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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/10/26 14:05, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> 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.

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)
>                                                                     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@unisoc.com>

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?

-- 
Cheers,

David