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(p200300d82f2c1e001e1e7a32e7986457.dip0.t-ipconnect.de. [2003:d8:2f2c:1e00:1e1e:7a32:e798:6457]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4532e24420csm10772795e9.20.2025.06.12.00.06.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 12 Jun 2025 00:06:05 -0700 (PDT) Message-ID: <0302ec30-856d-4e4b-be7b-1105966733e8@redhat.com> Date: Thu, 12 Jun 2025 09:06:04 +0200 Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio_pmd() To: Alistair Popple Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Dan Williams , Oscar Salvador References: <20250611120654.545963-1-david@redhat.com> <20250611120654.545963-3-david@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 0ZBDc1-uxtI1uSNjOGPJ_IBB--YM9jcu22lGW69St1w_1749711967 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12.06.25 04:17, Alistair Popple wrote: > On Wed, Jun 11, 2025 at 02:06:53PM +0200, David Hildenbrand wrote: >> Marking PMDs that map a "normal" refcounted folios as special is >> against our rules documented for vm_normal_page(). >> >> Fortunately, there are not that many pmd_special() check that can be >> mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that >> would get this wrong right now are rather harmless: e.g., none so far >> bases decisions whether to grab a folio reference on that decision. >> >> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big >> implications as it seems. >> >> Getting this right will get more important as we use >> folio_normal_page_pmd() in more places. >> >> Fix it by teaching insert_pfn_pmd() to properly handle folios and >> pfns -- moving refcount/mapcount/etc handling in there, renaming it to >> insert_pmd(), and distinguishing between both cases using a new simple >> "struct folio_or_pfn" structure. >> >> Use folio_mk_pmd() to create a pmd for a folio cleanly. >> >> Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()") >> Signed-off-by: David Hildenbrand >> --- >> mm/huge_memory.c | 58 ++++++++++++++++++++++++++++++++---------------- >> 1 file changed, 39 insertions(+), 19 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 49b98082c5401..7e3e9028873e5 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -1372,9 +1372,17 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf) >> return __do_huge_pmd_anonymous_page(vmf); >> } >> >> -static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, >> - pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write, >> - pgtable_t pgtable) >> +struct folio_or_pfn { >> + union { >> + struct folio *folio; >> + pfn_t pfn; >> + }; >> + bool is_folio; >> +}; > > I know it's simple, but I'm still not a fan particularly as these types of > patterns tend to proliferate once introduced. See below for a suggestion. It's much better than abusing pfn_t for folios -- and I don't particularly see a problem with this pattern here as long as it stays in this file. > >> +static int insert_pmd(struct vm_area_struct *vma, unsigned long addr, >> + pmd_t *pmd, struct folio_or_pfn fop, pgprot_t prot, >> + bool write, pgtable_t pgtable) >> { >> struct mm_struct *mm = vma->vm_mm; >> pmd_t entry; >> @@ -1382,8 +1390,11 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, >> lockdep_assert_held(pmd_lockptr(mm, pmd)); >> >> if (!pmd_none(*pmd)) { >> + const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) : >> + pfn_t_to_pfn(fop.pfn); >> + >> if (write) { >> - if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) { >> + if (pmd_pfn(*pmd) != pfn) { >> WARN_ON_ONCE(!is_huge_zero_pmd(*pmd)); >> return -EEXIST; >> } >> @@ -1396,11 +1407,19 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, >> return -EEXIST; >> } >> >> - entry = pmd_mkhuge(pfn_t_pmd(pfn, prot)); >> - if (pfn_t_devmap(pfn)) >> - entry = pmd_mkdevmap(entry); >> - else >> - entry = pmd_mkspecial(entry); >> + if (fop.is_folio) { >> + entry = folio_mk_pmd(fop.folio, vma->vm_page_prot); >> + >> + folio_get(fop.folio); >> + folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma); >> + add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR); >> + } else { >> + entry = pmd_mkhuge(pfn_t_pmd(fop.pfn, prot)); >> + if (pfn_t_devmap(fop.pfn)) >> + entry = pmd_mkdevmap(entry); >> + else >> + entry = pmd_mkspecial(entry); >> + } > > Could we change insert_pfn_pmd() to insert_pmd_entry() and have callers call > something like pfn_to_pmd_entry() or folio_to_pmd_entry() to create the pmd_t > entry as appropriate, which is then passed to insert_pmd_entry() to do the bits > common to both? Yeah, I had that idea as well but discarded it, because the refcounting+mapcounting handling is better placed where we are actually inserting the pmd (not possibly only upgrading permissions of an existing mapping). Avoid 4-line comments as the one we are removing in patch #3 ... -- Cheers, David / dhildenb