From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 D80963FF889 for ; Wed, 13 May 2026 11:36:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778672212; cv=none; b=q15BAVGPHIjPz1vLFuyxmdG2Oc+92x/4Jsj+1xlxP+gGwjNBKZdfk+uR99QecPid/eOkOhxRnelMgA2Q54WN4JnK6oG3JGoX4sh14g6sQA5bH6tiFtuTMoDQRJZH1uvzaignJtf116pSvCbKabLsGSwEBeUYPEM4xfaFZ3ctEAc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778672212; c=relaxed/simple; bh=2tHDcgBhOPqCxLRSH0z7aiqpxuZha9FjNCPAxIgFceM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QdfuikQYwJC6NkYk0+8bMFKZ4+8XfzG5L8NsyUnIzMxoU9eLGCNp8B/DgJeotbMgCPaM+EtH4lKQcMVGj7AtyByBlTTK7ZCfSo2TkM4elBE8Vj14+5WLxzL1bdxyeGBDPGwAU2NXszyfIJ8AcEYvhUvzb6zt2q9gz/JnO13gQs0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=rjvhZCn+; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="rjvhZCn+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CCA3AC2BCB7; Wed, 13 May 2026 11:36:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778672212; bh=2tHDcgBhOPqCxLRSH0z7aiqpxuZha9FjNCPAxIgFceM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=rjvhZCn+OILCOcPitf+xlGukPANWJrni1RXlRxJCk5OMAi5L55NTTTKbSCQa6Rv7k 1SHhFoasRhjslQ0LmtB6xGftHLBEuUuiGge+jqLJ4imw52X6Ab4zEY9RaDSfdyOlj0 BYJbONskSA59FBV1yEHKMHNOfqQ7wPTHrXNw9rOGTuMJuTUogPN79k7Zi+P5CFVl2f Paa+zv+4FTCwpRaTJZ8cr34FsSct1gtU1p6PqDoA4cvj7xwg1Z7GN+QVgD/wuf0kax FDw1zeuOJ1morPSGBohUzET9BzkyGKZdTiwDSvo/YDIN1GUIA+twQnV0bIMcoGDF1V glyYROEm3Jjeg== Message-ID: Date: Wed, 13 May 2026 13:36:43 +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 Subject: Re: [PATCH 1/2] mm/shmem: add shmem_insert_folio() To: =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , =?UTF-8?Q?Christian_K=C3=B6nig?= , intel-xe@lists.freedesktop.org Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , Brendan Jackman , Johannes Weiner , Zi Yan , Huang Rui , Matthew Auld , Matthew Brost , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , dri-devel@lists.freedesktop.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260512110339.6244-1-thomas.hellstrom@linux.intel.com> <20260512110339.6244-2-thomas.hellstrom@linux.intel.com> <26479389-459b-4cc4-914d-e7d29d5e5cc9@kernel.org> <1ce447ceea88bbe56c7d28654a51fc2856a7f986.camel@linux.intel.com> <65596f86-e6a3-48b0-aa3d-2e608964e29d@kernel.org> 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: 8bit On 5/13/26 12:37, Thomas Hellström wrote: > On Wed, 2026-05-13 at 12:03 +0200, David Hildenbrand (Arm) wrote: >> On 5/13/26 10:51, Thomas Hellström wrote: >>> >>> FWIW, the i915 driver which uses shmem "natively" uses a special >>> mount >>> here that gives back THPs. >>> >>> >>> Currently the drivers that use shmem in this way use >>> "mapping_set_unevictable()" as long as the object is bound to the >>> GPU. >>> Then shrinkers can unbind from GPU and revert that setting. >> >> Right, but mapping_set_unevictable() only affects folio_evictable() - >> -reclaim >> behavior. Not other properties (such as folio migration). > > Interesting. Does that imply that a shmem folio can be replaced > underneath without additional measures? It looks like most DRM > call sites imply that mapping_set_unevictable() pins underlying shmem > folios I don't think there is anything preventing folio migration. shmem implement the migrate_folio() callback simply by wiring up migrate_folio(). However, any raised reference on a shmem folio would prevent migration. However, taking longterm references on folios that are allocated as being movable (shmem default) breaks CMA, memory hotunplug, compaction... drm_gem_get_pages()/drm_gem_put_pages() seem to handle some part of that ... by grabbing/putting references. So if DRM actually might hold these references for a longer time, I suspect this breaks CMA etc.. We have the memfd_pin_folios() interface that takes care of handling that properly by only allowing longterm references if longterm references are actually allowed -- and otherwise migrates pages to physical memory areas where longterm pinning is allowed. [...] > x86 implementation is here: > https://elixir.bootlin.com/linux/v7.1-rc3/source/arch/x86/mm/pat/set_memory.c#L2556 > > TTM calls it here: > https://elixir.bootlin.com/linux/v7.1-rc3/source/drivers/gpu/drm/ttm/ttm_pool.c#L249 > Ah, the calls to set_pages_array_wc/set_pages_array_uc. > And there are actually shmem helpers that do this as well, without > pooling. > https://elixir.bootlin.com/linux/v7.1-rc3/source/drivers/gpu/drm/drm_gem_shmem_helper.c#L212 Thanks for pointing me at them. Right, after grabbing a reference, the folio is unmovable and we can modify the directmap. > > >> >>> That's an >>> extremely costly operation so TTM needs to pool such allocations. >>> That's where using shmem natively becomes very ugly, because you >>> can't >>> really use a 1:1 mapping between shmem objects and DRM objects >>> anymore. >> >> So you would require different caching attributes within a DRM >> object? > > The way the TTM pools work are that there are separate pools for each > allocation order and caching modes. That would essentially mean > allocations from a single shmem object would be spread out across > different pools, and we'd loose the 1:1 mapping between DRM objects and > shmem objects. Right. > > One alternative would be a single large sparse shmem object common for > all DRM objects, with a range allocator, but that also got pretty ugly > when I tried to implement that. Does not sound too crazy, though. > > Finally, (and I think that might be what Christian was getting at as > well) Without CONFIG_TRANSPARENT_HUGEPAGE, we'd only see order 0 shmem > folios, right? Right. Because large folios do not exist in such a world. So this is expected. If you don't set CONFIG_TRANSPARENT_HUGEPAGE, expect the system to have bad performance. -- Cheers, David