The documentation is wrong and relying on it almost resulted in BUGs
in new callers: we return -EAGAIN on unexpected folio references, not
-EBUSY.
Let's fix that and also document which other return values we can
currently see and why they could happen.
Cc: John Hubbard <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
---
mm/huge_memory.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ee12726291f1b..824eff9211db8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
*
* 3) The folio must not be pinned. Any unexpected folio references, including
* GUP pins, will result in the folio not getting split; instead, the caller
- * will receive an -EBUSY.
+ * will receive an -EAGAIN.
*
* 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not
* supported for non-file-backed folios, because folio->_deferred_list, which
@@ -2975,8 +2975,15 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
*
* Returns 0 if the huge page was split successfully.
*
- * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared
- * from under us.
+ * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP).
+ *
+ * Returns -EBUSY when trying to split the huge zeropage, if the folio is
+ * under writeback, if fs-specific folio metadata cannot currently be
+ * released, or if some unexpected race happened (e.g., anon VMA disappeared,
+ * truncation).
+ *
+ * Returns -EINVAL when trying to split to an order that is incompatible
+ * with the folio. Splitting to order 0 is compatible with all folios.
*/
int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
unsigned int new_order)
--
2.44.0
On 18 Apr 2024, at 11:18, David Hildenbrand wrote:
> The documentation is wrong and relying on it almost resulted in BUGs
> in new callers: we return -EAGAIN on unexpected folio references, not
> -EBUSY.
+Baolin
The code was changed at the commit fd4a7ac32918 ("mm: migrate: try again
if THP split is failed due to page refcnt") without changing the comment.
>
> Let's fix that and also document which other return values we can
> currently see and why they could happen.
>
> Cc: John Hubbard <[email protected]>
> Cc: Zi Yan <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Signed-off-by: David Hildenbrand <[email protected]>
> ---
> mm/huge_memory.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
The changes look good to me. Thanks. Reviewed-by: Zi Yan <[email protected]>
--
Best Regards,
Yan, Zi
On 19.04.24 02:15, John Hubbard wrote:
> On 4/18/24 8:18 AM, David Hildenbrand wrote:
>> The documentation is wrong and relying on it almost resulted in BUGs
>> in new callers: we return -EAGAIN on unexpected folio references, not
>> -EBUSY.
>>
>> Let's fix that and also document which other return values we can
>> currently see and why they could happen.
>>
>> Cc: John Hubbard <[email protected]>
>> Cc: Zi Yan <[email protected]>
>> Cc: Matthew Wilcox <[email protected]>
>> Cc: Andrew Morton <[email protected]>
>> Signed-off-by: David Hildenbrand <[email protected]>
>> ---
>> mm/huge_memory.c | 13 ++++++++++---
>> 1 file changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ee12726291f1b..824eff9211db8 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>> *
>> * 3) The folio must not be pinned. Any unexpected folio references, including
>> * GUP pins, will result in the folio not getting split; instead, the caller
>> - * will receive an -EBUSY.
>> + * will receive an -EAGAIN.
>> *
>> * 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not
>> * supported for non-file-backed folios, because folio->_deferred_list, which
>> @@ -2975,8 +2975,15 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>
> As an aside, the use of unconditional local_irq_disable() / local_irq_enable()
> calls in this routine almost makes me believe that we should have:
>
> 5) Local IRQs should be enabled. Because this routine may enable them.
>
> ...but I can't imagine a way to end up calling this with interrupts
> disabled, so it seems like documentation overkill. Just thought I'd mention
> it, though.
Yes, I think there might be more issues lurking with disabled interrupts.
anon_vma_lock_write() and i_mmap_lock_read() might even sleep ... so we
must not be in any atomic context. that's why relevant page table walkers drop the PTL.
>
>
>> *
>> * Returns 0 if the huge page was split successfully.
>> *
>> - * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared
>> - * from under us.
>> + * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP).
>
> ...or if the folio was removed from the page cache before this routine
> got a chance to lock it, right? (See the "fail:" path.)
Right, that is sneaky. Let me extend to cover that case as well.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 824eff9211db8..a7406267323ed 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2975,7 +2975,8 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
*
* Returns 0 if the huge page was split successfully.
*
- * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP).
+ * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP) or if
+ * the folio was concurrently removed from the page cache.
*
* Returns -EBUSY when trying to split the huge zeropage, if the folio is
* under writeback, if fs-specific folio metadata cannot currently be
Naive me would assume that this happens rarely ... but not an expert :)
>
>> + *
>> + * Returns -EBUSY when trying to split the huge zeropage, if the folio is
>> + * under writeback, if fs-specific folio metadata cannot currently be
>> + * released, or if some unexpected race happened (e.g., anon VMA disappeared,
>> + * truncation).
>> + *
>> + * Returns -EINVAL when trying to split to an order that is incompatible
>> + * with the folio. Splitting to order 0 is compatible with all folios.
>> */
>> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>> unsigned int new_order)
>
> Otherwise, looks good.
Thanks!
--
Cheers,
David / dhildenb