Re: [PATCH] mm/huge_memory.c: introduce folio_split_unmapped

Balbir Singh
2025-11-19T23:58:25Z

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 <lkp@intel.com>
> | Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
> | 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

>
> 391dc7f40590d7 Zi Yan 2025-07-18 4045 fail:
> 6c7de9c83be68b Zi Yan 2025-07-18 @4046 if (mapping)
> 6c7de9c83be68b Zi Yan 2025-07-18 4047 xas_unlock(&xas);
> 6c7de9c83be68b Zi Yan 2025-07-18 4048
> 6c7de9c83be68b Zi Yan 2025-07-18 4049 local_irq_enable();
> 6c7de9c83be68b Zi Yan 2025-07-18 4050
> 391dc7f40590d7 Zi Yan 2025-07-18 4051 if (nr_shmem_dropped)
> 391dc7f40590d7 Zi Yan 2025-07-18 @4052 shmem_uncharge(mapping->host, nr_shmem_dropped);
>
> Smatch complains that mapping can be NULL, but this is false positive
> because nr_shmem_dropped is always zero.
>

Agreed

> 6c7de9c83be68b Zi Yan 2025-07-18 4053
> 958fea4c1e2eb6 Balbir Singh 2025-10-01 4054 if (!ret && is_anon && !folio_is_device_private(folio))
> 391dc7f40590d7 Zi Yan 2025-07-18 4055 remap_flags = RMP_USE_SHARED_ZEROPAGE;
> 958fea4c1e2eb6 Balbir Singh 2025-10-01 4056
> 8ec26327c18e1d Wei Yang 2025-10-10 4057 remap_page(folio, 1 << old_order, remap_flags);
> 6c7de9c83be68b Zi Yan 2025-07-18 4058
> 6c7de9c83be68b Zi Yan 2025-07-18 4059 /*
> 6c7de9c83be68b Zi Yan 2025-07-18 4060 * Unlock all after-split folios except the one containing
> 6c7de9c83be68b Zi Yan 2025-07-18 4061 * @lock_at page. If @folio is not split, it will be kept locked.
> 6c7de9c83be68b Zi Yan 2025-07-18 4062 */
> 391dc7f40590d7 Zi Yan 2025-07-18 4063 for (new_folio = folio; new_folio != end_folio; new_folio = next) {
> 6c7de9c83be68b Zi Yan 2025-07-18 4064 next = folio_next(new_folio);
> 6c7de9c83be68b Zi Yan 2025-07-18 4065 if (new_folio == page_folio(lock_at))
> 6c7de9c83be68b Zi Yan 2025-07-18 4066 continue;
> 6c7de9c83be68b Zi Yan 2025-07-18 4067
> 6c7de9c83be68b Zi Yan 2025-07-18 4068 folio_unlock(new_folio);
> 6c7de9c83be68b Zi Yan 2025-07-18 4069 /*
> 6c7de9c83be68b Zi Yan 2025-07-18 4070 * Subpages may be freed if there wasn't any mapping
> 6c7de9c83be68b Zi Yan 2025-07-18 4071 * like if add_to_swap() is running on a lru page that
> 6c7de9c83be68b Zi Yan 2025-07-18 4072 * had its mapping zapped. And freeing these pages
> 6c7de9c83be68b Zi Yan 2025-07-18 4073 * requires taking the lru_lock so we do the put_page
> 6c7de9c83be68b Zi Yan 2025-07-18 4074 * of the tail pages after the split is complete.
> 6c7de9c83be68b Zi Yan 2025-07-18 4075 */
> 6c7de9c83be68b Zi Yan 2025-07-18 4076 free_folio_and_swap_cache(new_folio);
> 6c7de9c83be68b Zi Yan 2025-07-18 4077 }
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4078
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4079 out_unlock:
> baa355fd331424 Kirill A. Shutemov 2016-07-26 4080 if (anon_vma) {
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4081 anon_vma_unlock_write(anon_vma);
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4082 put_anon_vma(anon_vma);
> baa355fd331424 Kirill A. Shutemov 2016-07-26 4083 }
> baa355fd331424 Kirill A. Shutemov 2016-07-26 4084 if (mapping)
> baa355fd331424 Kirill A. Shutemov 2016-07-26 4085 i_mmap_unlock_read(mapping);
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4086 out:
> 69a37a8ba1b408 Matthew Wilcox (Oracle 2022-06-08 4087) xas_destroy(&xas);
> 8ec26327c18e1d Wei Yang 2025-10-10 4088 if (old_order == HPAGE_PMD_ORDER)
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4089 count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
> 8ec26327c18e1d Wei Yang 2025-10-10 4090 count_mthp_stat(old_order, !ret ? MTHP_STAT_SPLIT : MTHP_STAT_SPLIT_FAILED);
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4091 return ret;
> e9b61f19858a5d Kirill A. Shutemov 2016-01-15 4092 }
>

Balbir