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 96F9E2EAE6 for ; Tue, 26 Nov 2024 14:26:35 +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=1732631198; cv=none; b=TnPorswsjpRrOabvAWYzM19oxdWBphbHwKfbGj3pU62Km2CaIdupw4Hg7otPVoIIyXeygAno8IwI15zfwetT5IAKMqHzCj0sk7PCcJxjhxyn3EPVzwb18E/5o36BMQlRACIQgHAR3RfQpohk7wJJSv+OppKiBTPL3n51D1yIdew= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732631198; c=relaxed/simple; bh=+ML6lXgDwquaXKIIp4MwMHohMsTX5RT5aqMCa30JQtc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ovjGwdphtlXLWtT6RSLgDW9o0h/vZI55wbQ1t6Dg/s3plNgXhP1taXiZo3XUkVYRI37xXjO6gxFduY08vLZHXAySvsey0gkh7bgULED1rSMetcx0Gw/pyH/WaLIcwJbVcepqfJ48efZPK2HpmvqnM3T41qjGdAD3cgBJKy7S7Ps= 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 9A8891A00; Tue, 26 Nov 2024 06:27:03 -0800 (PST) Received: from [10.1.29.199] (XHFQ2J9959.cambridge.arm.com [10.1.29.199]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A8A543F5A1; Tue, 26 Nov 2024 06:26:32 -0800 (PST) Message-ID: Date: Tue, 26 Nov 2024 14:26:31 +0000 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] mm/slab: Avoid build bug for calls to kmalloc with a large constant Content-Language: en-GB To: Vlastimil Babka , Dave Kleikamp , Andrew Morton Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20241014105514.3206191-1-ryan.roberts@arm.com> <20241014105912.3207374-1-ryan.roberts@arm.com> <20241014105912.3207374-6-ryan.roberts@arm.com> <44312f4a-8b9c-49ce-9277-5873a94ca1bb@oracle.com> <7fb6c5a2-b9ae-4a29-a871-2f0bdc636e41@arm.com> <9675f4f0-6290-43aa-bf17-6b9c2b461485@suse.cz> From: Ryan Roberts In-Reply-To: <9675f4f0-6290-43aa-bf17-6b9c2b461485@suse.cz> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 26/11/2024 12:36, Vlastimil Babka wrote: > On 11/26/24 13:18, Ryan Roberts wrote: >> On 14/11/2024 10:09, Vlastimil Babka wrote: >>> On 11/1/24 21:16, Dave Kleikamp wrote: >>>> When boot-time page size is enabled, the test against KMALLOC_MAX_CACHE_SIZE >>>> is no longer optimized out with a constant size, so a build bug may >>>> occur on a path that won't be reached. >>> >>> That's rather unfortunate, the __builtin_constant_p(size) part of >>> kmalloc_noprof() really expects things to resolve at compile time and it >>> would be better to keep it that way. >>> >>> I think it would be better if we based KMALLOC_MAX_CACHE_SIZE itself on >>> PAGE_SHIFT_MAX and kept it constant, instead of introducing >>> KMALLOC_SHIFT_HIGH_MAX only for some sanity checks. >>> >>> So if the kernel was built to support 4k to 64k, but booted as 4k, it would >>> still create and use kmalloc caches up to 128k. SLUB should handle that fine >>> (if not, please report it :) >> >> So when PAGE_SIZE_MAX=64K and PAGE_SIZE=4K, kmalloc will support up to 128K >> whereas before it only supported up to 8K. I was trying to avoid that since I >> assumed that would be costly in terms of extra memory allocated for those higher >> order buckets that will never be used. But I have no idea how SLUB works in >> practice. Perhaps memory for the cache is only lazily allocated so we won't see >> an issue in practice? > > Yes the e.g. 128k slabs themselves will be lazily allocated. There will be > some overhead with the management structures (struct kmem_cache etc) but > much smaller. > To be completely honest, some extra overhead might come to be when the slabs > are allocated ans later the user frees those allocations. kmalloc_large() > wwould return them immediately, while a regular kmem_cache will keep one or > more per cpu for reuse. But if that becomes a visible problem we can tune > those caches to discard slabs more aggressively. > >> I'm happy to make this change if you're certain it's the right approach; please >> confirm. > > Yes it's much better option than breaking the build-time-constant part of > kmalloc_noprof(). OK, I'll take this approach as you suggest. Thanks, Ryan > >>> >>> Maybe we could also stop adding + 1 to PAGE_SHIFT_MAX if it's >=64k, so the >>> cache size is max 64k and not 128k but that should be probably evaluated >>> separately from this series. >> >> I'm inferring from this that perhaps there is a memory cost with having the >> higher orders defined but unused. > > Yeah as per above, should not be too large and we could tune it down if > necessary. > >> Thanks, >> Ryan >> >>> >>> Vlastimil >>> >>>> Found compiling drivers/net/ethernet/qlogic/qed/qed_sriov.c >>>> >>>> Signed-off-by: Dave Kleikamp >>>> --- >>>> >>>> Ryan, >>>> >>>> Please consider incorporating this fix or something similar into your >>>> mm patch in the boot-time pages size patches. >>>> >>>> include/linux/slab.h | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/include/linux/slab.h b/include/linux/slab.h >>>> index 9848296ca6ba..a4c7507ab8ec 100644 >>>> --- a/include/linux/slab.h >>>> +++ b/include/linux/slab.h >>>> @@ -685,7 +685,8 @@ static __always_inline unsigned int __kmalloc_index(size_t size, >>>> if (size <= 1024 * 1024) return 20; >>>> if (size <= 2 * 1024 * 1024) return 21; >>>> >>>> - if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant) >>>> + if (!IS_ENABLED(CONFIG_ARM64_BOOT_TIME_PAGE_SIZE) && >>>> + !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant) >>>> BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()"); >>>> else >>>> BUG(); >>> >> >