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a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1787321909; b=fCCxtkTl0gOsxgwQFrq8QAn9nXFxyA8IyMeXmMKQyuc5eFGz1eQdO+WOKSCQa96PwNM7dH NQuB77jRXKCjLV16MGrwBVeVabjbdsVt04PMwm8kIh0r/mFEllX0+8FXCHj7DOPGzKkIyG ku6w3/f9HpuXz7MoNS9pqTQT8hiz4Dk= ARC-Authentication-Results: i=1; imf07.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=L7kjw09V; spf=pass (imf07.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 6EE856001D; Fri, 21 Aug 2026 14:18:28 +0000 (UTC) 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 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; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260820190825.221308-1-ferran.duarri@me.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: uyzwn47p9oq6td85si6r97ffhrnuqufb X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 12C7740006 X-HE-Tag: 1787321908-838124 X-HE-Meta: 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 HV1N4yAk Y5PalsMAKJt7cGIZISXQasmlfe4lNyYV+CdYbz1EbNASWv5p7m4SxGvdJ3tmivuBvktDn0C6lXl0yIN2T8pr90h6RSxpc0v4AU4ty3J9/Kba7QIOb/q5D71mpgwvTTXomikO5UCqik6B8UABUvQsk2KZKv+SWyqBi7n3qVzjh4JhG7qYR9S9rXsVUZVgsfoN7G1uceuEfMDYAz4ipuY95UEaCPdl3wY15CaHHIFFIIcCSiqhVRPEOgw8lk5lpkk6uZoUVpuVsRaDro79624u1+fpcJA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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