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 D0DE833F58B for ; Mon, 8 Jun 2026 11:14:30 +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=1780917271; cv=none; b=iMYPbbydIXtaQnpf7pKwQgithMlP4xUBEjaKwkVBdhcqSm+9NpPBeO4rCniiG0jN6XHFdUYjmUCR1odQCRJpuUNPoPxppCq5D2BQGXT+8/U9eNmUKvwFMIyVU8soUjsjm98kOSrEAqZQHczhEQNfXEzJ/zB3OJewQPUSljjeUTQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780917271; c=relaxed/simple; bh=Lt9ak9zGJeYwTBhihPKDwGcmREask8sqggzPL/W6epo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kCnNokme3awgB9g4rBy0Eoh5a7xvzLh1KAj+/R5wsPNcEAUEftoT8VzMNgm0mskbklBOgyGFIPgVcahv6u7BWAmwXnHkIECLzTUKK01M8p3aVlp+J5B1vIKSYxvP9uq7pC+kNvqAaUDvAIK5ztxqU5aGnsn7IBWpmZz8shbNfiI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Va/1owc0; 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="Va/1owc0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 317721F00893; Mon, 8 Jun 2026 11:14:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780917270; bh=IDPAFI5FQvvWZFGQ97+UGzUve2NfcJnlbnzcH8NKyw8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Va/1owc0KxajEPlJS5BuC5H59P2uclmX6uYMA+EIQhUf+tXtOtZzwyVznn82I3hmC vopYHWEYb8gfVuGAAzCA9HeC1CRFd34EuS3Q23KrscL8B4eCVr0tvvUAjFfDwrKpPE o9dfY7YHAkCggUZuxvUdPzkK1zGU9TMcgq6lVZQZS+V5TkYJKfqWtTScNp69+wNbEt LnTDWT5SRlXwVv3RBf83r/zz9sg5fpxsKo8kNPWaLfPXd7JGjVhgO5YX2b9vvII0ig vUQ7Rde46+pEbB1tiiuLwjEe9Pz+D89TdO4thiY4lmtUeur9i/XmCmxN1tklVgupiQ ylCNIaXfpCoLg== Message-ID: <377d571d-0c41-4d08-90a8-d198e1658358@kernel.org> Date: Mon, 8 Jun 2026 13:14:24 +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: [RFC] mm: restrict zero-page remapping to underused THP splits To: Nico Pache Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, yuzhao@google.com, usamaarif642@gmail.com, lance.yang@linux.dev, baohua@kernel.org, dev.jain@arm.com, ryan.roberts@arm.com, liam@infradead.org, baolin.wang@linux.alibaba.com, ziy@nvidia.com, ljs@kernel.org, akpm@linux-foundation.org References: <20260508170509.640851-1-npache@redhat.com> <04ea0e68-de56-49c4-8c9f-1734139d5e7f@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/8/26 12:34, Nico Pache wrote: > On Fri, May 8, 2026 at 3:32 PM David Hildenbrand (Arm) wrote: >> >> On 5/8/26 19:05, Nico Pache wrote: >>> Since commit b1f202060afe ("mm: remap unused subpages to shared zeropage >>> when splitting isolated thp"), splitting an anonymous THP remaps all >>> zero-filled subpages to the shared zeropage via TTU_USE_SHARED_ZEROPAGE. >>> This flag is set unconditionally for every anonymous folio split, >>> including splits triggered by KSM. >> >> And even when the underused scanner is effectively disabled on a system. Hm. >> >> I don't quite like that we scan for zeropages when nobody even requested us to >> split because of zeropages. >> >> I can see why we would want to scan for zeropages in a setup where the underused >> scanner is active, even when the split was triggered by someone/something else >> (below). >> >> [...] >> >>> /** >>> @@ -4340,7 +4341,13 @@ int folio_split(struct folio *folio, unsigned int new_order, >>> struct page *split_at, struct list_head *list) >>> { >>> return __folio_split(folio, new_order, split_at, &folio->page, list, >>> - SPLIT_TYPE_NON_UNIFORM); >>> + SPLIT_TYPE_NON_UNIFORM, false); >>> +} >>> + >>> +int folio_split_underused(struct folio *folio) >>> +{ >>> + return __folio_split(folio, 0, &folio->page, &folio->page, >>> + NULL, SPLIT_TYPE_NON_UNIFORM, true); >>> } >>> >>> /** >>> @@ -4559,7 +4566,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, >>> } >>> if (!folio_trylock(folio)) >>> goto requeue; >>> - if (!split_folio(folio)) { >>> + if (!folio_split_underused(folio)) { >>> did_split = true; >>> if (underused) >>> count_vm_event(THP_UNDERUSED_SPLIT_PAGE); >> >> In general, this looks clean. >> >> But imagine the following: someone splits the THP for another reason: for >> example, because migration is unable to allocate a 2M THP, or because we have to >> split on swapout etc. >> >> Not freeing the zero-filled pages means that these pages cannot be reclaimed >> anymore easily. We split a possibly underused THP but didn't free the memory. >> >> The only way to free the memory would be to wait for another collapse, and then >> have the new THP be detected as underused. >> >> Hm. >> >> (1) As you say, the alternative is to let KSM say that it wants to handle the >> zero-filled pages itself. I'm not a the biggest fan of that approach. We still >> have two mechanisms interacting to some degree. >> >> (2) Another approach is to just let KSM handle this in VMAs that are marked as >> mergable while KSM is active. That is, we check for VM_MERGABLE and ksm_run == >> KSM_RUN_MERGE in try_to_map_unused_to_zeropage() to just let KSM do its thing. >> >> That really just stops both mechanisms from interacting. >> >> (3) Yet another approach I could think of (in general) is to disable the >> underused handling in a system where the underused splitting is entirely disabled. >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index e9d499da0ac7..5eca99271957 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -82,6 +82,14 @@ unsigned long huge_anon_orders_madvise __read_mostly; >> unsigned long huge_anon_orders_inherit __read_mostly; >> static bool anon_orders_configured __initdata; >> >> +static bool thp_underused_split_active(void) >> +{ >> + if (!split_underused_thp) >> + return false; >> + >> + return khugepaged_max_ptes_none != HPAGE_PMD_NR - 1; >> +} >> + >> static inline bool file_thp_enabled(struct vm_area_struct *vma) >> { >> struct inode *inode; >> @@ -4188,7 +4196,8 @@ static int __folio_split(struct folio *folio, unsigned int >> new_order, >> if (nr_shmem_dropped) >> shmem_uncharge(mapping->host, nr_shmem_dropped); >> >> - if (!ret && is_anon && !folio_is_device_private(folio)) >> + if (!ret && is_anon && !folio_is_device_private(folio) && >> + thp_underused_split_active()) >> ttu_flags = TTU_USE_SHARED_ZEROPAGE; >> >> remap_page(folio, 1 << old_order, ttu_flags); >> @@ -4497,7 +4506,7 @@ static bool thp_underused(struct folio *folio) >> int num_zero_pages = 0, num_filled_pages = 0; >> int i; >> >> - if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) >> + if (!thp_underused_split_active()) >> return false; >> >> if (folio_contain_hwpoisoned_page(folio)) >> >> >> >> I tend to like (2), and maybe (3) on top. Opinions? > > Coming back to this. > > for (2), I have to export the KSM run state, which may be fine, but it Right, you just need a simple helper. > introduces a race window. If a user disables KSM, a split occurs, and > then re-enables it, the bug will present itself again. Who cares? > > Would it be better to just check if its VM_MERGEABLE? Some user space unconditionally sets VM_MERGEABLE, even if KSM is never enabled (IIRC QEMU, for example). -- Cheers, David