Re: [PATCHv2] mm/huge_memory: do not add dropped split tail folios to LRU

[Zhaoyang Huang]

On Sat, Jun 13, 2026 at 7:46 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On 12 Jun 2026, at 19:38, Zhaoyang Huang wrote:
>
> > On Fri, Jun 12, 2026 at 10:12 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
> >>
> >> On 11 Jun 2026, at 22:34, zhaoyang.huang wrote:
> >>
> >>> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >>>
> >>> The kernel panics are keeping to be reported especially when the f2fs
> >>> partition get almost full. By investigation, we find that the reason is
> >>> one f2fs page got freed to buddy without being deleted from LRU and the
> >>> root cause is the race happened in [2] which is enrolled by this commit.
> >>> We solve this issue by reverting a f2fs commit 9609dd704725 ("f2fs: remove
> >>> non-uptodate folio from the page cache in move_data_block").
> >>>
> >>> There are 3 race processes in this scenario, please find below for their
> >>> main activities. However, by further investigation over the code, I
> >>> think there is a common race window for the truncated folios between
> >>> split_folio_to_order and folio_isolate_lru, where the folios lost the
> >>> refcount on page cache and remains the transient one of the split
> >>> caller, under which the folio could enter free path and compete with the
> >>> isolation process. This commit would like to suggest to have the folios
> >>> beyond EOF stay out of LRU.
> >>>
> >>> Split:
> >>> split_folio_to_order() can split the big folio into individual pages and
> >>> put the resulting subpages back on the LRU.  For tail pages beyond EOF,
> >>> split removes them from the page cache and drops their page-cache
> >>> references.  A tail page can then remain on the LRU with PG_lru set while
> >>> holding only the split caller's temporary reference.  When
> >>> free_folio_and_swap_cache() drops that final reference, the page enters
> >>> the final folio_put() release path.
> >>>
> >>> Truncate:
> >>> The changed code in move_data_block() lets the GC path evict the tail-end
> >>> folio from the page cache through folio_end_dropbehind().  Once
> >>> folio_unmap_invalidate() removes the folio from mapping->i_pages, the
> >>> page-cache references for all pages in the folio are dropped.  The folio
> >>> is then kept alive only by temporary external references, which allows a
> >>> later split to operate on a folio whose subpages are no longer protected
> >>> by page-cache references.
> >>>
> >>> Isolate:
> >>> In parallel, folio_isolate_lru() can observe the same tail page with a
> >>> non-zero refcount and PG_lru set.  It clears PG_lru before taking its own
> >>> reference.  If this races with the final folio_put() from the split path,
> >>> __folio_put() sees PG_lru already cleared and skips lruvec_del_folio().
> >>> The page is then freed back to the allocator while its lru links are
> >>> still present in the LRU list.  A later LRU operation on a neighboring
> >>> page detects the stale link and reports list corruption.
> >>>
> >>> [1]
> >>> [   22.486082] list_del corruption. next->prev should be fffffffec10e0ac8, but was dead000000000122. (next=fffffffec10e0a88)
> >>> [   22.486130] ------------[ cut here ]------------
> >>> [   22.486134] kernel BUG at lib/list_debug.c:67!
> >>> [   22.486141] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
> >>> [   22.488502] Tainted: [W]=WARN, [O]=OOT_MODULE
> >>> [   22.488506] Hardware name: Spreadtrum UMS9230 1H10 SoC (DT)
> >>> [   22.488511] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >>> [   22.488517] pc : __list_del_entry_valid_or_report+0x14c/0x154
> >>> [   22.488531] lr : __list_del_entry_valid_or_report+0x14c/0x154
> >>> [   22.488539] sp : ffffffc08006b830
> >>> [   22.488542] x29: ffffffc08006b868 x28: 0000000000003020 x27: 0000000000000000
> >>> [   22.488553] x26: 0000000000000000 x25: 0000000000000004 x24: fffffffec10e0ac0
> >>> [   22.488564] x23: 00000000000000e8 x22: 0000000000000024 x21: dead000000000122
> >>> [   22.488574] x20: fffffffec10e0a88 x19: fffffffec10e0ac8 x18: ffffffc080061060
> >>> [   22.488585] x17: 20747562202c3863 x16: 6130653031636566 x15: 0000000000000058
> >>> [   22.488595] x14: 0000000000000004 x13: ffffff80f91e0000 x12: 0000000000000003
> >>> [   22.488605] x11: 0000000000000003 x10: 0000000000000001 x9 : ffe85721f0e25f00
> >>> [   22.488615] x8 : ffe85721f0e25f00 x7 : 0000000000000000 x6 : 6c65645f7473696c
> >>> [   22.488625] x5 : ffffffed39b23026 x4 : 0000000000000000 x3 : 0000000000000010
> >>> [   22.488636] x2 : 0000000000000000 x1 : 0000000000000000 x0 : 000000000000006d
> >>> [   22.488647] Call trace:
> >>> [   22.488651]  __list_del_entry_valid_or_report+0x14c/0x154 (P)
> >>> [   22.488661]  __folio_put+0x2bc/0x434
> >>> [   22.488670]  folio_put+0x28/0x58
> >>> [   22.488678]  do_garbage_collect+0x1a34/0x2584
> >>> [   22.488689]  f2fs_gc+0x230/0x9b4
> >>> [   22.488697]  f2fs_fallocate+0xb90/0xdf4
> >>> [   22.488706]  vfs_fallocate+0x1b4/0x2bc
> >>> [   22.488716]  __arm64_sys_fallocate+0x44/0x78
> >>> [   22.488725]  invoke_syscall+0x58/0xe4
> >>> [   22.488732]  do_el0_svc+0x48/0xdc
> >>> [   22.488739]  el0_svc+0x3c/0x98
> >>> [   22.488747]  el0t_64_sync_handler+0x20/0x130
> >>> [   22.488754]  el0t_64_sync+0x1c4/0x1c8
> >>>
> >>> [2]
> >>> *F: big folio before split
> >>> *T: tail folio after split
> >>> CPU0 (f2fs GC)              CPU1 (split_folio_to_order)          CPU2 (folio_isolate_lru)
> >>> *F: pagecache refs = n
> >>> *F: extra refs = split
> >>> *F: PG_lru set, mapping != NULL
> >>>                             split_folio_to_order(F)
> >>>                               folio_ref_freeze(F, 1)
> >>>                               ...
> >>>                               lru_add_split_folio(T)
> >>>                                 list_add_tail(&T->lru, &F->lru)
> >>>                                 folio_set_lru(T)
> >>>                                 folio_unlock(T)
> >>>                               /* T PageLRU set */
> >>>
> >>> *T: pagecache refs = 1
> >>> *T: extra refs = GC + split
> >>> *T: PG_lru set, mapping != NULL
> >>>
> >>> move_data_block()
> >>> folio = f2fs_grab_cache_folio(T)
> >>
> >> Getting T from f2fs_grab_cache_folio() via __filemap_get_folio() should
> >> be impossible, since the xarray should either have the original folio F
> >> or folios are not EOF. But it reminds me a bug I fixed recently.
> > The folio be out of EOF is caused by the coming folio_end_dropbehind, right?
>
> To be specific, __filemap_get_folio() can only see:
> 1. the original folio before the split, because only the original folio
> is present in the xarray as a multi-index entry;
>
> 2. after-split folios that are not EOF, because EOF folios are not added
> to the xarray[1] for __filemap_get_folio() to get.
>
> That is why I said getting EOF tail folios from __filemap_get_folio()
> is impossible.
>
> [1] https://elixir.bootlin.com/linux/v7.0.12/source/mm/huge_memory.c#L3884
understood. Could the truncate of the file happen after the split
since the callstack is launched by vfs_fallocate.

> >>
> >> Does your v6.18 kernel have this commit[1], which fixes an xarray issue
> >> in folio_split()? If not, can you give it a try and see if the issue
> >> goes away?
> > Yes, this patch is on the tree.
> >>
> >>
> >> [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v6.18.35&id=08b2b65c63bb26dbb2a4e2adc2ce96e2929b8b60
> >>
> >>> ...
> >>> __folio_set_dropbehind(T)
> >>> folio_unlock(T)
> >>> folio_end_dropbehind(T)
> >>>   folio_unmap_invalidate(T)
> >>>     __filemap_remove_folio(T)
> >>>     folio_put_refs(T, 1)
> >>> folio_put(T)
> >>>
> >>> *T: pagecache refs = 0
> >>> *T: extra refs = split
> >>> *T: PG_lru set, mapping == NULL
> >>>                             free_folio_and_swap_cache(T)
> >>>                               folio_put_testzero(T)
> >>>                                 /* refcount: 1 -> 0 */
> >>>
> >>> *T: pagecache refs = 0
> >>> *T: extra refs = isolate
> >>> *T: PG_lru set, mapping == NULL
> >>>                                                                   folio_isolate_lru(T)
> >>>                                                                     folio_test_clear_lru(T)
> >>>                                 __folio_put(T)
> >>>                                   __page_cache_release(T)
> >>>                                     folio_test_lru(T) == false
> >>>                                     /* skip lruvec_del_folio(T) */
> >>>                                   free_frozen_pages(T)
> >>>                                                                   folio_get(T)
> >>>                                                                   lruvec_del_folio(T)
> >>> later:
> >>>   list_del(adjacent->lru)
> >>>     next == &T->lru
> >>>     next->prev == LIST_POISON / PCP freelist
> >>>     BUG
> >>>
> >>> Assisted-by: Cursor:claude-opus-4-8
> >>> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >>> ---
> >>> patchv2: update codes to eliminate bad page status
> >>> ---
> >>> ---
> >>>  mm/huge_memory.c | 22 +++++++++++++++++++++-
> >>>  1 file changed, 21 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >>> index 970e077019b7..c24c12f71157 100644
> >>> --- a/mm/huge_memory.c
> >>> +++ b/mm/huge_memory.c
> >>> @@ -3878,6 +3878,23 @@ static unsigned int folio_cache_ref_count(const struct folio *folio)
> >>>       return folio_nr_pages(folio);
> >>>  }
> >>>
> >>> +static void clear_dropped_split_folio_lru_flags(struct folio *folio)
> >>> +{
> >>> +     /*
> >>> +      * __split_folio_to_order() clones these LRU state bits from the
> >>> +      * original folio.  A folio that is dropped instead of being added to
> >>> +      * the LRU will not pass through lruvec_del_folio() and
> >>> +      * __folio_clear_lru_flags(), so clear the cloned state before it is
> >>> +      * freed back to the page allocator.
> >>> +      */
> >>> +     set_mask_bits(&folio->flags.f,
> >>> +                   (1UL << PG_referenced) | (1UL << PG_active) |
> >>> +                           (1UL << PG_workingset) |
> >>> +                           (1UL << PG_unevictable) | __PG_MLOCKED |
> >>> +                           LRU_GEN_MASK | LRU_REFS_MASK,
> >>> +                   0);
> >>> +}
> >>> +
> >>>  static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
> >>>                                            struct page *split_at, struct xa_state *xas,
> >>>                                            struct address_space *mapping, bool do_lru,
> >>> @@ -3958,6 +3975,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
> >>>               for (new_folio = folio_next(folio); new_folio != end_folio;
> >>>                    new_folio = next) {
> >>>                       unsigned long nr_pages = folio_nr_pages(new_folio);
> >>> +                     bool drop = mapping && new_folio->index >= end;
> >>>
> >>>                       next = folio_next(new_folio);
> >>>
> >>> @@ -3966,7 +3984,9 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
> >>>                       folio_ref_unfreeze(new_folio,
> >>>                                          folio_cache_ref_count(new_folio) + 1);
> >>>
> >>> -                     if (do_lru)
> >>> +                     if (drop)
> >>> +                             clear_dropped_split_folio_lru_flags(new_folio);
> >>> +                     else if (do_lru)
> >>>                               lru_add_split_folio(folio, new_folio, lruvec, list);
> >>>
> >>>                       /*
> >>> --
> >>> 2.25.1
> >>
> >>
> >> Best Regards,
> >> Yan, Zi
>
>
> --
> Best Regards,
> Yan, Zi