From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 9C4C419C54B for ; Thu, 11 Sep 2025 03:05:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757559927; cv=none; b=PaN6XaEbj7VewHFhaMf+dUtkWREzJ4TbDN6RmVj+UgTIAtHAXZQGAQSb4sLwoQii8aA1tA2jVaLOxYI6c5yApDwR1BCjaqM8hB1ZAxvQFrE6qLG4av+/qlkPayiszQz1wtYszxrDFMK7e0FoxD6qr3017cFlcpYJB1qX2dCwe1A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757559927; c=relaxed/simple; bh=HMnumFLvNGXbXkmzpFc98aXEhY6M+nuGVu7h1AVxYjE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Gmr5Qzp0xYGUVCXvgDe7KvU/yU6vRE5BK0KURWyACMjjkdchfjpOgu8jO7+vnSp2SbeXrCyRP6hbRAkCJafGDd/DK9h0ikKci/fYQBK+wRHvhqAamZVjC9pHnHoUGRw/YEh3o+uIw9MeRydgjKDo96fv7KlRugwiak1LoEJ2FY0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=mq1KZfJy; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="mq1KZfJy" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1757559913; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9T9VUgZB4FkXCXqmZCRFSIgZN/NB+DPn0gz88Z9lruY=; b=mq1KZfJySR1GsrdLS/vyU1/YzvGVT859oA5mxcagNE4baLrGHdnZHnTOwg/0IGaV075piA RATnyx24do8iwRkHvAEVXabMu0YLLSWSz2uR6KaKXiP+rxVO8WxCS8kFcxHvsdzsaWlaCD 1vz/9+SZQcqDilMFaMIE2QeNQD6fpLM= Date: Thu, 11 Sep 2025 11:04:58 +0800 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v7 mm-new 02/10] mm: thp: add support for BPF based THP order selection Content-Language: en-US To: Yafang Shao Cc: akpm@linux-foundation.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, hannes@cmpxchg.org, usamaarif642@gmail.com, gutierrez.asier@huawei-partners.com, willy@infradead.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, ameryhung@gmail.com, rientjes@google.com, corbet@lwn.net, 21cnbao@gmail.com, shakeel.butt@linux.dev, bpf@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org References: <20250910024447.64788-1-laoar.shao@gmail.com> <20250910024447.64788-3-laoar.shao@gmail.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/9/11 10:48, Yafang Shao wrote: > On Wed, Sep 10, 2025 at 9:57 PM Lance Yang wrote: [...] >>>>> +/** >>>>> + * @thp_order_fn_t: Get the suggested THP orders from a BPF program for allocation >>>>> + * @vma: vm_area_struct associated with the THP allocation >>>>> + * @vma_type: The VMA type, such as BPF_THP_VM_HUGEPAGE if VM_HUGEPAGE is set >>>>> + * BPF_THP_VM_NOHUGEPAGE if VM_NOHUGEPAGE is set, or BPF_THP_VM_NONE if >>>>> + * neither is set. >>>>> + * @tva_type: TVA type for current @vma >>>>> + * @orders: Bitmask of requested THP orders for this allocation >>>>> + * - PMD-mapped allocation if PMD_ORDER is set >>>>> + * - mTHP allocation otherwise >>>>> + * >>>>> + * Return: The suggested THP order from the BPF program for allocation. It will >>>>> + * not exceed the highest requested order in @orders. Return -1 to >>>>> + * indicate that the original requested @orders should remain unchanged. >>>> >>>> A minor documentation nit: the comment says "Return -1 to indicate that the >>>> original requested @orders should remain unchanged". It might be slightly >>>> clearer to say "Return a negative value to fall back to the original >>>> behavior". This would cover all error codes as well ;) > > will change it. Please feel free to change it ;) > >>>> [...] >>>> >>>> Also, for future extensions, it might be a good idea to add a reserved >>>> flags argument to the thp_order_fn_t signature. >>>> >>>> For example thp_order_fn_t(..., unsigned long flags). >>>> >>>> This would give us aforward-compatible way to add new semantics later >>>> without breaking the ABI and needing a v2. We could just require it to be >>>> 0 for now. > > That makes sense. However, as Lorenzo mentioned previously, we should > keep the interface as minimal as possible. Got it. [...] >> Forgot to add: >> >> Noticed that if the hook returns 0, bpf_hook_thp_get_orders() falls >> back to 'orders', preventing us from dynamically disabling mTHP >> allocations. > > Could you please clarify what you mean by that? > > + thp_order = bpf_hook_thp_get_order(vma, vma_type, tva_type, orders); > + if (thp_order < 0) > + goto out; > > In my implementation, it only falls back to @orders if the return > value is negative. If the return value is 0, it uses BIT(0): My bad, I completely misread the code last night ... I see now that returning 0 forces a base page (order-0) > > + if (thp_order <= highest_order(orders)) > + thp_orders = BIT(thp_order); Yes, this is exactly the behavior we need. It will allow us to dynamically disable mTHP for low-priority containers when we need to, which is perfect for our use case! > >> >> Honoring a return of 0 is critical for our use case, which is to >> dynamically disable mTHP for low-priority containers when memory gets >> low in mixed workloads. >> >> And then re-enable it for them when memory is back above the low >> watermark. > > Thank you for detailing your use case; that context is very helpful. Cheers, Lance