On 8/6/20 3:48 PM, Matthew Wilcox wrote: > On Thu, Aug 06, 2020 at 01:45:11PM +0200, Vlastimil Babka wrote: >> How about this additional patch now that we have head_mapcoun()? (I wouldn't >> go for squashing as the goal and scope is too different). > > I like it. It bothers me that the compiler doesn't know that > compound_head(compound_head(x)) == compound_head(x). I updated > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911 with a request to be > able to tell the compiler that compound_head() is idempotent. Yeah it would be nice to get the benefits everywhere automatically. But I guess the compiler would have to discard the idempotence assumptions if there are multiple consecutive (perhaps hidden behind page flag access) compound_head(page) from a function, as soon as we modify the struct page somewhere. >> The bloat-o-meter difference without DEBUG_VM is the following: >> >> add/remove: 0/0 grow/shrink: 1/4 up/down: 32/-56 (-24) >> Function old new delta >> __split_huge_pmd 2867 2899 +32 >> shrink_page_list 3860 3847 -13 >> reuse_swap_page 762 748 -14 >> page_trans_huge_mapcount 153 139 -14 >> total_mapcount 187 172 -15 >> Total: Before=8687306, After=8687282, chg -0.00% > > That's great. I'm expecting improvements from my thp_head() macro when > that lands (currently in Andrew's tree). I have been reluctant to replace > current callers of compound_head() with thp_head(), but I suspect PF_HEAD > could use thp_head() and save a few bytes on a tinyconfig build. > >> +++ b/mm/huge_memory.c >> @@ -2125,7 +2125,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> * Set PG_double_map before dropping compound_mapcount to avoid >> * false-negative page_mapped(). >> */ >> - if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) { >> + if (head_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) { > > I'm a little nervous about this one. The page does actually come from > pmd_page(), and today that's guaranteed to be a head page. But I'm > not convinced that's going to still be true in twenty years. With the > current THP patchset, I won't allocate pages larger than PMD order, but > I can see there being interest in tracking pages in chunks larger than > 2MB in the future. And then pmd_page() might well return a tail page. > So it might be a good idea to not convert this one. Hmm the function converts the compound mapcount of the whole page to a HPAGE_PMD_NR of base pages. If suddenly the compound page was bigger than a pmd, then I guess this wouldn't work properly anymore without changes anyway? Maybe we could stick something like VM_BUG_ON(PageTransHuge(page)) there as "enforced documentation" for now?
