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 E0B7337AA98 for ; Thu, 9 Apr 2026 07:52:13 +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=1775721134; cv=none; b=ctpa3q5qKelvsFdMIQz5NhRwWLmpXyPGDJgHOnbVDMqIJ7Ld3pQ2lYKzZOQqH1rgB5E5kEQ5/twMYhqmFFW1RZV+Gk47FMvWi63tLP4jFK/L5QklS3ocSFvZKhMqK1klDqujQf6B8o14i4BTNITOu521wtspF/NRTN3YuStDPWU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775721134; c=relaxed/simple; bh=yGi65EzNbfPcmaJHDWorm0OFGsNjNwfdp9HFY6agVns=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ITOY4kodLDLgayjcs7frUoDIwUfrka/LV6l1gAdphNtQ6BbkbKID+MCV+ZLzVJKMwtBIPSpDdgW1yGwVSwM5WBvNvuohkFwdbQc7lGIrTVuoHwYbPjBqqG4o7Bz5Vv17xx/bojSaVTyypGZZgLB4Hea2S1Yn8WqIMaCqvWZBxdk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RvVJ8Uzm; 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="RvVJ8Uzm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85DCEC4CEF7; Thu, 9 Apr 2026 07:52:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775721133; bh=yGi65EzNbfPcmaJHDWorm0OFGsNjNwfdp9HFY6agVns=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RvVJ8UzmOEqrs0G14kFM++8rr6iSHglz2PZ93CS4iIyeprIkvH8VqMiq/edzOnkCw Si4666iPcgT3pRU8sE0Wf+cubbsOxmSkc58sEXEE0TCWStDdwgdVmxRQmfBm8wrY71 mzI3VspNd6nW1JYnfViIiONMRI2Kej7tVv4FKomuFTLENDR/OcJ18kSdXGarSyzoR5 r118jxfBv6pJ7S8C5KL9DbZVwIWGBoDobp64ckPeC4QjdTdxu0/gPXSdcQNREJvO9n jJLUS6GwJ0Vjf3l1frsawwNuCCzoXfWttFhEnBiG3G2B7XWsVMhPxhMs+7qvRANkx0 J1LsyFry/mCQw== Message-ID: <4d6c1af0-dee5-4f1b-b74c-2ebc23de7baf@kernel.org> Date: Thu, 9 Apr 2026 09:52:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios To: John Hubbard , Andrew Morton Cc: Jason Gunthorpe , Peter Xu , Mike Rapoport , LKML , linux-mm@kvack.org, Sourab Gupta , Zi Yan References: <20260409014647.397515-1-jhubbard@nvidia.com> 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 4/9/26 03:46, John Hubbard wrote: > Since commit f002882ca369 ("mm: merge folio_is_secretmem() and > folio_fast_pin_allowed() into gup_fast_folio_allowed()"), > gup_fast_folio_allowed() falls back to the slow path for any order-0 > folio with a NULL mapping when CONFIG_SECRETMEM=y. This causes a > performance regression for drivers that allocate pages with alloc_page() > and insert them into VMAs via vm_insert_page(). These pages legitimately > have a NULL folio->mapping, but they cannot be secretmem pages. > > Secretmem pages are always added to the secretmem inode's page cache via > filemap_add_folio(), which sets folio->mapping to the inode's i_mapping. > A folio with a NULL mapping can never be a secretmem folio. The > NULL-mapping check was intended to handle truncated file-backed pages (a > reject_file_backed concern), not secretmem detection. > > When only check_secretmem is true (and reject_file_backed is false), a > NULL mapping is sufficient to prove the folio is not secretmem, so the > fast path can proceed. Hm, what if secretmem folio just got truncated? I hate to rely on some handling in the caller to detect truncation differently during GUP-fast, but this function returning "true". Zi is working on a way to distinguish folios from non-folio things: that we can identify whatever was added through vm_insert_page(). Because that's really the key problem here: vm_insert_page() pages are not actually folios, they just look like a folio today, but looking at fields like ->mapping does not make any sense. > > Tested-by: Sourab Gupta > Fixes: f002882ca369 ("mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()") > Signed-off-by: John Hubbard > --- > mm/gup.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 8e7dc2c6ee73..3ea661e67eea 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2784,12 +2784,17 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags) > mapping = READ_ONCE(folio->mapping); > > /* > - * The mapping may have been truncated, in any case we cannot determine > - * if this mapping is safe - fall back to slow path to determine how to > - * proceed. > + * If the mapping is NULL (truncated, or never set), we cannot > + * determine whether the folio is file-backed, so a long-term writable > + * pin must fall back to the slow path. > + * > + * Otherwise, a NULL mapping proves this is not a secretmem folio > + * (secretmem folios always have a valid mapping to the secretmem > + * inode's address_space), so in that case, we can continue with the > + * fast path. > */ > if (!mapping) > - return false; > + return !reject_file_backed; > > /* Anonymous folios pose no problem. */ > mapping_flags = (unsigned long)mapping & FOLIO_MAPPING_FLAGS; > > base-commit: 7f87a5ea75f011d2c9bc8ac0167e5e2d1adb1594 -- Cheers, David