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 DE5C051990F; Mon, 7 Sep 2026 16:17:42 +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=1788797865; cv=none; b=RLJModKLA4b5Y41w9dlZ15ocjq/FEDLsHuZCVwNMEtBKXCIE0Jof2DflVSHJmOY9EqgNIkr2w4n/xf1mkG8GsHz6jTYK2of2AP9hpL8TBVIZSedWXVT2ze5I0uHgGVaB87+UF1yD1jnQ2M/PfZIgdW1soSykU53jDmw7gS+n94A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788797865; c=relaxed/simple; bh=IBUKXyVBs3ELYeNrmV5yec13Og1+lO7CD5WRBnH53nQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=AnA3RRhXZuYM4Cu+5tMwoT53eqKqG9kNAWDflNk6NjScIgt2IlVAt/bntFT/chrXjXFVrrVKm8HgTs7hnBY/T+5dsCShoSp0VQ1PHKMKy27XNbjSRxJ3E17bruwTFyYi/4OLlrU9uGFN4xawZHzOP8nhs8lx4X/O+/g1lQzmyiQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=chjSiu0k; 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="chjSiu0k" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 65D921F00A3A; Mon, 7 Sep 2026 16:17:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788797860; bh=wOnQahPO1TmCL6FmC3Riu7SZQYrSuY8UOh5mIJltyRE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=chjSiu0kxNy55FySd80YLuWpGKduvYPpWOIeXBDAEDtKqnVBNh9d2zpIacBhCSdaA /hPjIqOuSUHj8c/P9fQmZl5QsN4WOnfiIOfKBSlpTDf98LalsExTWUCBri/U/G11Qu 463UFNmJOoS1/LOyoX+MydBYhJ4deFChj18UBUl7lXOC4G9EfqkcCO0FWNUFnMgVSu ZJ0dq8yg+/MGoN7WleKNCrwoFHxPeyLQn9DKBpOtPSgfQA6XKTE7mNpQtFaKmgn8pj cXgDuC+tRseK5NaxjJcE836EtT/y3wIWapSvHcDps3dvye1fYFKYf3q119INcb4rPb /Z9QHNIdzdHYA== Date: Mon, 7 Sep 2026 17:17:33 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Peter Xu , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Cedric Le Goater , Saravanan D , stable@vger.kernel.org Subject: Re: [PATCH] mm/huge_memory: bypass THP tuneables for huge pfnmap mappings Message-ID: References: <20260827-hugepfn-allowable-orders-v1-1-94819c8807c8@kernel.org> <034a2687-3551-485a-a382-e7e77aa3919b@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@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: <034a2687-3551-485a-a382-e7e77aa3919b@kernel.org> On Mon, Sep 07, 2026 at 06:14:14PM +0200, David Hildenbrand (Arm) wrote: > On 8/27/26 21:55, Lorenzo Stoakes (ARM) wrote: > > The sysfs THP tuneables at /sys/kernel/mm/transparent_huge_pages/ rather > > confusingly only control the behaviour of THP in some instances. > > > > They are not applicable to MADV_COLLAPSE operations, nor to DAX mappings. > > > > Long-term, THP is predicated upon compaction being able to obtain large > > folios to populate THP ranges. > > > > However, vm_normal_folio() returns NULL for PFN map mappings, thus their > > reference count is maintained by the driver, not core mm. > > > > As a consequence, the folios are not subject to reclaim nor compaction, so > > are not truly part of the THP mechanism at all. > > > > However, since commit 5dd40721f147 ("mm: allow THP orders for PFNMAPs") > > introduced the ability to establish huge PFN maps, they have been subject > > to THP tuneables. > > > > This is incorrect - if a huge PFN map is available (defined by > > vma->vm_ops->huge_fault being non-NULL for a VMA_PFNMAP_BIT VMA), then it > > should be mapped huge upon fault-in. > > > > Correct this by explicitly checking for this while ensuring that smaps > > continues to accurately report THPeligible statistics. > > > > While here, abstract the entire file-backed THP check in > > vma_can_map_huge_file(), with sensible separation of logic into helper > > functions. > > > > Note that drm_gem_shmem_mmap() and panthor_gem_mmap() establish huge PFN > > maps of shmem folios, however they are marked unevictable in > > drm_gem_get_pages(), and in any case would fail the reference check in > > __remove_mapping() even if they weren't. > > > > Failing to map huge PFN maps has resulted in significant real-world > > performance degradation, see links for details. > > > > Reported-by: Cedric Le Goater > > Closes: https://lore.kernel.org/linux-mm/20260805055544.1568534-1-clg@redhat.com/ > > Reported-by: Saravanan D > > Closes: https://lore.kernel.org/linux-mm/20260821070520.25759-1-saravanand@crusoe.ai/ > > Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs") > > Cc: stable@vger.kernel.org > > Signed-off-by: Lorenzo Stoakes (ARM) > > --- > > mm/huge_memory.c | 86 +++++++++++++++++++++++++++++++++++++++++--------------- > > 1 file changed, 64 insertions(+), 22 deletions(-) > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > index afbb5974bd22..4bf7b670586d 100644 > > --- a/mm/huge_memory.c > > +++ b/mm/huge_memory.c > > @@ -92,7 +92,7 @@ unsigned long huge_anon_orders_madvise __read_mostly; > > unsigned long huge_anon_orders_inherit __read_mostly; > > static bool anon_orders_configured __initdata; > > > > -static inline bool file_thp_enabled(struct vm_area_struct *vma) > > +static inline bool file_thp_enabled(const struct vm_area_struct *vma) > > { > > struct inode *inode; > > > > @@ -118,6 +118,67 @@ static bool vma_is_special_huge(const struct vm_area_struct *vma) > > return vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT); > > } > > > > +static bool vma_bypass_thp_tuneables_file(const struct vm_area_struct *vma, > > + enum tva_type type) > > +{ > > + const bool has_huge_fault = vma->vm_ops->huge_fault; > > + > > + /* MADV_COLLAPSE ignores tuneables. */ > > + if (type == TVA_FORCED_COLLAPSE) > > + return true; > > + /* Huge PFN mappings are uncompactable so the policy doesn't apply. */ > > + if (vma_test(vma, VMA_PFNMAP_BIT) && has_huge_fault) > > + return true; > > + return false; > > +} > > + > > +static bool vma_thp_tuneables_allow_file(vm_flags_t vm_flags) > > +{ > > + /* THP=always? */ > > + if (hugepage_global_always()) > > + return true; > > + /* THP=madvise and marked MADV_HUGEPAGE? */ > > + if (hugepage_global_enabled() && (vm_flags & VM_HUGEPAGE)) > > + return true; > > + return false; > > +} > > + > > +static bool vma_check_thp_tuneables_file(const struct vm_area_struct *vma, > > + vm_flags_t vm_flags, enum tva_type type) > > +{ > > + return vma_bypass_thp_tuneables_file(vma, type) || > > + vma_thp_tuneables_allow_file(vm_flags); > > +} > > + > > +static bool vma_can_map_huge_file(const struct vm_area_struct *vma, > > + vm_flags_t vm_flags, enum tva_type type) > > +{ > > + const bool has_huge_fault = vma->vm_ops->huge_fault; > > + > > + /* > > + * Enforce THP collapse requirements as necessary. Anonymous vmas > > + * were already handled in thp_vma_allowable_orders(). > > + */ > > + if (!vma_check_thp_tuneables_file(vma, vm_flags, type)) > > + return false; > > + > > + switch (type) { > > + case TVA_PAGEFAULT: > > + /* > > + * Trust that ->huge_fault() handlers know what they are doing > > + * in fault path. > > + */ > > + return has_huge_fault; > > + case TVA_SMAPS: > > + if (has_huge_fault) > > + return true; > > + fallthrough; > > + default: > > + /* Only regular file is valid in collapse path. */ > > + return file_thp_enabled(vma); > > + } > > +} > > + > > unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, > > vm_flags_t vm_flags, > > enum tva_type type, > > @@ -190,27 +251,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, > > vma, vma_start_pgoff(vma), 0, > > forced_collapse); > > > > - if (!vma_is_anonymous(vma)) { > > - /* > > - * Enforce THP collapse requirements as necessary. Anonymous vmas > > - * were already handled in thp_vma_allowable_orders(). > > - */ > > - if (!forced_collapse && > > - (!hugepage_global_enabled() || (!(vm_flags & VM_HUGEPAGE) && > > - !hugepage_global_always()))) > > - return 0; > > - > > - /* > > - * Trust that ->huge_fault() handlers know what they are doing > > - * in fault path. > > - */ > > - if (((in_pf || smaps)) && vma->vm_ops->huge_fault) > > - return orders; > > - /* Only regular file is valid in collapse path */ > > - if (((!in_pf || smaps)) && file_thp_enabled(vma)) > > - return orders; > > - return 0; > > - } > > + if (!vma_is_anonymous(vma)) > > + return vma_can_map_huge_file(vma, vm_flags, type) ? orders : 0; > > I don't have the time to find out how these functions ended up being named, I'll > have to trust that it will be alright. :-O > > Adding to what Zi say, vma_can_map_huge_file is also confusing, no? Yeah I agree, I had a model in my head as to why it was an OK name then when Zi raised it I realised that my naming scheme was, in fact, terrible :) > > are we mapping a huge file? :) Lol... hugetlb maybe? HW poision too? Perhaps? OK you've only been back for a day I should stop using these swear words :P > > -- > Cheers, > > David -- Cheers, Lorenzo