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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Ralph Campbell , Mika =?iso-8859-1?Q?Penttil=E4?= , Matthew Brost , Francois Dugast Subject: Re: [PATCH] mm/huge_memory.c: introduce folio_split_unmapped Message-ID: References: <202511151216.rhK2ItOb-lkp@intel.com> <661d1f3b-0f04-43fc-85ef-d29d492a2b06@nvidia.com> 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: <661d1f3b-0f04-43fc-85ef-d29d492a2b06@nvidia.com> On Thu, Nov 20, 2025 at 10:58:07AM +1100, Balbir Singh wrote: > On 11/19/25 23:32, Dan Carpenter wrote: > > Hi Balbir, > > > > kernel test robot noticed the following build warnings: > > > > url: https://github.com/intel-lab-lkp/linux/commits/Balbir-Singh/mm-huge_memory-c-introduce-folio_split_unmapped/20251114-093541 > > base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything > > patch link: https://lore.kernel.org/r/20251114012228.2634882-1-balbirs%40nvidia.com > > patch subject: [PATCH] mm/huge_memory.c: introduce folio_split_unmapped > > config: i386-randconfig-141-20251115 (https://download.01.org/0day-ci/archive/20251115/202511151216.rhK2ItOb-lkp@intel.com/config) > > compiler: gcc-12 (Debian 12.4.0-5) 12.4.0 > > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > > the same patch/commit), kindly add following tags > > | Reported-by: kernel test robot > > | Reported-by: Dan Carpenter > > | Closes: https://lore.kernel.org/r/202511151216.rhK2ItOb-lkp@intel.com/ > > > > smatch warnings: > > mm/huge_memory.c:4044 __folio_split() error: uninitialized symbol 'end'. > > mm/huge_memory.c:4052 __folio_split() error: we previously assumed 'mapping' could be null (see line 4046) > > > > Thanks for the report! > > If mapping is not NULL, end is initialized. More comments on UBSan below > > > vim +/end +4044 mm/huge_memory.c > > > > 6384dd1d18de7b Zi Yan 2025-03-07 3908 static int __folio_split(struct folio *folio, unsigned int new_order, > > 58729c04cf1092 Zi Yan 2025-03-07 3909 struct page *split_at, struct page *lock_at, > > f6b1f167ffe29f Balbir Singh 2025-11-14 3910 struct list_head *list, enum split_type split_type) > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3911 { > > 58729c04cf1092 Zi Yan 2025-03-07 3912 XA_STATE(xas, &folio->mapping->i_pages, folio->index); > > 6c7de9c83be68b Zi Yan 2025-07-18 3913 struct folio *end_folio = folio_next(folio); > > 5d65c8d758f259 Barry Song 2024-08-24 3914 bool is_anon = folio_test_anon(folio); > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3915 struct address_space *mapping = NULL; > > 5d65c8d758f259 Barry Song 2024-08-24 3916 struct anon_vma *anon_vma = NULL; > > 8ec26327c18e1d Wei Yang 2025-10-10 3917 int old_order = folio_order(folio); > > 6c7de9c83be68b Zi Yan 2025-07-18 3918 struct folio *new_folio, *next; > > 391dc7f40590d7 Zi Yan 2025-07-18 3919 int nr_shmem_dropped = 0; > > 391dc7f40590d7 Zi Yan 2025-07-18 3920 int remap_flags = 0; > > 504e070dc08f75 Yang Shi 2021-06-15 3921 int extra_pins, ret; > > 006d3ff27e884f Hugh Dickins 2018-11-30 3922 pgoff_t end; > > 478d134e9506c7 Xu Yu 2022-04-28 3923 bool is_hzp; > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3924 > > 714b056c832106 Zi Yan 2025-07-17 3925 VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); > > 714b056c832106 Zi Yan 2025-07-17 3926 VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio); > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3927 > > 58729c04cf1092 Zi Yan 2025-03-07 3928 if (folio != page_folio(split_at) || folio != page_folio(lock_at)) > > 1412ecb3d256e5 Zi Yan 2024-03-07 3929 return -EINVAL; > > 1412ecb3d256e5 Zi Yan 2024-03-07 3930 > > 8ec26327c18e1d Wei Yang 2025-10-10 3931 if (new_order >= old_order) > > c010d47f107f60 Zi Yan 2024-02-26 3932 return -EINVAL; > > 58729c04cf1092 Zi Yan 2025-03-07 3933 > > aa27253af32c74 Wei Yang 2025-11-06 3934 if (!folio_split_supported(folio, new_order, split_type, /* warn = */ true)) > > 6a50c9b512f773 Ran Xiaokai 2024-06-07 3935 return -EINVAL; > > c010d47f107f60 Zi Yan 2024-02-26 3936 > > 5beaee54a324ba Matthew Wilcox (Oracle 2024-03-26 3937) is_hzp = is_huge_zero_folio(folio); > > 4737edbbdd4958 Naoya Horiguchi 2023-04-06 3938 if (is_hzp) { > > 4737edbbdd4958 Naoya Horiguchi 2023-04-06 3939 pr_warn_ratelimited("Called split_huge_page for huge zero page\n"); > > 478d134e9506c7 Xu Yu 2022-04-28 3940 return -EBUSY; > > 4737edbbdd4958 Naoya Horiguchi 2023-04-06 3941 } > > 478d134e9506c7 Xu Yu 2022-04-28 3942 > > 3e9a13daa61253 Matthew Wilcox (Oracle 2022-09-02 3943) if (folio_test_writeback(folio)) > > 59807685a7e77e Ying Huang 2017-09-06 3944 return -EBUSY; > > 59807685a7e77e Ying Huang 2017-09-06 3945 > > 5d65c8d758f259 Barry Song 2024-08-24 3946 if (is_anon) { > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3947 /* > > c1e8d7c6a7a682 Michel Lespinasse 2020-06-08 3948 * The caller does not necessarily hold an mmap_lock that would > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3949 * prevent the anon_vma disappearing so we first we take a > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3950 * reference to it and then lock the anon_vma for write. This > > 2f031c6f042cb8 Matthew Wilcox (Oracle 2022-01-29 3951) * is similar to folio_lock_anon_vma_read except the write lock > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3952 * is taken to serialise against parallel split or collapse > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3953 * operations. > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3954 */ > > 29eea9b5a9c9ec Matthew Wilcox (Oracle 2022-09-02 3955) anon_vma = folio_get_anon_vma(folio); > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3956 if (!anon_vma) { > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3957 ret = -EBUSY; > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3958 goto out; > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3959 } > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 3960 anon_vma_lock_write(anon_vma); > > 3d4c0d98eb8572 Balbir Singh 2025-10-01 3961 mapping = NULL; > > > > end is not initialized for anonymous folios. > > > > Yes > > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3962 } else { > > e220917fa50774 Luis Chamberlain 2024-08-22 3963 unsigned int min_order; > > 6a3edd29395631 Yin Fengwei 2022-08-10 3964 gfp_t gfp; > > 6a3edd29395631 Yin Fengwei 2022-08-10 3965 > > 3e9a13daa61253 Matthew Wilcox (Oracle 2022-09-02 3966) mapping = folio->mapping; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3967 > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3968 /* Truncated ? */ > > 6384dd1d18de7b Zi Yan 2025-03-07 3969 /* > > 6384dd1d18de7b Zi Yan 2025-03-07 3970 * TODO: add support for large shmem folio in swap cache. > > 6384dd1d18de7b Zi Yan 2025-03-07 3971 * When shmem is in swap cache, mapping is NULL and > > 6384dd1d18de7b Zi Yan 2025-03-07 3972 * folio_test_swapcache() is true. > > 6384dd1d18de7b Zi Yan 2025-03-07 3973 */ > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3974 if (!mapping) { > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3975 ret = -EBUSY; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3976 goto out; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3977 } > > baa355fd331424 Kirill A. Shutemov 2016-07-26 3978 > > e220917fa50774 Luis Chamberlain 2024-08-22 3979 min_order = mapping_min_folio_order(folio->mapping); > > e220917fa50774 Luis Chamberlain 2024-08-22 3980 if (new_order < min_order) { > > e220917fa50774 Luis Chamberlain 2024-08-22 3981 ret = -EINVAL; > > e220917fa50774 Luis Chamberlain 2024-08-22 3982 goto out; > > e220917fa50774 Luis Chamberlain 2024-08-22 3983 } > > e220917fa50774 Luis Chamberlain 2024-08-22 3984 > > 6a3edd29395631 Yin Fengwei 2022-08-10 3985 gfp = current_gfp_context(mapping_gfp_mask(mapping) & > > 6a3edd29395631 Yin Fengwei 2022-08-10 3986 GFP_RECLAIM_MASK); > > 6a3edd29395631 Yin Fengwei 2022-08-10 3987 > > 0201ebf274a306 David Howells 2023-06-28 3988 if (!filemap_release_folio(folio, gfp)) { > > 6a3edd29395631 Yin Fengwei 2022-08-10 3989 ret = -EBUSY; > > 6a3edd29395631 Yin Fengwei 2022-08-10 3990 goto out; > > 6a3edd29395631 Yin Fengwei 2022-08-10 3991 } > > 6a3edd29395631 Yin Fengwei 2022-08-10 3992 > > 3c844d850e4486 Wei Yang 2025-11-06 3993 if (split_type == SPLIT_TYPE_UNIFORM) { > > 58729c04cf1092 Zi Yan 2025-03-07 3994 xas_set_order(&xas, folio->index, new_order); > > 8ec26327c18e1d Wei Yang 2025-10-10 3995 xas_split_alloc(&xas, folio, old_order, gfp); > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 3996) if (xas_error(&xas)) { > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 3997) ret = xas_error(&xas); > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 3998) goto out; > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 3999) } > > 58729c04cf1092 Zi Yan 2025-03-07 4000 } > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 4001) > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4002 anon_vma = NULL; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4003 i_mmap_lock_read(mapping); > > 006d3ff27e884f Hugh Dickins 2018-11-30 4004 > > 006d3ff27e884f Hugh Dickins 2018-11-30 4005 /* > > 58729c04cf1092 Zi Yan 2025-03-07 4006 *__split_unmapped_folio() may need to trim off pages beyond > > 58729c04cf1092 Zi Yan 2025-03-07 4007 * EOF: but on 32-bit, i_size_read() takes an irq-unsafe > > 58729c04cf1092 Zi Yan 2025-03-07 4008 * seqlock, which cannot be nested inside the page tree lock. > > 58729c04cf1092 Zi Yan 2025-03-07 4009 * So note end now: i_size itself may be changed at any moment, > > 58729c04cf1092 Zi Yan 2025-03-07 4010 * but folio lock is good enough to serialize the trimming. > > 006d3ff27e884f Hugh Dickins 2018-11-30 4011 */ > > 006d3ff27e884f Hugh Dickins 2018-11-30 4012 end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE); > > d144bf6205342a Hugh Dickins 2021-09-02 4013 if (shmem_mapping(mapping)) > > d144bf6205342a Hugh Dickins 2021-09-02 4014 end = shmem_fallocend(mapping->host, end); > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4015 } > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4016 > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4017 /* > > 684555aacc90d7 Matthew Wilcox (Oracle 2022-09-02 4018) * Racy check if we can split the page, before unmap_folio() will > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4019 * split PMDs > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4020 */ > > 8710f6ed34e7bc David Hildenbrand 2024-08-02 4021 if (!can_split_folio(folio, 1, &extra_pins)) { > > fd4a7ac32918d3 Baolin Wang 2022-10-24 4022 ret = -EAGAIN; > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4023 goto out_unlock; > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4024 } > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4025 > > 684555aacc90d7 Matthew Wilcox (Oracle 2022-09-02 4026) unmap_folio(folio); > > e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4027 > > b6769834aac1d4 Alex Shi 2020-12-15 4028 /* block interrupt reentry in xa_lock and spinlock */ > > b6769834aac1d4 Alex Shi 2020-12-15 4029 local_irq_disable(); > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4030 if (mapping) { > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4031 /* > > 3e9a13daa61253 Matthew Wilcox (Oracle 2022-09-02 4032) * Check if the folio is present in page cache. > > 3e9a13daa61253 Matthew Wilcox (Oracle 2022-09-02 4033) * We assume all tail are present too, if folio is there. > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4034 */ > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 4035) xas_lock(&xas); > > 6b24ca4a1a8d4e Matthew Wilcox (Oracle 2020-06-27 4036) xas_reset(&xas); > > 391dc7f40590d7 Zi Yan 2025-07-18 4037 if (xas_load(&xas) != folio) { > > 391dc7f40590d7 Zi Yan 2025-07-18 4038 ret = -EAGAIN; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4039 goto fail; > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4040 } > > 391dc7f40590d7 Zi Yan 2025-07-18 4041 } > > baa355fd331424 Kirill A. Shutemov 2016-07-26 4042 > > f6b1f167ffe29f Balbir Singh 2025-11-14 4043 ret = __folio_freeze_and_split_unmapped(folio, new_order, split_at, &xas, mapping, > > f6b1f167ffe29f Balbir Singh 2025-11-14 @4044 true, list, split_type, end, extra_pins); > > ^^^ > > Passing uninitialized variables isn't allowed unless the function is > > inlined. It triggers a UBSan warning at runtime as well. > > end is expected to be unused when uninitialized, are you suggesting we need to have a default value even if unused inside the function. > My daily build has UBSan enabled, I'll try again > > CONFIG_UBSAN=y > CONFIG_CC_HAS_UBSAN_ARRAY_BOUNDS=y > # CONFIG_UBSAN_BOUNDS is not set > CONFIG_UBSAN_SHIFT=y > # CONFIG_UBSAN_UNREACHABLE is not set > # CONFIG_UBSAN_BOOL is not set > # CONFIG_UBSAN_ENUM is not set > Huh. It's interesting that UBSan doesn't complain. Potentially, the function is inlined? Technically, passing uninitialized variables to a function is undefined behavior in C. But in practical terms (and it make Linus cross that the C standard doesn't account for this), when a function is inlined then that shouldn't really be undefined. But otherwise, yes, passing uninitialized variables is not allowed, even when they're not used. Everyone sensible is going to set CONFIG_INIT_STACK_ALL_ZERO for production systems so initializing variables to zero doesn't affect anything. regards, dan carpenter