From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 8199249504C for ; Fri, 21 Aug 2026 14:18:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787321909; cv=none; b=HkjYm6WxkHfOhDmK4v0JpdfZQPclelsWK6S13zM0yfUVI1Z29TEfHqjtAP0wUn/VkpdLamqsAB+Un5rZq00WFL5Z4yefQmB925Z/Y2XszFrmnOj6V3dMzy+yAe5pRK2CwypnpOh8Q0rx8JvVEO6lQGA+sqr1lq4fx4YEIBqPkWQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787321909; c=relaxed/simple; bh=Aau50aAYB8JIudTHDqWrTfRubzdz5fr45ItcXWxOXtE=; h=Message-ID:Date:MIME-Version:From:Subject:To:Cc:References: In-Reply-To:Content-Type; b=PwoKABzGMfvxXP2kAP1YaZxcKc5f6ILv/DXaAsReIUM7kSuK74kNR9EqKBTXtqDgE47Hy8vxM+whZBWOZDf3bUPL3W92O4kPH2tantF8X3XLsswPzC+vQwbZIkWqSpDzQRFKg9Z/ZlIqc7P2sYh3VfHx6zoeLkWTEybvSlbx6Wg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=L7kjw09V; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="L7kjw09V" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E95F71F000E9; Fri, 21 Aug 2026 14:18:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787321908; bh=GCzpH+AkCY0aeIXoDOfaDvPOR2Q5CwIwfkAmh6a+U3A=; h=Date:From:Subject:To:Cc:References:In-Reply-To; b=L7kjw09VgZa7fNcZgNUg45LTk4MaCo7uJlNzZyYyGZoptUJTMEMqkd59WinlTzTab a2rH/NDJUMr8KNppsYDuNXhpQXNkP4gns6May87olh1mnEhRPOPhfqo3IhcgR5j8Nd k7IU2HBwGcyJXEzXboUYXxOfQM8zkaca1r6DWZugDFxgcrQ67gwDku/tlB88i/sUpI dHnGyH0TmiLA/Q0uVJZftozjtk62ZbhFOxGSbCHXMYfWEhdLaK3cZMbF4syKc67Bq4 RGoxDkLU60YT9DvWKukOdkwbaqK60n8uSpejwEhJsdTo1AECVidsGwXcz07K5aka5O qkQ+/iESsp6Xg== Message-ID: <06f5eea7-c6e7-49ed-8f45-0c6c86a04c26@kernel.org> Date: Fri, 21 Aug 2026 16:18:23 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: "David Hildenbrand (Arm)" Subject: Re: [PATCH] mm: thp: default defrag mode to defer+madvise To: Ferran Duarri , Andrew Morton , Lorenzo Stoakes Cc: Zi Yan , Baolin Wang , Ryan Roberts , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260820190825.221308-1-ferran.duarri@me.com> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/20/26 21:08, Ferran Duarri wrote: > transparent_hugepage_flags unconditionally sets > TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, so the built-in defrag mode > is always "madvise" regardless of which of CONFIG_TRANSPARENT_HUGEPAGE_ > ALWAYS or _MADVISE was selected. Those Kconfig options choose whether > THP is applied by default; they do not express a preference about how > hard the allocator should work to produce a huge page, yet the defrag > default is pinned as if they did. > > In "madvise" mode a fault on a MADV_HUGEPAGE region performs direct > compaction in the fault path. Paired with transparent_hugepage=madvise > that is a bounded cost, since only regions that asked for it are > affected. Paired with transparent_hugepage=always every anonymous fault > becomes eligible, and under memory pressure the faulting thread can > stall in compaction. > > "defer+madvise" keeps the same set of regions eligible for huge pages > and the same allocation attempt, but on failure it wakes kswapd and > khugepaged to compact in the background instead of doing it inline. The > fault proceeds with small pages and the region is collapsed later. This > is the configuration long recommended to users running THP=always with > large anonymous working sets, and it is a strictly weaker stall > guarantee to make the default. > > This changes only the compiled-in default; > /sys/kernel/mm/transparent_hugepage/defrag continues to accept every > existing mode, including the current "madvise" behaviour. > > No stall measurement is offered with this patch. On the machine that > prompted it the fault path never reaches direct compaction at all: > thp_fault_alloc is 60682 against thp_fault_fallback 0, and compact_stall > is 0, because memory has stayed abundant enough that no huge-page > allocation has had to fall back. That makes the box unable to testify > either way, and a number gathered under those conditions would measure > nothing. The argument above is a correctness one about which knob the > Kconfig choice is entitled to set, and it should be judged on that. > > Signed-off-by: Ferran Duarri > --- > mm/huge_memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index b118bcd392cb..749f669cca56 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -63,7 +63,7 @@ unsigned long transparent_hugepage_flags __read_mostly = > #ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE > (1< #endif > - (1< + (1< (1< (1< Why are such defaults not better changed in the distributions or by the users in their boot system config? Why is there a need for us to change the default in the kernel? -- Cheers, David