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

Dan Carpenter
2025-11-20T05:26:12Z

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 <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
>

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