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

[Zi Yan]

On 10 Jun 2026, at 13:25, Zi Yan wrote:

> On 10 Jun 2026, at 10:38, Zi Yan wrote:
>
>> 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.

Something is wrong here with the caller of folio_isolate_lru(), since
folio_isolate_lru() requires the caller to take an elevated refcount.
This means when entering folio_isolate_lru(), the EOF folio should have
at least refcount == 2, 1 from folio_split(), 1 from the caller of
folio_isolate_lru(). This should prevent the EOF folio being freed
by the parallel __folio_put().

Hi Zhaoyang, can you elaborate on the folio_isolate_lru() caller?

In addition (with the help of Claude), the race trace[2] below
looks invalid. It says split happens after folio_end_dropbehind(),
which sets folio->mapping to NULL, but __folio_split() returns -EBUSY
when folio->mapping is NULL in filemap_release_folio() check.
So the split cannot happen.

Now I am not sure if the bug report is valid or not. At least for
folio_split() and folio_isolate_lru(), the race should not exist.
But let me know if I miss anything.

>>>
>>> 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);
>
> Talked to Claude and find an accounting issue with this. Without putting
> EOF after-split folios back to LRU, they are not going through lruvec_del_folio(),
> which decreases NR_*_LRU counter along with removing the folio from LRU
> and it causes NR_*_LRU accounting errors. Note that the original folio
> is on LRU all the time and LRU counters are not modified and after the split
> the original folio size is decreased and the after-split folios need to
> be added back to LRU to keep the LRU counters right. We will need to adjust
> LRU accounting for (!mapping || new_folio->index < end) if we decide to
> not add them back to LRU.
>
>
> Best Regards,
> Yan, Zi


Best Regards,
Yan, Zi