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 3FCF33B71CF for ; Thu, 1 Oct 2026 08:35:02 +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=1790843703; cv=none; b=EyeLzW+obS/l8rh8eG6WTJdS79zG13GxOOEYltY0Vsgei9iurDyfHqAenxfSGBvj8sgd11VRIGlR1OAgEM1enSh8pnve8r0uj/nWenXvcRTB7P+pZLw1AIkYGCdzmmGyMUJDD5oYEoU6TOuwmD33sZ7xt+L8vZVhDQQz25Yt+t4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790843703; c=relaxed/simple; bh=RUcCmJXMafO+1xVxRi9lTxkn8DsNoOeV88OTvIJCCIg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iSTya2Bqoeb5iRuA5GTp7rmB2r4LjBHKbmGFV753TNHpP773jasM1i5H8W2FAvjeKshCAXqNV4X6cGRwGkwMhEm3fny8csMPFpiMv1OLWuDUENrKCWu86DrZ3V5jGZQBD+KXywGZB+XmMmCoqNdZjSa9F6Gy4RYsZbYsjQqN53A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YpwGDrY5; 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="YpwGDrY5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C07121F000FF; Thu, 1 Oct 2026 08:34:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790843702; bh=Beu3PTH0hGN+3Qc/F8w3XSUtWyseEXXZRw/+4Um0mjo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YpwGDrY5ZFPShy38M7zct/x7SvYt5v87AXBKQjVt4OTx5QQT/w1dOKlAk8T0WW1Qb +y6KZLI1PD6p622hdijB9L5s07rsZ1UAI6d/OJQ+225ZHG3bHxaPrizZ9HMkvM2IKa tmwOlJ5lzOU3krwHs8bLTr4Qw7/YQgmevD6Lg5Sl1QP7wjY0L2dMyp5Nwl6C3z3PUA 436dXWUSBlCeC2NZRFNziVzDm2AogHd11r5/kZCsVU6XzNqeMLqo1zI/tf768cj4k4 OJ5KQBdY9cqUHsC5gfaRBEDArH+rqSrVTOf1ivoWmHnj61ZoW7ISTjAveduCY9Oxmh GvMoH78UTDN7g== Message-ID: <9bf6623e-ad1b-415c-9720-6063a1ac11ea@kernel.org> Date: Thu, 1 Oct 2026 10:34:54 +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 v4 09/13] mm/collapse: separate scanning a PTE table from collapsing it To: Kiryl Shutsemau , Andrew Morton , Lorenzo Stoakes , Zi Yan , Baolin Wang Cc: "Kiryl Shutsemau (Meta)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Vlastimil Babka , Jann Horn References: <20260928100630.21870-1-kirill@shutemov.name> <20260928100630.21870-10-kirill@shutemov.name> 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: <20260928100630.21870-10-kirill@shutemov.name> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/28/26 12:06, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" > > A collapse is two jobs. One reads a PTE table under mmap_lock and decides > whether the range is worth collapsing. The other allocates, isolates, > copies and flushes, and wants the lock given up first. > > collapse_single_pmd() did both, so the boundary between them was somewhere > in the middle of a function. > > Give each half its own function: > > - collapse_scan_pmd() scans one table. The anonymous scan that used to > carry that name keeps its body as collapse_scan_anon_pmd(), and > collapse_scan_pmd() is now the entry that picks the anonymous or the > file side. > > - collapse_run_pmd() does the collapse the scan asked for, and is > handed what the scan returned. SCAN_SUCCEED means there is > something to collapse. SCAN_PTE_MAPPED_HUGEPAGE means the page > cache already holds the PMD folio and only the PTE table is left to > retract. Both are work for the run; anything else is why there is > nothing to do. I raised that I don't like the "run" in collapse_run_pmd(). That hasn't changed, as we are adding more inconsistency. We do have after your patch set collapse_scan_pmd calling -> collapse_scan_file -> collapse_scan_anon_pmd Inconsistency: why not simply collapse_scan_anon? Alternatively collapse_scan_file_pmd, although for this internal helper including the "pmd" is not particularly helpful. The we have collapse_run_pmd calling -> mthp_collapse -> collapse_file Inconsistency: a lot, including the "run" thing. So I suggest the following in the context of this patch here: collapse_scan_pmd -> collapse_scan_file -> collapse_scan_anon collapse_pmd -> collapse_anon -> collapse_file (as I previously said, "scanned" could be thrown in there, to make the expectation clearer, but I don't particularly care about that) > > collapse_single_pmd() is now the two of them with the mmap_lock drop in > between, so its callers see what they saw before. collapse_control_init() > sets a control up before its first scan. You should mention that collapse_single_pmd() is only temporary. > > What the scan found and the run needs travels in collapse_control. For > an anonymous table that is the orders and the referenced and swapped-out > counts, which mthp_collapse() and collapse_huge_page() now read from > there instead of taking as arguments. For a file it is the file itself > and the offset in it: a file collapse works on the page cache and never > sees a VMA, so the scan takes the reference while it still has one and > the run gives it back. > > The file scan moves under mmap_lock with the anonymous one, where before > the lock was given up first. The lock is now held over the page cache > walk, an RCU walk over one table's worth of slots with no PTL, and taken > fewer times. collapse_scan_mm_slot() ends its walk whenever the lock was > dropped, so a refused file table used to cost khugepaged an unlock, a > trip back through khugepaged_do_scan(), a relock and a VMA lookup. Now > only a table that goes on to be collapsed does. [...] > > +/* > + * Try to collapse a single PMD starting at a PMD aligned addr, and return > + * the results. > + */ I'd just not add that doc if you intend to remove the whole thing in the same comment. -- Cheers, David