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 9E47C3F7E9C; Fri, 27 Mar 2026 14:31:13 +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=1774621873; cv=none; b=V/ELPGJvy2w1LV74rgdg2ienVc1GWzuciTbxrisi/ID96dAhuaMxf3qYwDdzcbl/Rt6aRGG6oX8XtZBiYWdHdHfUPOHQNTeAUrx+ceiE/v7/sVEBUpzmitKj/PElhsOmBtmX0g/cPPcogkJAOCCX/7/2cQiCEvW8cF0ZareRiwQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774621873; c=relaxed/simple; bh=MrvKjRWkl1Y6HHbAPwGm0kWozY8T3Glyl78Hgi7yspY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aROwFMGDBJhBez+iVFVDQ7dyPpEZxFiQYcoKcfFWdSqE9r9BSUywzMWnyITOxSZJEBMNXhuJY32QjYIL7f14/Td5nMgeP+uNxV4fjjWxCaGRK0K2BwZ5Ou0d2R4a5bsNiWVRb6aXVkBeixDwNAZwf18qUWLY0rmn2KfNvbgaRXM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FA65YRld; 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="FA65YRld" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9B8E8C2BC87; Fri, 27 Mar 2026 14:31:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774621873; bh=MrvKjRWkl1Y6HHbAPwGm0kWozY8T3Glyl78Hgi7yspY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=FA65YRldQYG5G3drQMMoAc8SfQ4mmeIUA90nBgmb1APmS8p6BhrzYu9nrBDDjJfto tMBA9seiAdGawC246pLjV+Fl5yut7bDI2tSd1BoHbsW+K3JnTkXzANWLMFai+Z0b+8 +lyTw8FpFfTPmDRtT9Qo8aGRXT3lo236j74uh/mLZvP2qfIlCIGdmBFY+f1STVdcdV i7GtH/LWRU0i++pN+8Bt945zXeQXk5IQKYViC1aplops7W8LmQ8KPMcMekKZnq2GHT wZX83qWDlNnCQT3ZAIXM+3nmM7S475HJjU6WRuj+KtTSvG7Mw46mbRhKWnsePUXjna 7p88Q+af6JbLw== Date: Fri, 27 Mar 2026 14:31:05 +0000 From: "Lorenzo Stoakes (Oracle)" To: Baolin Wang 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 Subject: Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check Message-ID: <6442d533-27ae-4e2d-b8d1-64acdd2dfbd9@lucifer.local> 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> Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote: > > > 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(). It sucks not to have an assert. Maybe in that case make it a VM_WARN_ON_ONCE(). I hate that you're left tracing things back like that... > > 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; 2 < 5, we won't tolerate bisection hazards. > > > > > 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 > Cheers, Lorenzo