From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 823A73E8688; Tue, 7 Jul 2026 14:58:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783436326; cv=none; b=a61cDLOuCfH508pxsiDhzQgVwovIwmdk2GPmcphCArxCFt9USSPoCZrMfEBQufJI4SMr64fU3IjYkOSMhyQWmS5VMaCzYogRyps6qq9n1nwfaANq/fBotZtHSbDIvcL2yej2BlC0shcOblIhKb6bILrQhcFdMKqWC+sJAoDRBAI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783436326; c=relaxed/simple; bh=PlMxsGARKkgfcMvVXTnKC8/T3l8zRC4VArlMr7lNFQw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OW6Hofc0i52a1wNgU0ZnUY360Jd7/XgC0MZUCS6q6+lf1ZerTdTl3STt+HOnXNzn/0snKxPjrkMz6r94w8tih/UWt3U/q4tSz3a9bYExlX5u1Spmn/5B/Q4VmIbeoAmCu7pDAnBHOjBshiSIj3qCteTBelkIWwsMOqA/BVgysbU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kfDIla2s; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kfDIla2s" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8F4C71F00A3A; Tue, 7 Jul 2026 14:58:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783436323; bh=ygpMICMoTnkpL1/1oxwmS7t/GpkK91ripgRv9mbioew=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kfDIla2sbEsBl3KxEfELr0718lUqk8sYVGFqtQmOcUJdRMzBbBOAovYXorXdGZoFO g7NineCVlSviBjqaUlWQ0OfCKobp0+HEbPnZ0z9/f+d/xWFZugdwHunU4/0lIeRaY3 sOyMTlWG1wwWFtHZj7kz7BhgrEHYHRYOXM0c85aMjDdD2ptPSgvD4HunFJ8k2D4L7l Yw9OIZ726jyBAATK0QzS/KoTfDfA4cJpVxXKrmb8sK2bTKtoH1wj/Q5De0WOeEWsll opt2e8PlkYvAzCPH2R1YNEpxY6qNj2GeOt5sfy+JcjJiOx4tQV0H/3+uu2lA5IkYG/ +7EP7wFlVczLw== Message-ID: <27ebe8f0-78b6-402a-a2e7-4e807251d20a@kernel.org> Date: Tue, 7 Jul 2026 16:58:39 +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] KVM: SEV: drop FOLL_LONGTERM for encrypted region registration To: "Gupta, Pankaj" , seanjc@google.com, pbonzini@redhat.com, tglx@kernel.org, mingo@redhat.com, dave.hansen@linux.intel.com Cc: bp@alien8.de, x86@kernel.org, thomas.lendacky@amd.com, hpa@zytor.com, yangge1116@126.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, "Lorenzo Stoakes (Oracle)" References: <20260701144543.39582-1-pankaj.gupta@amd.com> <1cc159b9-5f94-4524-8e03-efe91601ccfc@kernel.org> <46f19bd8-0d43-4b0e-a8ab-0ef9d3b8bd1a@kernel.org> <2bd89e95-9c15-4a3a-916d-0d71a92d8b02@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 7/7/26 15:45, Gupta, Pankaj wrote: > >>> Hi David, >>> >>> Yes, it fails in this path but for file backed mapping, vma_is_fsdax() returns >>> false because >>> >>> vma_is_dax() returns false: >> Ah, okay, so fsdax is not involved and we really only fail because of the >> writable_file_mapping_allowed() check. >> >> I was for a second thinking in terms of nested virt :) >> >>> Host side backend is regular file backed memory (no fsdax). >> Okay, so we'll end up mapping an ordinary file into VM memory, and expose that >> to the VM as part of virtio-pmem device. >> >> That also means that vfio etc. won't be able to longterm-pin such device memory. >> So this is not a problem isolated to SEV. >> >> Forbidding to longterm pin is actually the right thing to do if the filesystem >> relies on writenotify, as spelled out by Lorenzo's commit: >> >> " >>      Writing to file-backed mappings which require folio dirty tracking using >>      GUP is a fundamentally broken operation, as kernel write access to GUP >>      mappings do not adhere to the semantics expected by a file system. >> >>      A GUP caller uses the direct mapping to access the folio, which does not >>      cause write notify to trigger, nor does it enforce that the caller marks >>      the folio dirty. >> >>      The problem arises when, after an initial write to the folio, writeback >>      results in the folio being cleaned and then the caller, via the GUP >>      interface, writes to the folio again. >> " >> >> Hmmm > > Yes. For file based mapping we don't allow long term pinning. > > If we take into account the fragmentation concerns for MIGRATE_CMA and > ZONE_MOVABLE allocations > > solvable with FOLL_LONGTERM, I can think of two options(tested) to allow file > based mappings as well: > > 1. Fallback on FOLL_WRITE when FOLL_LONGTERM fails as suggested by Sean. That is just not acceptable, as it breaks random other stuff (MIGRATE_CMA, as one example) besides the file-pinning problems that Lorenzo added. If we're going to hack something in, then that we bypass the file writeback check. Not that we don't use FOLL_LONGTERM. I'd hate to use a GUP flag to indicate "this is a legacy hack", but it clearly isolates the issue (needs a better name obviously): diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index ae9bca4eda5ca..e2c531f914d44 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1912,6 +1912,9 @@ enum { */ FOLL_HONOR_NUMA_FAULT = 1 << 12, + /* TODO */ + FOLL_LONGTERM = 1 << 13, + /* See also internal only FOLL flags in mm/internal.h */ }; diff --git a/mm/gup.c b/mm/gup.c index 0692119b79043..1fa0aa0cdc99d 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1186,8 +1186,8 @@ static bool writable_file_mapping_allowed(struct vm_area_struct *vma, * If we aren't pinning then no problematic write can occur. A long term * pin is the most egregious case so this is the case we disallow. */ - if ((gup_flags & (FOLL_PIN | FOLL_LONGTERM)) != - (FOLL_PIN | FOLL_LONGTERM)) + if ((gup_flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_LONGTERM_HACK)) != + (FOLL_PIN | FOLL_LONGTERM | FOLL_LONGTERM_HACK)) return true; /* @@ -2746,7 +2746,7 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags) * If we aren't pinning then no problematic write can occur. A long term * pin is the most egregious case so this is the one we disallow. */ - if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) == + if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE | FOLL_LONGTERM_HACK)) == (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) reject_file_backed = true; @@ -3180,7 +3180,7 @@ static int gup_fast_fallback(unsigned long start, unsigned long nr_pages, int locked = 0; int ret; - if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM | + if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM | FOLL_LONGTERM_HACK | FOLL_FORCE | FOLL_PIN | FOLL_GET | FOLL_FAST_ONLY | FOLL_NOFAULT | FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT))) -- Cheers, David