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@xxxxxxxxxx>
> > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >    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