On 12/18/19 7:52 AM, Kirill A. Shutemov wrote:
On Mon, Dec 16, 2019 at 02:25:13PM -0800, John Hubbard wrote:
+static void put_compound_head(struct page *page, int refs)
+{
+ /* Do a get_page() first, in case refs == page->_refcount */
+ get_page(page);
+ page_ref_sub(page, refs);
+ put_page(page);
+}
It's not terribly efficient. Maybe something like:
VM_BUG_ON_PAGE(page_ref_count(page) < ref, page);
if (refs > 2)
page_ref_sub(page, refs - 1);
put_page(page);
?
OK, but how about this instead? I don't see the need for a "2", as that
is a magic number that requires explanation. Whereas "1" is not a magic
number--here it means: either there are "many" (>1) refs, or not.
And the routine won't be called with refs less than about 32 (2MB huge
page, 64KB base page == 32 subpages) anyway.
VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
/*
* Calling put_page() for each ref is unnecessarily slow. Only the last
* ref needs a put_page().
*/
if (refs > 1)
page_ref_sub(page, refs - 1);
put_page(page);
thanks,
--
John Hubbard
NVIDIA
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