On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
Remove an unnecessary VM_BUG_ON by handling pages both with and without
a pincount field in page_pincount_sub().
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
mm/gup.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 3ed9907f3c8d..aed48de3912e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -48,12 +48,15 @@ static void page_pincount_add(struct page *page, int refs)
page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
}
-static void hpage_pincount_sub(struct page *page, int refs)
OK, this is a correct transformation. But it does make the function, as
small as it is, rather hard to understand. It is now factored out at a
strange place, as evidenced in the difficulty in documenting it. Here's
my best attempt so far at documenting it here:
/*
* Subtract @refs from the compound_pincount, if applicable. Otherwise, do
* nothing. And in all cases, return the number of pages that should be passed
* to put_page_refs().
*/
...which I think is worth adding, if we can't find a better factoring.
Either way, it's correct, so:
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
thanks,
--
John Hubbard
NVIDIA
+static int page_pincount_sub(struct page *page, int refs)
{
- VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
VM_BUG_ON_PAGE(page != compound_head(page), page);
- atomic_sub(refs, compound_pincount_ptr(page));
+ if (hpage_pincount_available(page))
+ atomic_sub(refs, compound_pincount_ptr(page));
+ else
+ refs *= GUP_PIN_COUNTING_BIAS;
+ return refs;
}
/* Equivalent to calling put_page() @refs times. */
@@ -177,11 +180,7 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
if (flags & FOLL_PIN) {
mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
refs);
-
- if (hpage_pincount_available(page))
- hpage_pincount_sub(page, refs);
- else
- refs *= GUP_PIN_COUNTING_BIAS;
+ refs = page_pincount_sub(page, refs);
}
put_page_refs(page, refs);
