pfn(pmde); > else > pfn = softleaf_to_pfn(softleaf_from_pmd(pmde)); > > if (!check_pmd(pfn, pvmw)) > return not_found(pvmw); > } else if (!pmd_present(pmde)) { It's more compact, but not necessarily cleaner. In particular, I detest pmd_trans_huge(), we should phase it out. if (pmd_present(pmde) && !pmd_leaf(pmde)) { goto pte_table; } else if (pmd_present(pmde) || pmd_is_valid_softleaf(pmde)) ... Might work as well. But once we add support for other softleaf types, we'll have to touch it again. So I'd rather just list what we actually expect. -- Cheers, David