[PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[David Hildenbrand]

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 <jhubbard@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 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

Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[John Hubbard]

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 <jhubbard@nvidia.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   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.


>    *
>    * 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.)

> + *
> + * 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,
-- 
John Hubbard
NVIDIA

Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[David Hildenbrand]

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 <jhubbard@nvidia.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>    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

Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[John Hubbard]

On 4/22/24 12:31 PM, David Hildenbrand wrote:
> On 19.04.24 02:15, John Hubbard wrote:
>> On 4/18/24 8:18 AM, David Hildenbrand wrote:
...
>>>     *
>>>     * 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.

Looks good,

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[Zi Yan]

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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 <jhubbard@nvidia.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/huge_memory.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)

The changes look good to me. Thanks. Reviewed-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi

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