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 5AB2E38BF9C for ; Thu, 9 Apr 2026 10:10:53 +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=1775729455; cv=none; b=MH03qmNqXKpHDNuvjniGqsYvK+hEpQFea58kq2eLVMA+Nh5v7pzRjyy+iYgyM7v/A2Ah4Ya/ljlrkccETPMrI0tBtvpun3Juyb6eyZgakHcDBojo2PWbAXMk0cijMWfCtAhWGSyOp5+GHDYQgHxDrSgcebDLZtSoWkAC0qrHld4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775729455; c=relaxed/simple; bh=xIGIsShfGZskdnuFPNvuPLkTAert80RGZC2aB1q7CjI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TAfjVF2joWhuV1dce/cOKBgD19bGugiKhaMqQB+bJGaxWJ3c86yM7JCcWmSqM0dURW01eWvBxowvNvf6RFPOySfS1kx+X1O/JnSMO/oj+gpvRwUkQQHHk+qHKz2lnvHGLKtY1oO1Jm1oZiIgi1YuFQl+XqwpWphyqCPoU/8VzEc= 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; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=DoXJefwm; 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="DoXJefwm" 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 73DAE3297; Thu, 9 Apr 2026 03:10:46 -0700 (PDT) Received: from [10.164.148.43] (unknown [10.164.148.43]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5A9393F632; Thu, 9 Apr 2026 03:10:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1775729452; bh=xIGIsShfGZskdnuFPNvuPLkTAert80RGZC2aB1q7CjI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=DoXJefwmEF4Xv70LZdkYk6MtFwB06HynMMQiS9dxHAGyDBhDs6tIQxU6JqGlaxxT1 5l0KuSbVXUsmv7dZFj7pBdLci58Ix28p+xTpPcqqgCgREtAbIg718vw3jXbj/yM9py 6vibhorVUqWAuDsmXYGl7v3TKTbr8WA+aB2BJdKI= Message-ID: <46fbd241-4d64-409a-b9dc-77e778ca088e@arm.com> Date: Thu, 9 Apr 2026 15:40:38 +0530 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: [RFC PATCH 5/8] mm/vmalloc: map contiguous pages in batches for vmap() if possible To: Barry Song Cc: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, urezki@gmail.com, linux-kernel@vger.kernel.org, anshuman.khandual@arm.com, ryan.roberts@arm.com, ajd@linux.ibm.com, rppt@kernel.org, david@kernel.org, Xueyuan.chen21@gmail.com References: <20260408025115.27368-1-baohua@kernel.org> <20260408025115.27368-6-baohua@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 09/04/26 3:24 am, Barry Song wrote: > On Wed, Apr 8, 2026 at 10:03 PM Dev Jain wrote: >> >> >> >> On 08/04/26 8:21 am, Barry Song (Xiaomi) wrote: >>> In many cases, the pages passed to vmap() may include high-order >>> pages allocated with __GFP_COMP flags. For example, the systemheap >>> often allocates pages in descending order: order 8, then 4, then 0. >>> Currently, vmap() iterates over every page individually—even pages >>> inside a high-order block are handled one by one. >>> >>> This patch detects high-order pages and maps them as a single >>> contiguous block whenever possible. >>> >>> An alternative would be to implement a new API, vmap_sg(), but that >>> change seems to be large in scope. >>> >>> Signed-off-by: Barry Song (Xiaomi) >>> --- >>> mm/vmalloc.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++-- >>> 1 file changed, 49 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>> index eba436386929..e8dbfada42bc 100644 >>> --- a/mm/vmalloc.c >>> +++ b/mm/vmalloc.c >>> @@ -3529,6 +3529,53 @@ void vunmap(const void *addr) >>> } >>> EXPORT_SYMBOL(vunmap); >>> >>> +static inline int get_vmap_batch_order(struct page **pages, >>> + unsigned int max_steps, unsigned int idx) >>> +{ >>> + unsigned int nr_pages; >>> + >>> + if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP) || >>> + ioremap_max_page_shift == PAGE_SHIFT) >>> + return 0; >>> + >>> + nr_pages = compound_nr(pages[idx]); >>> + if (nr_pages == 1 || max_steps < nr_pages) >>> + return 0; >> >> This assumes that the page array passed to vmap() will have compound pages >> if it is a higher order allocation. >> >> See rb_alloc_aux_page(). It gets higher-order allocations without passing >> GFP_COMP. >> >> That is why my implementation does not assume anything about the property >> of the pages. > > If you’re asking about support for non-compound pages, I think > that’s fine. My current use case is dma-buf, where pages are > compound. I recall discussing this previously with David and > Uladzislau. > > If you’re working with non-compound pages, I’m happy to add > support in the next version. I’m also happy to reuse some of your > code and credit you as Co-developed-by if you’re willing. I actually > prefer your __vmap_huge() name over my > vmap_contig_pages_range(). > > Does that make sense to you? Yeah it will perhaps be better to have a fast-path detecting compound pages, and if not then checking contiguity. So sure please go ahead sharing some of my code and you can co-credit me. > >> >> Also it may be useful to do regression-testing for the common case of >> vmap() with a single page (assuming it is common, I don't know), in >> which case we may have to special case it. > > I agree, so I had Xueyuan test single pages and highlighted this > in the cover letter. There is no regression: "vmap() is 5.6× > faster when memory includes some order-8 pages, with no > regression observed for order-0 pages." > >> >> My implementation requires opting in with VM_ALLOW_HUGE_VMAP - I suspect >> you may run into problems if you make vmap() do huge-mappings as best-effort >> by default. I am guessing this because ... >> >> Drivers can operate on individual pages, so vmalloc() calls split_page() >> and then does the block/cont mappings. This same issue should be present >> with vmap() too? In which case if we are to do huge-mappings by default >> then we can do split_page() after detecting contiguous chunks. >> >> But ... that may create problems for the caller of vmap() - vmap now >> has the changed the properties of the pages. > > I don’t see this as a problem at all. Splitting pages does not > affect physical or virtual contiguity; it only changes the > contents of struct page objects, not the PTE/PMD mappings. > For ioremap, there isn’t even a struct page, yet the mappings > can still be huge. Okay so I was under the impression that *not* splitting the page will be problematic. But, vmalloc splits pages because the caller can operate on individual struct pages by vmalloc_to_page(). To the contrary, since the caller of vmap() decides what kind of pages to virtually-map, we don't have the problem I was raising. So I guess we are fine by making vmap do huge-mappings by default. > > Thanks > Barry