Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 18 Apr 2024 17:15:43 -0700 | Subject | Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation | From | John Hubbard <> |
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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
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