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
