On 19 Oct 2025, at 18:49, Balbir Singh wrote:
> On 10/19/25 19:19, Wei Yang wrote:
>> On Wed, Oct 01, 2025 at 04:57:02PM +1000, Balbir Singh wrote:
>> [...]
>>> static int __folio_split(struct folio *folio, unsigned int new_order,
>>> struct page *split_at, struct page *lock_at,
>>> - struct list_head *list, bool uniform_split)
>>> + struct list_head *list, bool uniform_split, bool unmapped)
>>> {
>>> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>> XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>> @@ -3765,13 +3757,15 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> * is taken to serialise against parallel split or collapse
>>> * operations.
>>> */
>>> - anon_vma = folio_get_anon_vma(folio);
>>> - if (!anon_vma) {
>>> - ret = -EBUSY;
>>> - goto out;
>>> + if (!unmapped) {
>>> + anon_vma = folio_get_anon_vma(folio);
>>> + if (!anon_vma) {
>>> + ret = -EBUSY;
>>> + goto out;
>>> + }
>>> + anon_vma_lock_write(anon_vma);
>>> }
>>> mapping = NULL;
>>> - anon_vma_lock_write(anon_vma);
>>> } else {
>>> unsigned int min_order;
>>> gfp_t gfp;
>>> @@ -3838,7 +3832,8 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> goto out_unlock;
>>> }
>>>
>>> - unmap_folio(folio);
>>> + if (!unmapped)
>>> + unmap_folio(folio);
>>>
>>> /* block interrupt reentry in xa_lock and spinlock */
>>> local_irq_disable();
>>> @@ -3925,10 +3920,13 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>>
>>> next = folio_next(new_folio);
>>>
>>> + zone_device_private_split_cb(folio, new_folio);
>>> +
>>> expected_refs = folio_expected_ref_count(new_folio) + 1;
>>> folio_ref_unfreeze(new_folio, expected_refs);
>>>
>>> - lru_add_split_folio(folio, new_folio, lruvec, list);
>>> + if (!unmapped)
>>> + lru_add_split_folio(folio, new_folio, lruvec,
>>> list);
>>>
>>> /*
>>> * Anonymous folio with swap cache.
>>> @@ -3959,6 +3957,8 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>> __filemap_remove_folio(new_folio, NULL);
>>> folio_put_refs(new_folio, nr_pages);
>>> }
>>> +
>>> + zone_device_private_split_cb(folio, NULL);
>>> /*
>>> * Unfreeze @folio only after all page cache entries, which
>>> * used to point to it, have been updated with new folios.
>>> @@ -3982,6 +3982,9 @@ static int __folio_split(struct folio *folio,
>>> unsigned int new_order,
>>>
>>> local_irq_enable();
>>>
>>> + if (unmapped)
>>> + return ret;
>>
>> As the comment of __folio_split() and __split_huge_page_to_list_to_order()
>> mentioned:
>>
>> * The large folio must be locked
>> * After splitting, the after-split folio containing @lock_at remains locked
>>
>> But here we seems to change the prerequisites.
>>
>> Hmm.. I am not sure this is correct.
>>
>
> The code is correct, but you are right in that the documentation needs to be
> updated.
> When "unmapped", we do want to leave the folios locked after the split.
Sigh, this "unmapped" code needs so many special branches and a different
locking
requirement. It should be a separate function to avoid confusions.
--
Best Regards,
Yan, Zi
