From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id C3A28606A32D for ; Mon, 2 Nov 2026 13:59:05 +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 kanga.kvack.org (Postfix) id DEA736B05BC; Mon, 2 Nov 2026 08:59:04 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id DC2546B05BD; Mon, 2 Nov 2026 08:59:04 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CB1246B05BE; Mon, 2 Nov 2026 08:59:04 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 9D3786B05BC for ; Mon, 2 Nov 2026 08:59:04 -0500 (EST) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 450241A00E8 for ; Mon, 2 Nov 2026 13:59:04 +0000 (UTC) X-FDA: 85390335408.17.3F3E854 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf09.hostedemail.com (Postfix) with ESMTP id 59499140008 for ; Mon, 2 Nov 2026 13:59:02 +0000 (UTC) Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=FzcHPsnW; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf09.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1793627942; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=z636TFZ8J/efy6KD/Zz0HvIWw5738nMcZMSYjKsuO7c=; b=LzvNQvQmVveinR55ybTF4Fwhk6u6PKMv27S6bfMpFlNn0Ku752UqYQmJVpDUtqkuzrpZMe kqWKxwOFhPkdxsPWcQeUA6RpbRuh79hIyGd/zpxjrpT8fsPklEQHkOZDKWmIFkNgqHZBP3 fcXdOsaUeJS7t/SdvoPwKPke/zyomjA= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=FzcHPsnW; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf09.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1793627942; b=cpBnQq8pZS7d0xPA2F1XItRrxGUFN2yn4DeF8WO8MYFkb46g0WcNdzYTOzsht3zhmpRvte k0ofytN+NimSEWYhY7q5EOqvKC57kE3H3+O0NjTYIOdD+EKJl/BR3d4jefXz601w5E3Mei GtqdMJSAkHokuRXuqpxq5+rgJMfRwcQ= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 75D4C40A6A; Mon, 2 Nov 2026 13:59:01 +0000 (UTC) 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-Archived-By: 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 X-Rspamd-Queue-Id: 59499140008 X-Stat-Signature: 5ayzwf4z3deh6ne85smn1rm1ahz5ddr1 X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1793627942-624965 X-HE-Meta: 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 C4SIOGQ8 sd+xQg1ngpPjf9Z437RJ7MxaHqiods9qjCX5XpaydQaf7zhEjDh4CCbh9raBsVPp0Uj467aM786u1ChdNt7vxVQFjPqbubstEYZBRgpMpFumoAQQbGYzIfbgyDzR94nxHnIj30i7pc1PDWbyhsIZ58Q8yk9dFm7P1vHsZm82S9taB894d3oaecY5jI3gvILzi9zb3YhHIL0vrN/fWoKx0QhPpyCg4jMY33tCeubWpDhL6xOzh1r8x8/3P5SfwGRXm0H9x Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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