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

[Zi Yan]

On 10 Jun 2026, at 8:50, David Hildenbrand (Arm) wrote:

> On 6/10/26 14:05, 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").
>
> But I assume, that other FSes can trigger this as well? Any insights?
>
>>
>> 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.
>>
>> 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.
>>
>> Split:
>> After the page-cache references are gone, 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.
>>
>> 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.
>
> Complicated mess :(
>
> So, folio_isolate_lru() really only requires the caller to hold a folio
> reference, which can happen given that we did the folio_ref_unfreeze(). It can,
> for example, be triggered by memory offlining or page migration.
>
> So we really want to not allow folio_isolate_lru() while we are still processing
> the folio.

Or we should defer adding split folios to LRU after unfreeze.

>
> What your patch does is, simply not add folios that we will drop from the page
> cache to the LRU?
>
>
> You should describe here how you are fixing it: "Let's fix it by..."
>
>>
>> [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]
>> CPU0 (f2fs GC)              CPU1 (split_folio_to_order)          CPU2 (folio_isolate_lru)
>>
>> F: pagecache refs = n
>> F: extra refs = GC + split
>> F: PG_lru set
>> move_data_block()
>> folio = f2fs_grab_cache_folio(F)
>> ...
>> __folio_set_dropbehind(F)
>> folio_unlock(F)
>> folio_end_dropbehind(F)
>>   folio_unmap_invalidate(F)
>>     __filemap_remove_folio(F)
>>     folio_put_refs(F, n)
>> folio_put(F)
>>                             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)
>>                               __filemap_remove_folio(T)
>>                               folio_put_refs(T, 1)
>>                               /* T refcount == 1, PageLRU set */
>>                             free_folio_and_swap_cache(T)
>>                               folio_put(T)
>>                                 /* refcount: 1 -> 0 */
>>                                                                   folio_isolate_lru(T)

If refcount is 0 at this point, VM_BUG_ON_FOLIO(!folio_ref_count(folio), folio) in
folio_isolate_lru() would be triggered. Maybe we could just return false in that case.

>>                                                                     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)

But in CPU2 (folio_isolate_lru), lruvec_del_folio(T) should remove T from LRU list.

>> later:
>>   list_del(adjacent->lru)
>>     next == &T->lru
>>     next->prev == LIST_POISON / PCP freelist
>>     BUG
>>

Why does CPU0 still see the stale link from adjacent?

>> Assisted-by: Cursor:claude-opus-4-8
>> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> I'm wondering if this has been broken the whole time, or if some rework allowed
> this to trigger.
>
> I assume the issue can be triggered for other FSes, and we want Fixes: + CC: stable?
>
> Looking into the history, I think we always unconditionally did the
> lru_add_split_folio()/lru_add_page_tail().
>
>> ---
>>  mm/huge_memory.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 970e077019b7..7465525a94a8 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3966,7 +3966,7 @@ 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 (do_lru && !(mapping && new_folio->index >= end))
>
> It might be clearer to write this as
>
> 	do_lru && (!mapping || new_folio->index < end)
>
> To match the page-cache check further below
>
> 	if (!mapping)
> 		continue
>
> 	...
> 	if (new_folio->index < end)
> 		...
>
>>  				lru_add_split_folio(folio, new_folio, lruvec, list);
>>
>>  			/*
>
> folio_check_splittable() makes sure that we have a mapping for non-anon folios.
> (no truncation). end is then only set for non-anon folios.
>
> @Zi, any thoughts?

The fix works but I feel that it is masking the race between folio_isolate_lru() and
folio_put(). I worry that the same issue might be triggered in other ways or
in new code if we do not fix the race.

To summarize my thoughts above:
1. adding frozen folios in LRU might be problematic, since folio_isolate_lru()
has a VM_BUG_ON_FOLIO() for it but still chooses to proceed the isolation.

2. the race analysis is not clear, since both folio_isolate_lru() and folio_put()
do lruvec_del_folio() if folio is on LRU. When list_del(adjacent->lru) sees
the stale link, the folio is already in buddy and page->lru is modified for
PageBuddy use? So even without CPU0, folio_isolate_lru()'s lruvec_del_folio()
can do the wrong thing on pages on buddy?


--
Best Regards,
Yan, Zi