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

Dan Carpenter
2025-11-19T12:33:07Z

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)

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.

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.

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.

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 }

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