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
