From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793627941; b=olafHkxJMgAG3Mq+J2OvTV+4t+iJT1IHAjbn7oYFfx1ZUFFhIWHqlq/tFU6HlqO1kv6b O3O6hxnrDcDwMrobImg/zXa5sJz2JljsIfCmCEgw2uFbX8vPVzefxZGTGYc/Z8UmkXYFT kiRJRy1R/YX3kst7bnhafOu/2mUxjYX0wT98vSEGkdtKwpUAg/fnLE0s00UobLxe1OUce 4sMlHuDoybo2DB9MXAZgXHJ+qV2QiT0XZPGVthceasjYCE9PujqteobCh5fUQMDxRVghM B4n2LlbS/ZNUFe5Mzla76ygwSlMtRY7r1sqZ0st7ehiQgPRp8Sd7h0B1ezXjXt87+iA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793627941; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=z636TFZ8J/efy6KD/Zz0HvIWw5738nMcZMSYjKsuO7c=; b=0BT8sldzWT6BFN0Ogd3s5VnF+xicK5sVGxoYyUB6VzftrKe2oVa0KuDi/2lj3bucMLB9 uV65JECLudNG7SAZcOorx2aWSaj+J9SY716qvy0QLMSqvM8JapSRsP88GH8m9qUQ60U+l c8g5BJeb1VB8K7BaqDpVXmHxTe7vnWfOaXNeeMEpyozH2g8CNNgsM3msl3tNuWtTASW2F vBqPA1UpoD5UV+0uOXoindULVzwwyriVIOHrOX/n8RRSb2Xrem6grsSpMdVBS6/1ntHLR 31BKHyD9i/RWxATD9d1rH4hXuocIzajvBppdGM9sRX39xUcUb49/X1DURJ6IhlbEsWQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=FzcHPsnW; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7C5D3606A32D for ; Mon, 2 Nov 2026 13:59:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=FzcHPsnW Received: by smtp.kernel.org (Postfix) with ESMTPSA id 937F81F00A16; Mon, 2 Nov 2026 13:59:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793627941; bh=z636TFZ8J/efy6KD/Zz0HvIWw5738nMcZMSYjKsuO7c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=FzcHPsnW3q2Ej26Y0zaezRttXCpKyXZgRQDIf+oVdbAVrSZaxa1hA/SK5uxM8Rvm1 GlxTZeqVxaGdnr0YMOrx+aT+lopuJLxihXARc85ukL3YNWVWpR5L0XcmA7mfHXvCZY tR0thba+KGUt3MK+UIAJdOHxdOXhuLGy3yPb0X9eZIb8Lj82rNQAAWX+1xQ+6aiurr jatLzVB4HkltRc2BiHB220Su1xQU+VGmGNrXwa1B11UVDtNPJ5z3fkqPSMCWHAPuOk eOUNc1cJ+V201OX46HZdhjN17mVI15v6lKpWnyLaONRebUkhyD48rIgyeBRYlnCjcF G70TdC+KKv87g== Message-ID: <804d9f27-bfb0-4621-b394-fa88709a90d3@kernel.org> Date: Mon, 2 Nov 2026 14:58:58 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: mm: tmpfs: huge=always Kconfig help understates severe space amplification To: Dmitry Ilvokhin , "Artem S. Tashkinov" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: From: "David Hildenbrand (Arm)" 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/2/26 13:53, Dmitry Ilvokhin wrote: > On Sat, Oct 31, 2026 at 03:19:08PM +0000, Artem S. Tashkinov wrote: >> Hi Dmitry, >> >> I'd like to raise a documentation issue with the Kconfig option introduced >> by your patch: >> >> mm: shmem/tmpfs hugepage defaults config choice >> >> Specifically, CONFIG_TRANSPARENT_HUGEPAGE_TMPFS_HUGE_ALWAYS. >> >> The help text currently says: >> >> Always attempt to allocate hugepage for tmpfs mount, can >> increase the memory footprint of applications without a >> guaranteed benefit but it will work automatically for all >> applications. >> >> I believe this substantially understates the potential consequences of >> selecting this option, particularly on general-purpose systems where /tmp is >> mounted as tmpfs. >> >> I recently encountered the following situation while extracting a Fedora >> filesystem tree under /tmp on an x86-64 system running Linux 7.3. >> >> The tmpfs mount had huge=always enabled. >> >> $ df -hT /tmp/Fedora >> Filesystem Type Size Used Avail Use% >> tmpfs tmpfs 48G 47G 925M 99% >> >> $ du -sh /tmp/Fedora/usr >> 45G /tmp/Fedora/usr >> >> $ du -sh --apparent-size /tmp/Fedora/usr >> 1.4G /tmp/Fedora/usr >> >> Even more strikingly: >> >> $ du -sh /tmp/Fedora/usr/share/man/man3 >> 7.5G /tmp/Fedora/usr/share/man/man3 >> >> $ du -sh --apparent-size /tmp/Fedora/usr/share/man/man3 >> 7.8M /tmp/Fedora/usr/share/man/man3 >> >> Thus, a directory containing just 7.8 MiB of apparent file data occupied >> approximately 7.5 GiB of allocated space. >> >> The overall tree occupied approximately 45 GiB despite containing only 1.4 >> GiB of apparent file data. >> >> This is consistent with severe allocation amplification caused by >> transparent huge pages, although I have not yet established the exact >> breakdown of huge-page allocations and other tmpfs overhead. > > Yeah, I think one can expect up to 512 amplification on x86 in the > absolutely worst case, based on the ratio of pages sizes: > 2 MiB to 4 KiB. > > I suspect the magnitude can be even bigger on aarch64. > > [...] > >> >> For example, the help might say: >> >> Always attempt hugepage allocation for tmpfs mounts. >> >> WARNING: This policy can substantially increase memory and >> tmpfs space consumption, particularly for workloads creating >> large numbers of small files. Allocated space may greatly >> exceed apparent file sizes, potentially exhausting tmpfs >> capacity even when relatively little file data is stored. > > I would probably drop "WARNING", since there nothing to warn about yet. > Also, second sentence communicating the same idea as the first one, but > with different words. > > Probably, something like this will be enough: "This policy can > substantially increase memory and tmpfs space consumption, particularly > for workloads creating large numbers of small files." > >> >> This setting does not guarantee improved performance and >> should only be selected when its memory overhead is >> understood and acceptable for the intended workload. > > This part is already mentioned in the current wording: "increase the > memory footprint of applications without a guaranteed benefit". > > Anyway, I think documentation edits are always welcome, please, submit a > patch :) Agreed. BUT, one interesting detail: On small files, the shmem unused shrinker shmem_unused_huge_shrink() should try reclaiming such THPs that expands beyond EOF to reclaim their memory. Artem, does that work in you scenario when there is actually memory pressure? -- Cheers, David