Re: [PATCH v2] mm: Optimise put_pages_list()

[Matthew Wilcox <willy@infradead.org>]

On Fri, Oct 22, 2021 at 04:26:59PM -0700, Anthony Yznaga wrote:
> 
> On 10/7/21 12:21 PM, Matthew Wilcox (Oracle) wrote:
> > Instead of calling put_page() one page at a time, pop pages off
> > the list if their refcount was too high and pass the remainder to
> > put_unref_page_list().  This should be a speed improvement, but I have
> > no measurements to support that.  Current callers do not care about
> > performance, but I hope to add some which do.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> > v2:
> >   - Handle compound pages (Mel)
> >   - Comment why we don't need to handle PageLRU
> >   - Added call to __ClearPageWaiters(), matching that in release_pages()
> > 
> >   mm/swap.c | 23 ++++++++++++++++-------
> >   1 file changed, 16 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index af3cad4e5378..9f334d503fd2 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -134,18 +134,27 @@ EXPORT_SYMBOL(__put_page);
> >    * put_pages_list() - release a list of pages
> >    * @pages: list of pages threaded on page->lru
> >    *
> > - * Release a list of pages which are strung together on page.lru.  Currently
> > - * used by read_cache_pages() and related error recovery code.
> > + * Release a list of pages which are strung together on page.lru.
> >    */
> >   void put_pages_list(struct list_head *pages)
> >   {
> > -	while (!list_empty(pages)) {
> > -		struct page *victim;
> > +	struct page *page, *next;
> > -		victim = lru_to_page(pages);
> > -		list_del(&victim->lru);
> > -		put_page(victim);
> > +	list_for_each_entry_safe(page, next, pages, lru) {
> > +		if (!put_page_testzero(page)) {
> > +			list_del(&page->lru);
> > +			continue;
> > +		}
> 
> 
> I know that compound pages are not currently passed to put_pages_list(),
> but I assume the put_page_testzero() should only be done on the head
> page similar to release_pages()?

Fun fact about pages: You can't put a tail page on an LRU list.  Why?

struct page {
...
        union {
                struct {        /* Page cache and anonymous pages */
                        struct list_head lru;
...
                struct {        /* Tail pages of compound page */
                        unsigned long compound_head;    /* Bit zero is set */

so if you try to put a tail page on the LRU list, it becomes no longer
a tail page.