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 522A539023C for ; Thu, 9 Apr 2026 15:11:35 +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=1775747495; cv=none; b=oq9BSKziOpfxiaQspf/CHQDP/g2IbZWEHL97oqgl6pBWOsbnVraWVbO/8oANdYp8WqxUaiZgMiPKqUiEmIrj92tg8RHVB1e/cTAs2PBAg3S9Ivmlm4AWOd0zXCWYIneO6i1Ef+k1Pa9YEmRongxxDiJHWLmobfYnBKBmoo5BF8s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775747495; c=relaxed/simple; bh=g2ADslYAl1gkG6B2ryk7AAj+udO2hcR5FE/jpdgcyXE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=b0C0HqIkmVE+fzDGw1H/JudL4O2DyDHDJ288eEPk92XrtO0NAV6xLLnrdLlBMmQIWEGzETjJWq5MiZryxA0BNgo+NWqusuEMmH+Cmr2Xq+Sqkx3xIBvLbq/tK7KOBYM+28MVpqrnC5aEJJZbrOWvTr/PMREF6G/z5D2vvFNVW40= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VlUcElYu; 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="VlUcElYu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 86E9BC4CEF7; Thu, 9 Apr 2026 15:11:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775747494; bh=g2ADslYAl1gkG6B2ryk7AAj+udO2hcR5FE/jpdgcyXE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=VlUcElYub9DBYJ7OhfBzT83mIYtFZk+b7GV8Yc2OxMb55U279PAMlUoIYKbc+lP9k IcL9R3S0tfkvClz9l31/nYbK7bZIuLUQeXG0tqu7GlemTGalRrhbUGz9B8Ghepg597 WHxO0dyzH2GABw8ZivJkcL/syeN8nIodknFT75lBFfJ20KomrrusjUMuxq5KNYk6+K 7vEpPVFEQ6+A14Jnnx3JW2tmOVFSpJ6u03nU46hTKoocNWGkAOCtSR7MV+QiaGYid/ PO2iY+AhpUY4s7odgDEREOI0vyJFSqeQRLjJUmuCdFq2bWfHRVSo6Lh6twSeohqTyT ipl4sc6yO2Zpg== Message-ID: Date: Thu, 9 Apr 2026 17:11:30 +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: Zi Yan Cc: John Hubbard , Andrew Morton , Jason Gunthorpe , Peter Xu , Mike Rapoport , LKML , linux-mm@kvack.org, Sourab Gupta , "Matthew Wilcox (Oracle)" References: <20260409014647.397515-1-jhubbard@nvidia.com> <4d6c1af0-dee5-4f1b-b74c-2ebc23de7baf@kernel.org> 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 17:05, Zi Yan wrote: > On 9 Apr 2026, at 3:52, David Hildenbrand (Arm) wrote: > >> 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(). > > The idea is to give every allocated page a NotRmappable page_type and > 1) let page_rmappable_folio() remove the page_type by setting mapcount > to 0, since page_type overlaps with mapcount; > 2) allow callers of alloc_page*() to change page_type from NotRmappable > to other types. Could we instead just set the special type in vm_insert_page(), after verifying that it does not have any other type? > > After these, rmappable folios with any order is a folio without any > page_type. HugeTLB is an exception, but rmap code has special handling > for them, so it should be fine. > > I have a draft[1], but have not had time to refresh it yet. I am planning > to work on this after my two other jobs (make READ_ONLY_THP_FOR_FS enabled > for all FS with large folio support and ensure ->private is 0 for tail pages). > > [1] https://github.com/x-y-z/linux-dev/tree/remove_mapcount_for_non_folio-2026-02-16-23-27 > >> >> 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. > > Based on my understanding of some callers of alloc_page()/get_free_pages(), > vm_insert_page(), they use GFP_COMP to get a compound page and vm_insert_page() > individual pages of the compound page. These pages might be pinned. > During past THP cabal meeting, we discussed about it if > 1. we should disallow such uses and only allow order-0 allocations, or Right. > 2. we need a new type like new_type_folio for them and the type does not > have folio fields except ->pincount. Right. -- Cheers, David