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 65FA72153FB; Fri, 19 Dec 2025 13:52:50 +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=1766152370; cv=none; b=kwjpiVlgIH/NbjiNYRkLPM7q3fO3WidP6z9+ZwZdxRHioW0whkMsY2Rs31nwpwxQFM6qqwOzWXeNWj5olu9V2cqGyVdL1Iy/vze9Pkoz0zRiF3YdI1x4NhpaFkUfD+tHt6csien7NJB00ZbCjovgGhOiTr0LaGukkyxMBpeuve0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766152370; c=relaxed/simple; bh=kp11YU2QH+WTDS76e4SebHd/dxUjhu1pW98eOwe8K0w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WXPklWEltYV+HoySDVpxDi9ive2pSRlMP7uZWVrv5FrFWV1EUqLIThRv1ewlsqb6AihtovZagMUOVyf037VIczLceJQqA6MHz1cIk9eu8UGfcdiOwz08Se2huqsdH5ckjuK1kWCNoyHmIqszwqI/RA+tFaNW69fpo6kaahOeWXQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BZTh/akB; 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="BZTh/akB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 840A5C4CEF1; Fri, 19 Dec 2025 13:52:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766152369; bh=kp11YU2QH+WTDS76e4SebHd/dxUjhu1pW98eOwe8K0w=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=BZTh/akBrk0EFUsB0mnnrGI4bSdbDF3g3y2Tib8PutDKdve9TMuIWjcv9u2LLNdIX ADG1T0x8r9IcGlyAB5/mldOvYLs5PCnr+afNXRsNJvt+VsnshEE3B7EhHkawPk/Bdy S8FORjtngC0ok6jYoVShmyBc4dYpbN45kXgzzGtFXA4LOp8ChVkABePp5Bj9KN6NTi WwpW+GdParMGATH8cbUsN4GqanpHT5ornzR3FJFIqS/7duGdYRphCyiiTquvWaMA8h 8fNb0YcKhcskSfvgYd7X5jdCHEkSirsMW40Bi0pn+61Me+MMQYToQG/Xxu8mLzUeEK pQ1h0rbMtfKRA== Message-ID: Date: Fri, 19 Dec 2025 14:52:41 +0100 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 4/4] mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather To: Harry Yoo Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, Will Deacon , "Aneesh Kumar K.V" , Andrew Morton , Nick Piggin , Peter Zijlstra , Arnd Bergmann , Muchun Song , Oscar Salvador , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , Rik van Riel , Laurence Oberman , Prakash Sangappa , Nadav Amit , stable@vger.kernel.org, Ryan Roberts , Catalin Marinas , Christophe Leroy References: <20251212071019.471146-1-david@kernel.org> <20251212071019.471146-5-david@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12/19/25 13:37, Harry Yoo wrote: > On Fri, Dec 12, 2025 at 08:10:19AM +0100, David Hildenbrand (Red Hat) wrote: >> As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix >> huge_pmd_unshare() vs GUP-fast race") we can end up in some situations >> where we perform so many IPI broadcasts when unsharing hugetlb PMD page >> tables that it severely regresses some workloads. >> >> In particular, when we fork()+exit(), or when we munmap() a large >> area backed by many shared PMD tables, we perform one IPI broadcast per >> unshared PMD table. >> > > [...snip...] > >> Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race") >> Reported-by: Uschakow, Stanislav" >> Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/ >> Tested-by: Laurence Oberman >> Cc: >> Signed-off-by: David Hildenbrand (Red Hat) >> --- >> include/asm-generic/tlb.h | 74 ++++++++++++++++++++++- >> include/linux/hugetlb.h | 19 +++--- >> mm/hugetlb.c | 121 ++++++++++++++++++++++---------------- >> mm/mmu_gather.c | 7 +++ >> mm/mprotect.c | 2 +- >> mm/rmap.c | 25 +++++--- >> 6 files changed, 179 insertions(+), 69 deletions(-) >> >> @@ -6522,22 +6511,16 @@ long hugetlb_change_protection(struct vm_area_struct *vma, >> pte = huge_pte_clear_uffd_wp(pte); >> huge_ptep_modify_prot_commit(vma, address, ptep, old_pte, pte); >> pages++; >> + tlb_remove_huge_tlb_entry(h, tlb, ptep, address); >> } >> >> next: >> spin_unlock(ptl); >> cond_resched(); >> } >> - /* >> - * There is nothing protecting a previously-shared page table that we >> - * unshared through huge_pmd_unshare() from getting freed after we >> - * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare() >> - * succeeded, flush the range corresponding to the pud. >> - */ >> - if (shared_pmd) >> - flush_hugetlb_tlb_range(vma, range.start, range.end); >> - else >> - flush_hugetlb_tlb_range(vma, start, end); >> + >> + tlb_flush_mmu_tlbonly(tlb); >> + huge_pmd_unshare_flush(tlb, vma); > > Shouldn't we teach mmu_gather that it has to call I hope not :) In the worst case we could keep the flush_hugetlb_tlb_range() in the !shared case in. Suboptimal but I am sick and tired of dealing with this hugetlb mess. Let me CC Ryan and Catalin for the arm64 pieces and Christophe on the ppc pieces: See [1] where we convert away from some flush_hugetlb_tlb_range() users to operate on mmu_gather using * tlb_remove_huge_tlb_entry() for mremap() and mprotect(). Before we would only use it in __unmap_hugepage_range(). * tlb_flush_pmd_range() for unsharing of shared PMD tables. We already used that in one call path. [1] https://lore.kernel.org/all/20251212071019.471146-5-david@kernel.org/ > flush_hugetlb_tlb_range() instead of ordinary TLB flush routine, > otherwise it will break ARCHes that has "special requirements" > for evicting hugetlb backing TLB entries? Yeah, I was briefly wondering about that myself (and the inconsistency we had in the code). I would hope that we're good, but maybe there are some nasty corner cases we're missing. So thanks for raising that. Given tlb_remove_huge_tlb_entry() exist (and is already getting used) I would assume that it does the right thing. In tlb_unshare_pmd_ptdesc(), I am now using tlb_flush_pmd_range(), because we know that we are dealing with PMD-sized hugetlb folios. And in fact, we were already doing that in case of __unmap_hugepage_range(), where we did exactly what I do now: tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE); So, again, something would already be broken there unless I am missing something important. Looking at it, I wonder whether we must do the tlb_remove_huge_tlb_entry() in move_hugetlb_page_tables() after the move_huge_pte(). Looks like tlb_remove_huge_tlb_entry() might do some flushing on ppc (and not just updating the mmu_gather) through __tlb_remove_tlb_entry(). But it's a bit confusing. -- Cheers David