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(p200300d82f3c3a00566226b33e5d438e.dip0.t-ipconnect.de. [2003:d8:2f3c:3a00:5662:26b3:3e5d:438e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b5e8e0d732sm4725648f8f.52.2025.07.11.07.40.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Jul 2025 07:40:50 -0700 (PDT) Message-ID: <3223bd3b-38cc-45d1-8080-d62117df0e85@redhat.com> Date: Fri, 11 Jul 2025 16:40:49 +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: [PATCH] mm/huge_memory: move unrelated code out of __split_unmapped_folio() To: Zi Yan Cc: Balbir Singh , linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , linux-kernel@vger.kernel.org References: <20250711030259.3574392-1-ziy@nvidia.com> <2AE055C4-BFFE-4B61-A96A-6DE227422C7B@nvidia.com> From: David Hildenbrand Content-Language: en-US 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: <2AE055C4-BFFE-4B61-A96A-6DE227422C7B@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 11.07.25 16:37, Zi Yan wrote: > On 11 Jul 2025, at 2:41, David Hildenbrand wrote: > >> On 11.07.25 05:02, Zi Yan wrote: >>> remap(), folio_ref_unfreeze(), lru_add_split_folio() are not related to >>> splitting unmapped folio operations. Move them out to the caller, so that >>> __split_unmapped_folio() only splits unmapped folios. This makes >>> __split_unmapped_folio() reusable. >>> >>> Convert VM_BUG_ON(mapping) to use VM_WARN_ON_ONCE_FOLIO(). >>> >>> Signed-off-by: Zi Yan >>> --- >>> Based on the prior discussion[1], this patch makes >>> __split_unmapped_folio() reusable for splitting unmapped folios without >>> adding a new boolean unmapped parameter to guard mapping related code. >>> >>> Another potential benefit is that __split_unmapped_folio() could be >>> called on after-split folios by __folio_split() to perform new split >>> methods. For example, at deferred split time, unmapped subpages can >>> scatter arbitrarily within a large folio, neither uniform nor non-uniform >>> split can maximize after-split folio orders for mapped subpages. >>> Hopefully, performing __split_unmapped_folio() multiple times can >>> achieve the optimal split result. >>> >>> It passed mm selftests. >>> >>> [1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/ >>> --- >>> >>> mm/huge_memory.c | 275 ++++++++++++++++++++++++----------------------- >>> 1 file changed, 139 insertions(+), 136 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 3eb1c34be601..d97145dfa6c8 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -3396,10 +3396,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order, >>> * order - 1 to new_order). >>> * @split_at: in buddy allocator like split, the folio containing @split_at >>> * will be split until its order becomes @new_order. >>> - * @lock_at: the folio containing @lock_at is left locked for caller. >>> - * @list: the after split folios will be added to @list if it is not NULL, >>> - * otherwise to LRU lists. >>> - * @end: the end of the file @folio maps to. -1 if @folio is anonymous memory. >>> * @xas: xa_state pointing to folio->mapping->i_pages and locked by caller >>> * @mapping: @folio->mapping >>> * @uniform_split: if the split is uniform or not (buddy allocator like split) >>> @@ -3425,51 +3421,27 @@ static void __split_folio_to_order(struct folio *folio, int old_order, >>> * @page, which is split in next for loop. >>> * >>> * After splitting, the caller's folio reference will be transferred to the >>> - * folio containing @page. The other folios may be freed if they are not mapped. >>> - * >>> - * In terms of locking, after splitting, >>> - * 1. uniform split leaves @page (or the folio contains it) locked; >>> - * 2. buddy allocator like (non-uniform) split leaves @folio locked. >>> - * >>> + * folio containing @page. The caller needs to unlock and/or free after-split >>> + * folios if necessary. >>> * >>> * For !uniform_split, when -ENOMEM is returned, the original folio might be >>> * split. The caller needs to check the input folio. >>> */ >>> static int __split_unmapped_folio(struct folio *folio, int new_order, >>> - struct page *split_at, struct page *lock_at, >>> - struct list_head *list, pgoff_t end, >>> - struct xa_state *xas, struct address_space *mapping, >>> - bool uniform_split) >>> + struct page *split_at, struct xa_state *xas, >>> + struct address_space *mapping, >>> + bool uniform_split) >> >> Use two-tabs indent please (like we already do, I assume). > > OK. I was using clang-format. It gave me this indentation. >> >> [...] >> >>> @@ -3706,11 +3599,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order, >>> { >>> struct deferred_split *ds_queue = get_deferred_split_queue(folio); >>> XA_STATE(xas, &folio->mapping->i_pages, folio->index); >>> + struct folio *next_folio = folio_next(folio); >>> bool is_anon = folio_test_anon(folio); >>> struct address_space *mapping = NULL; >>> struct anon_vma *anon_vma = NULL; >>> int order = folio_order(folio); >>> + struct folio *new_folio, *next; >>> int extra_pins, ret; >>> + int nr_shmem_dropped = 0; >>> pgoff_t end; >>> bool is_hzp; >>> @@ -3833,13 +3729,18 @@ static int __folio_split(struct folio *folio, unsigned int new_order, >>> */ >>> xas_lock(&xas); >>> xas_reset(&xas); >>> - if (xas_load(&xas) != folio) >>> + if (xas_load(&xas) != folio) { >>> + ret = -EAGAIN; >>> goto fail; >>> + } >>> } >>> /* Prevent deferred_split_scan() touching ->_refcount */ >>> spin_lock(&ds_queue->split_queue_lock); >>> if (folio_ref_freeze(folio, 1 + extra_pins)) { >>> + struct address_space *swap_cache = NULL; >>> + struct lruvec *lruvec; >>> + >>> if (folio_order(folio) > 1 && >>> !list_empty(&folio->_deferred_list)) { >>> ds_queue->split_queue_len--; >>> @@ -3873,18 +3774,120 @@ static int __folio_split(struct folio *folio, unsigned int new_order, >>> } >>> } >>> - ret = __split_unmapped_folio(folio, new_order, >>> - split_at, lock_at, list, end, &xas, mapping, >>> - uniform_split); >>> + if (folio_test_swapcache(folio)) { >>> + if (mapping) { >>> + VM_WARN_ON_ONCE_FOLIO(mapping, folio); >>> + ret = -EINVAL; >>> + goto fail; >>> + } >>> + >>> + /* >>> + * a swapcache folio can only be uniformly split to >>> + * order-0 >>> + */ >>> + if (!uniform_split || new_order != 0) { >>> + ret = -EINVAL; >>> + goto fail; >>> + } >>> + >>> + swap_cache = swap_address_space(folio->swap); >>> + xa_lock(&swap_cache->i_pages); >>> + } >>> + >>> + /* lock lru list/PageCompound, ref frozen by page_ref_freeze */ >>> + lruvec = folio_lruvec_lock(folio); >>> + >>> + ret = __split_unmapped_folio(folio, new_order, split_at, &xas, >>> + mapping, uniform_split); >>> + >>> + /* >>> + * Unfreeze after-split folios and put them back to the right >>> + * list. @folio should be kept frozon until page cache entries >>> + * are updated with all the other after-split folios to prevent >>> + * others seeing stale page cache entries. >>> + */ >>> + for (new_folio = folio_next(folio); new_folio != next_folio; >>> + new_folio = next) { >>> + next = folio_next(new_folio); >>> + >>> + folio_ref_unfreeze( >>> + new_folio, >>> + 1 + ((mapping || swap_cache) ? >>> + folio_nr_pages(new_folio) : >>> + 0)); >> >> While we are at it, is a way to make this look less than an artistic masterpiece? :) >> >> expected_refs = ... >> folio_ref_unfreeze(new_folio, expected_refs). >> >> >> Can we already make use of folio_expected_ref_count() at that point? Mapcount should be 0 and the folio should be properly setup (e.g., anon, swapcache) IIRC. >> >> So maybe >> >> expected_refs = folio_expected_ref_count(new_folio) + 1; >> folio_ref_unfreeze(new_folio, expected_refs). >> >> Would do? > > I think so. Even further, I think we probably can get rid of can_split_folio()’s > pextra_pins and use folio_expected_ref_count() too. That will only do the right think if we know that the folio is not mapped and that there is no way it can get mapped concurrently. Otherwise, when freezing, we might ignore a mapping (where we should fail freezing). -- Cheers, David / dhildenb