From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 5645B2C856; Tue, 20 Feb 2024 16:36:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708446973; cv=none; b=rtK9Tr2YP++XtSFpm1MRcKvW4KjUxBTKX8vj2XYF0DoMuConCW6kncQBU80nhoRG71o4L6iOeBnQxYGHm2oGSGiCHb+A8IW8qbRYxC2LMDAI7HskxTvx5qFv/NEaor/KIEYG5+IrEVi2SprKj7kF7wsQ6V095B55txbOeuFyJzc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708446973; c=relaxed/simple; bh=MfRrDubXEVjaGcbYyk2dr+f/x8bO34rZrjgFdcLHTmg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Uuy0NPkbHX9cDZlYoiL07sGB+2+OyL7ZBqmmtinuvy4AiX2ZNIjZGShDoH6vaDpZbVaMdWY/pPqmmCVsC7ihEGVli31ikHZIHB/mgYFiKGiYddKtVRQg6IeL4LUplQsZrM1pcoj5MC3sBtp3RDgAdBWdMw0zPhLJfCW3vLSNPYI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4F848FEC; Tue, 20 Feb 2024 08:36:49 -0800 (PST) Received: from raptor (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 20E3A3F762; Tue, 20 Feb 2024 08:36:06 -0800 (PST) Date: Tue, 20 Feb 2024 16:36:03 +0000 From: Alexandru Elisei To: David Hildenbrand Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev, maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, pcc@google.com, steven.price@arm.com, anshuman.khandual@arm.com, eugenis@google.com, kcc@google.com, hyesoo.yu@samsung.com, rppt@kernel.org, akpm@linux-foundation.org, peterz@infradead.org, konrad.wilk@oracle.com, willy@infradead.org, jgross@suse.com, hch@lst.de, geert@linux-m68k.org, vitaly.wool@konsulko.com, ddstreet@ieee.org, sjenning@redhat.com, hughd@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, alexandru.elisei@arm.com Subject: Re: arm64 MTE tag storage reuse - alternatives to MIGRATE_CMA Message-ID: References: <70d77490-9036-48ac-afc9-4b976433070d@redhat.com> Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Hi, On Tue, Feb 20, 2024 at 05:16:26PM +0100, David Hildenbrand wrote: > > > > > > I believe this is a very good fit for tag storage reuse, because it allows > > > > > > tag storage to be allocated even in atomic contexts, which enables MTE in > > > > > > the kernel. As a bonus, all of the changes to MM from the current approach > > > > > > wouldn't be needed, as tag storage allocation can be handled entirely in > > > > > > set_ptes_at(), copy_*highpage() or arch_swap_restore(). > > > > > > > > > > > > Is this a viable approach that would be upstreamable? Are there other > > > > > > solutions that I haven't considered? I'm very much open to any alternatives > > > > > > that would make tag storage reuse viable. > > > > > > > > > > As raised recently, I had similar ideas with something like virtio-mem in > > > > > the past (wanted to call it virtio-tmem back then), but didn't have time to > > > > > look into it yet. > > > > > > > > > > I considered both, using special device memory as "cleancache" backend, and > > > > > using it as backend storage for something similar to zswap. We would not > > > > > need a memmap/"struct page" for that special device memory, which reduces > > > > > memory overhead and makes "adding more memory" a more reliable operation. > > > > > > > > Hm... this might not work with tag storage memory, the kernel needs to > > > > perform cache maintenance on the memory when it transitions to and from > > > > storing tags and storing data, so the memory must be mapped by the kernel. > > > > > > The direct map will definitely be required I think (copy in/out data). But > > > memmap for tag memory will likely not be required. Of course, it depends how > > > to manage tag storage. Likely we have to store some metadata, hopefully we > > > can avoid the full memmap and just use something else. > > > > So I guess instead of ZONE_DEVICE I should try to use arch_add_memory() > > directly? That has the limitation that it cannot be used by a driver > > (symbol not exported to modules). > You can certainly start with something simple, and we can work on removing > that memmap allocation later. > > Maybe we have to expose new primitives in the context of such drivers. > arch_add_memory() likely also doesn't do what you need. > > I recall that we had a way of only messing with the direct map. > > Last time I worked with that was in the context of memtrace > (arch/powerpc/platforms/powernv/memtrace.c) > > There, we call arch_create_linear_mapping()/arch_remove_linear_mapping(). > > ... and now my memory comes back: we never finished factoring out > arch_create_linear_mapping/arch_remove_linear_mapping so they would be > available on all architectures. > > > Your driver will be very arm64 specific, so doing it in an arm64-special way > might be good enough initially. For example, the arm64-core could detect > that special memory region and just statically prepare the direct map and > not expose the memory to the buddy/allocate a memmap. Similar to how we > handle the crashkernel/kexec IIRC (we likely do not have a direct map for > that, though; ). > > [I was also wondering if we could simply dynamically map/unmap when required > so you can just avoid creating the entire direct map; might bot be the best > approach performance-wise, though] > > There are a bunch of details to be sorted out, but I don't consider the > directmap/memmap side of things a big problem. Sounds reasonable, thank you for the feedback! Thanks, Alex > > -- > Cheers, > > David / dhildenb >