From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (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 C22201F0991 for ; Mon, 1 Jun 2026 09:43:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780306993; cv=none; b=Oa6wJ3a9VEKspOWGR9v63ubEwkYIZD6ZUq0lJ0E4+20UeEyclNTIX4Mufci9It0QEKihtVh7J2zFNEn1PtnIrSfIgT7jIWKfZcTafhQPd/zg1kCFHpfVaRRGCRHZfDqlexhsDg7Sly6w/5lOkDFECUAR803UhtVyBznrvnktnTw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780306993; c=relaxed/simple; bh=vB9HAqr1io+JCVuMDocmxtnzHNpKH/qbAlYpybbwzfg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ItDlOfXWEjSDem0ZMG07bhCxxhSqzEcYw9bIxHcME3wuIgxOQCDUE7Rx0nRWEFfgerI+XwlF07sdDEDUphGbSCQRQX0Dbk+eNRyB8ehBuATaGPEQmbJNz7NHwQr5rwFRr7XrwYUdaRJpv1U5s02hrBnY/6CROZutpHLZLrwOakU= 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=p86NKhR5; arc=none smtp.client-ip=91.218.175.180 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="p86NKhR5" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780306989; 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=gZFkDlb0M/DK/+4npGDmhkRBwuIrbB14olid11Coyvg=; b=p86NKhR5i/z2Uh1CGWtYdPqyJsB+l0ogupMPW57FzLrGY97HrT45jbXoDRvfhPQV6Ko/kB 2hro5IL7lC1A5P78SDTfsqD7kOeIkD0+PPOkCGMw6SlHX/T7JIK1pyusIGtQ8D3kehnye3 M7VgyTjtAKFZd3DPmFiYvQRNTNJOE8E= Date: Mon, 1 Jun 2026 10:43:03 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v6 2/2] mm: use mapping_max_folio_order() for force_thp_readahead order To: Jan Kara Cc: Pedro Falcato , willy@infradead.org, Andrew Morton , david@kernel.org, ryan.roberts@arm.com, linux-mm@kvack.org, r@hev.cc, Andrew Donnellan , apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, brauner@kernel.org, catalin.marinas@arm.com, dev.jain@arm.com, kees@kernel.org, kevin.brodsky@arm.com, lance.yang@linux.dev, "Liam R. Howlett" , linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, npache@redhat.com, pasha.tatashin@soleen.com, rmclure@linux.ibm.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org, Al Viro , wilts.infradead.org@pedro-suse.lan, ziy@nvidia.com, hannes@cmpxchg.org, kas@kernel.org, shakeel.butt@linux.dev, kernel-team@meta.com References: <20260528165635.2068012-1-usama.arif@linux.dev> <20260528165635.2068012-3-usama.arif@linux.dev> <185f1caf-b33d-4467-beb5-51bd8520ac78@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 30/05/2026 16:16, Jan Kara wrote: > On Fri 29-05-26 15:11:54, Usama Arif wrote: >> On 29/05/2026 14:40, Pedro Falcato wrote: >>> On Fri, May 29, 2026 at 01:19:03PM +0100, Usama Arif wrote: >>>> >>>> which means mapping_max_folio_order(mapping) <= MAX_PAGECACHE_ORDER <= HPAGE_PMD_ORDER is always >>>> true, and you dont need the min3(..) in your diff. >>>> >>>> Now the question is if then why not just do: >>>> >>>> if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) { >>>> if (mapping_large_folio_support(mapping)) { >>>> force_thp_readahead = true; >>>> thp_order = min_t(unsigned int, >>>> mapping_max_folio_order(mapping), >>>> get_order(SZ_2M)); >>>> } >>>> } >>>> >>>> >>>> This is because this will regress the 16K ARM case where we already got 32M >>>> folios. Someone might upgrade the kernel and start getting 2M folios now. >>> >>> So maybe limit to 32MB? It's still arbitrary but at least you get simpler >>> logic. If the architecture does not support 32MiB folios, it will clamp >>> the maximum folio order to HPAGE_PMD_ORDER, and you get the same result. >>> >>> Does this sound correct? >>> >> >> Yes, so if we replace it with SZ_32M, it sounds correct. I just think >> the 32M size is too large. But as you pointed out, even 2M can be too large... > > So AFAIU the practical discussion is about two options: > > 1) limiting at 2MB with a slighly more complicated logic to keep mapping at > PMD order for 16k pagesize on ARM but use 2MB pages for 64k pagesize on ARM > > or > > 2) limit at 32MB with simple logic which results in larger (32MB) folios > with 16k and 64k pagesize on ARM and thus larger memory overhead. > > I'd like to maybe offer option 3): limit at 2MB with simple logic. This > will reduce folio size on 16k pagesize ARM compared to 1) but do we really > care? I.e., is there big enough practical performance impact with conpte > and other tricks ARM is playing? > I think the logic isn't that complicated for 1, but I am happy with option 3. >>> Bottom line is that changing things will always affect someone :) Particularly >>> since the logic we have is not too careful at deciding what should or should >>> not be a THP (both in anon and file cases). And if (once?) we make it smarter, >>> it will surely also regress someone! >> >> Yes completely agree on this as well. >> >> So personally I do have a preference of keeping the cap at 2M atleast initially >> while we currently try and solve the issues we see with 2M alone. As we are already >> seeing reports of thrashing and compaction with just 2M, I dont think the logic >> in this patch with just an if else is that complicated. >> >> Matthew, Jan, do you have any thoughts or strong preferences on cap size? > > Frankly, no strong opinion. I'd think 3) is worth trying for its simplicity > and seeing whether somebody complains, otherwise I can live with both 1) > and 2). Thanks! Yes, let me send this. Andrew I will send this as a new and hopefully last revision as I am not sure if you would like another fixup on top of this series! Thanks! > > Honza