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

[David Hildenbrand (Arm)]

On 6/12/26 04:34, 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)
> ...
> __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)

What I am still struggling with:

folio_isolate_lru() is documented to:

"Must be called with an elevated refcount on the folio.".

That is, there must be *something* keeping the folio alive before the
folio_get(). It could be a page table mapping with the PTL held (which does not
apply here).

Or it could be another prior folio_try_get() (e.g., do_migrate_range()).

But essentially, folio_isolate_lru() cannot race with __folio_put(). It would be
fundamentally broken.

-- 
Cheers,

David