From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com [115.124.30.130]) (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 E22F931ED93; Fri, 27 Mar 2026 14:26:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774621624; cv=none; b=E1l5oq+iWibusFwO7Rvoyuo953POT1bVPGg7b7lCG6vs1hu8HvsmnbOzTDTyH4x9J7B7rbdOw69LzrnGHDv/kZFtW9GoELtOFBfNMBvWT16vdduc7zYQ/qbhyT+ChRLsPuf3CxB37qBJnu0/LGOj8LF1sHwklIlnKbTHbbHFs+4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774621624; c=relaxed/simple; bh=S849YbsTewCwRIs4fC0CRDUyssNKK6CAaatHObMQ30o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Av9dzVB8hq0oPA0TRF4LN5hazw1YlZK2StYbnC1LA5OvKWPswiznfJ+tF+sg8uVC4ZW8PLLZlY6D2VYD7OW7wiCIEhHwuO3FrRyN+aDWbUtYw7/kl9TcQqrE+ZeVGHoY88dXwGoiZrEBFFcfnYrF026QNUiloq014OWwHv8o/4o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=yGZJYOyk; arc=none smtp.client-ip=115.124.30.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="yGZJYOyk" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1774621616; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=IGtPBcxw9SmT8YrZeAYAA/G+Qaiyn6Pw5UUHZsnXfKo=; b=yGZJYOykjFrtJk5RVWT0QEB4rhitDSFJXvWdfZeS42EY7Yl5co/4EuPYpWw0jRPcelfzfpXyAN7uVJWQb2og9RL15zpgBO8z+hY7ONbdKG9+orHLxI6Tzl8NcsBCHmVk05MwLxJ/WBQ7TZ/jfcdwvmmH08diklqb2ylHtBy8RN4= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R241e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045098064;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=27;SR=0;TI=SMTPD_---0X.oc15I_1774621613; Received: from 30.42.98.36(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0X.oc15I_1774621613 cluster:ay36) by smtp.aliyun-inc.com; Fri, 27 Mar 2026 22:26:54 +0800 Message-ID: Date: Fri, 27 Mar 2026 22:26:53 +0800 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check To: "Lorenzo Stoakes (Oracle)" Cc: Zi Yan , "Matthew Wilcox (Oracle)" , Song Liu , Chris Mason , David Sterba , Alexander Viro , Christian Brauner , Jan Kara , Andrew Morton , David Hildenbrand , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <20260327014255.2058916-1-ziy@nvidia.com> <20260327014255.2058916-3-ziy@nvidia.com> <7fd90f5e-65b5-4734-abb2-77b22c733af5@linux.alibaba.com> <8f5119a1-9aa9-4a39-ac94-ca1631db26e1@lucifer.local> <89c8b93c-f6dd-4d8e-bcee-3c1ff1c04295@lucifer.local> From: Baolin Wang In-Reply-To: <89c8b93c-f6dd-4d8e-bcee-3c1ff1c04295@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote: > On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote: >> >> >> On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote: >>> On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote: >>>> >>>> >>>> On 3/27/26 9:42 AM, Zi Yan wrote: >>>>> collapse_file() requires FSes supporting large folio with at least >>>>> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with >>>>> huge option turned on also sets large folio order on mapping, so the check >>>>> also applies to shmem. >>>>> >>>>> While at it, replace VM_BUG_ON with returning failure values. >>>>> >>>>> Signed-off-by: Zi Yan >>>>> --- >>>>> mm/khugepaged.c | 7 +++++-- >>>>> 1 file changed, 5 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index d06d84219e1b..45b12ffb1550 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, >>>>> int nr_none = 0; >>>>> bool is_shmem = shmem_file(file); >>>>> - VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem); >>>>> - VM_BUG_ON(start & (HPAGE_PMD_NR - 1)); >>>>> + /* "huge" shmem sets mapping folio order and passes the check below */ >>>>> + if (mapping_max_folio_order(mapping) < PMD_ORDER) >>>>> + return SCAN_FAIL; >>>> >>>> This is not true for anonymous shmem, since its large order allocation logic >>>> is similar to anonymous memory. That means it will not call >>>> mapping_set_large_folios() for anonymous shmem. >>>> >>>> So I think the check should be: >>>> >>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) >>>> return SCAN_FAIL; >>> >>> Hmm but in shmem_init() we have: >>> >>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >>> if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY) >>> SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge; >>> else >>> shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */ >>> >>> /* >>> * Default to setting PMD-sized THP to inherit the global setting and >>> * disable all other multi-size THPs. >>> */ >>> if (!shmem_orders_configured) >>> huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER); >>> #endif >>> >>> And shm_mnt->mnt_sb is the superblock used for anon shmem. Also >>> shmem_enabled_store() updates that if necessary. >>> >>> So we're still fine right? >>> >>> __shmem_file_setup() (used for anon shmem) calls shmem_get_inode() -> >>> __shmem_get_inode() which has: >>> >>> if (sbinfo->huge) >>> mapping_set_large_folios(inode->i_mapping); >>> >>> Shared for both anon shmem and tmpfs-style shmem. >>> >>> So I think it's fine as-is. >> >> I'm afraid not. Sorry, I should have been clearer. >> >> First, anonymous shmem large order allocation is dynamically controlled via >> the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) and >> the mTHP interfaces >> (/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled). >> >> This means that during anonymous shmem initialization, these interfaces >> might be set to 'never'. so it will not call mapping_set_large_folios() >> because sbinfo->huge is 'SHMEM_HUGE_NEVER'. >> >> Even if shmem large order allocation is subsequently enabled via the >> interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called >> again. > > I see your point, oh this is all a bit of a mess... > > It feels like entirely the wrong abstraction anyway, since at best you're > getting a global 'is enabled'. > > I guess what happened before was we'd never call into this with ! r/o thp for fs > && ! is_shmem. Right. > But now we are allowing it, but should STILL be gating on !is_shmem so yeah your > suggestion is correct I think actualyl. > > I do hate: > > if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) > > As a bit of code though. It's horrible. Indeed. > Let's abstract that... > > It'd be nice if we could find a way to clean things up in the lead up to changes > in series like this instead of sticking with the mess, but I guess since it > mostly removes stuff that's ok for now. I think this check can be removed from this patch. During the khugepaged's scan, it will call thp_vma_allowable_order() to check if the VMA is allowed to collapse into a PMD. Specifically, within the call chain thp_vma_allowable_order() -> __thp_vma_allowable_orders(), shmem is checked via shmem_allowable_huge_orders(), while other FSes are checked via file_thp_enabled(). For those other filesystems, Patch 5 has already added the following check, which I think is sufficient to filter out those FSes that do not support large folios: if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER) return false; >> Anonymous shmem behaves similarly to anonymous pages: it is controlled by >> the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to >> check for allowed large orders, rather than relying on >> mapping_max_folio_order(). >> >> The mapping_max_folio_order() is intended to control large page allocation >> only for tmpfs mounts. Therefore, I find the current code confusing and >> think it needs to be fixed: >> >> /* Don't consider 'deny' for emergencies and 'force' for testing */ >> if (sb != shm_mnt->mnt_sb && sbinfo->huge) >> mapping_set_large_folios(inode->i_mapping); > > Cheers, Lorenzo