On Mon, Oct 04, 2021 at 01:49:37PM +0100, Matthew Wilcox wrote: > On Mon, Oct 04, 2021 at 10:10:37AM +0100, Mel Gorman wrote: > > On Thu, Sep 30, 2021 at 05:32:58PM +0100, Matthew Wilcox (Oracle) wrote: > > > Instead of calling put_page() one page at a time, pop pages off > > > the list if there are other refcounts and pass the remainder > > > to free_unref_page_list(). This should be a speed improvement, > > > but I have no measurements to support that. It's also not very > > > widely used today, so I can't say I've really tested it. I'm only > > > bothering with this patch because I'd like the IOMMU code to use it > > > https://lore.kernel.org/lkml/20210930162043.3111119-1-willy@xxxxxxxxxxxxx/ > > > > > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > > > I see your motivation but you need to check that all users of > > put_pages_list (current and future) handle destroy_compound_page properly > > or handle it within put_pages_list. For example, the release_pages() > > user of free_unref_page_list calls __put_compound_page directly before > > freeing. put_pages_list as it stands will call dstroy_compound_page but > > free_unref_page_list does not destroy compound pages in free_pages_prepare > > Quite right. I was really only thinking about order-zero pages because > there aren't any users of compound pages that call this. But of course, > we should be robust against future callers. So the obvious thing to do > is to copy what release_pages() does: > > +++ b/mm/swap.c > @@ -144,6 +144,10 @@ void put_pages_list(struct list_head *pages) > list_for_each_entry_safe(page, next, pages, lru) { > if (!put_page_testzero(page)) > list_del(&page->lru); > + if (PageCompound(page)) { > + list_del(&page->lru); > + __put_compound_page(page); > + } > } > > free_unref_page_list(pages); That would be the most straight-forward > > But would it be better to have free_unref_page_list() handle compound > pages itself? > > +++ b/mm/page_alloc.c > @@ -3427,6 +3427,11 @@ void free_unref_page_list(struct list_head *list) > > /* Prepare pages for freeing */ > list_for_each_entry_safe(page, next, list, lru) { > + if (PageCompound(page)) { > + __put_compound_page(page); > + list_del(&page->lru); > + continue; > + } > pfn = page_to_pfn(page); > if (!free_unref_page_prepare(page, pfn, 0)) { > list_del(&page->lru); > > (and delete the special handling from release_pages() in the same patch) It's surprisingly tricky. Minimally, that list_del should be before __put_compound_page or you'll clobber whatever list the compound page destructor placed the free page on. Take care with how you remove the special handling and leave a comment explaining why __put_compound_page is not called and that PageLRU will be cleared when it falls through to add the page to pages_to_free. The tricky part is memcg uncharging because if mem_cgroup_uncharge_list() is called then the uncharging happens twice -- once in the destructor and again in mem_cgroup_uncharge_list. I guess you could use two lists and splice them after mem_cgroup_uncharge_list() and before free_unref_page_list. -- Mel Gorman SUSE Labs
