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=1793271720; b=2RRJAmeOLTtgmIOPRLh8yIAkut0sWjpAxBEXXrwF9+0ypx0sI/4/2urDKtrbhyTGcncy eDR0rgpXSdkjwHCqp60es9CbQD/SJtWlgFhrj+KAsh2nh8UL5RpoFMKBwFgk28W3CQEui PewZCQKMVF//H3f3sr+pVkc1BzsDruamDBHvYJHBGwxBHSS7K9fArKA5daU+js62VvtSy m12yGot6foTUlsW7DWWqTfydFOmIUkXo38AE6kxx2mAz82DHFlavv/hTiHYkos+D8owWu MVdgmRoME3y5kOFTJSX/ctZ65Tt0J9Xq8hh8Kkjcc5qya7CbjcQwNQ0z4e1ajAre34w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793271720; 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=OYmhv8HwvlOQOOI8sOhoeifNYQzQv5FQ9ix1RIz0byk=; b=LICyQ8uuriK9pfB/RKx6bfrUiq+vXxbZm1oWAdL1aK+YirPL0so0OkgfZGIl42SKCcjM ai/WpwNMIQhWZaczLSEGrf2fGBLj0w3qGjn6YMLUpsZ633GL8zWQa6ogqrEfZFYUbDvUs Z+TVz3NHlWe/Nnh13y1ZNou5HJLHXzq1F4baMDPw5jpqIkHUb+jDyqrUKSkMlWSVDWIpq nihZTM7GJtuzIPua5aMmdFVvLsr1cHyM/r1Ix31pC/XIJagaZAIDYj5WO+tL8MaK74HCn YlzFkzjr9YC6q7EqbdFKZNTvUujk/0hr7GbXHl3qizV83dejuUZj/9ADQXeCHC6t+tQ== 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=WrVYzA32; 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 76A98606A31A; Thu, 29 Oct 2026 11:02:00 +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=WrVYzA32 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 868E41F01825; Thu, 29 Oct 2026 11:01:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793271720; bh=OYmhv8HwvlOQOOI8sOhoeifNYQzQv5FQ9ix1RIz0byk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=WrVYzA32uJkCm0VS++wNZs3DP7uWRGdJEXgTe24KpJtmejSidIe8tQmWXTY33s0QG HWa5maFNgZ9hsLBs9p6tltp+e/BdFtOGCb9jz8qmS+Cp/6fRjZJQMmzLdLwR9TfBUQ sN03Re4Pf9uxJOc9kulo+S78oKrbtmqC1f9GNHIJfTsLmluP4xsQoE6hd9E9GfiCvb 17qs7MKo4Qvzf3fWJz3HgnZ6Q6xNSZmlIhFv32WzWDFBIhnqWNSRPfmP7RtFcf+bLQ l2Tpp3alC16EanFj658aO1knHP3qQXCiprCyZR3keIAic7R0SlOv9FIqNe2jn4U8+0 pf43loQgoyv0Q== Message-ID: Date: Thu, 29 Oct 2026 12:01:40 +0100 X-Mailing-List: linux-fsdevel@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: [PATCH 18/46] mm/khugepaged: check if PTE table is empty in try_collapse_pte_mapped_thp() To: Gregory Price Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-18-6f74bec9a5bb@kernel.org> <179323853182.453251.8510731747954180380.b4-reply@b4> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/29/26 02:48, Gregory Price wrote: > On 2026-10-27 00:17 +0100, David Hildenbrand (Arm) wrote: >> The existing code is rather horrible and already resulted in bugs in the >> past: we special-case UFFD inserting anonymous folios by grabbing the PMD >> lock early, but ignore any other markers (guard, poison through uffd) that >> could have been inserted into the page table in the meantime, effectively >> dropping them. >> >> Let's just unify and simplify the locking: try to grab the PMD table >> lock and if that fails, flush the TLB, drop the PTE lock, take PT locks in >> the right order and check if the page table is still empty. >> >> We still perform the heavy lifting (zapping the folio ptes) under PTE table >> lock only. >> >> Include for now, as x86's flush_tlb_range() macro >> depends on hugetlb helpers. This dependency needs a separate cleanup. >> >> Both issues this fixes are rather corner cases. Let's not CC stable for >> now. >> >> Fixes: f807123d578d ("mm: allow guard regions in file-backed and read-only mappings") >> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()") >> Signed-off-by: David Hildenbrand (Arm) > > This patch is painful. It's taking me a bit to wrap my head around. The key here is that we unfuck (sorry) what we over-engineered before. We just to the same locking independent of VMA flags. But if the trylock fails (which we have to do because of possible deadlocks) we'll drop both locks and just do another lookup. If we raced with someone messing with the page tables in the meantime (no longer empty), we give up. > > I get that you want to avoid taking both locks while zapping, but > wouldn't it be simpler? I suppose we don't know what kind of > regressions might pop up with that though, bah. Exactly, that's the spirit of the original code. We would just block a 1GiB range of va space for the entire operation. If you think there is a more elegant way of doing it (while minimizing holding the pmd lock), please shoot :) > > I can say that I've reproduced and tested the before/after state, > and it looks stable. Thanks! -- Cheers, David