On Tue, Jan 31, 2023 at 5:24 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Tue, Jan 31, 2023 at 04:24:15PM -0800, James Houghton wrote: > > On Mon, Jan 30, 2023 at 1:14 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > On Mon, Jan 30, 2023 at 10:38:41AM -0800, James Houghton wrote: > > > > On Mon, Jan 30, 2023 at 9:29 AM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > > > > > On Fri, Jan 27, 2023 at 01:02:02PM -0800, James Houghton wrote: [snip] > > > > > Another way to not use thp mapcount, nor break smaps and similar calls to > > > > > page_mapcount() on small page, is to only increase the hpage mapcount only > > > > > when hstate pXd (in case of 1G it's PUD) entry being populated (no matter > > > > > as leaf or a non-leaf), and the mapcount can be decreased when the pXd > > > > > entry is removed (for leaf, it's the same as for now; for HGM, it's when > > > > > freeing pgtable of the PUD entry). > > > > > > > > Right, and this is doable. Also it seems like this is pretty close to > > > > the direction Matthew Wilcox wants to go with THPs. > > > > > > I may not be familiar with it, do you mean this one? > > > > > > https://lore.kernel.org/all/Y9Afwds%2FJl39UjEp@xxxxxxxxxxxxxxxxxxxx/ > > > > Yep that's it. > > > > > > > > For hugetlb I think it should be easier to maintain rather than any-sized > > > folios, because there's the pgtable non-leaf entry to track rmap > > > information and the folio size being static to hpage size. > > > > > > It'll be different to folios where it can be random sized pages chunk, so > > > it needs to be managed by batching the ptes when install/zap. > > > > Agreed. It's probably easier for HugeTLB because they're always > > "naturally aligned" and yeah they can't change sizes. > > > > > > > > > > > > > Something I noticed though, from the implementation of > > > > folio_referenced()/folio_referenced_one(), is that folio_mapcount() > > > > ought to report the total number of PTEs that are pointing on the page > > > > (or the number of times page_vma_mapped_walk returns true). FWIW, > > > > folio_referenced() is never called for hugetlb folios. > > > > > > FWIU folio_mapcount is the thing it needs for now to do the rmap walks - > > > it'll walk every leaf page being mapped, big or small, so IIUC that number > > > should match with what it expects to see later, more or less. > > > > I don't fully understand what you mean here. > > I meant the rmap_walk pairing with folio_referenced_one() will walk all the > leaves for the folio, big or small. I think that will match the number > with what got returned from folio_mapcount(). See below. > > > > > > > > > But I agree the mapcount/referenced value itself is debatable to me, just > > > like what you raised in the other thread on page migration. Meanwhile, I > > > am not certain whether the mapcount is accurate either because AFAICT the > > > mapcount can be modified if e.g. new page mapping established as long as > > > before taking the page lock later in folio_referenced(). > > > > > > It's just that I don't see any severe issue either due to any of above, as > > > long as that information is only used as a hint for next steps, e.g., to > > > swap which page out. > > > > I also don't see a big problem with folio_referenced() (and you're > > right that folio_mapcount() can be stale by the time it takes the > > folio lock). It still seems like folio_mapcount() should return the > > total number of PTEs that map the page though. Are you saying that > > breaking this would be ok? > > I didn't quite follow - isn't that already doing so? > > folio_mapcount() is total_compound_mapcount() here, IIUC it is an > accumulated value of all possible PTEs or PMDs being mapped as long as it's > all or part of the folio being mapped. We've talked about 3 ways of handling mapcount: 1. The RFC v2 way, which is head-only, and we increment the compound mapcount for each PT mapping we have. So a PTE-mapped 2M page, compound_mapcount=512, subpage->_mapcount=0 (ignoring the -1 bias). 2. The THP-like way. If we are fully mapping the hugetlb page with the hstate-level PTE, we increment the compound mapcount, otherwise we increment subpage->_mapcount. 3. The RFC v1 way (the way you have suggested above), which is head-only, and we increment the compound mapcount if the hstate-level PTE is made present. With #1 and #2, there is no concern with folio_mapcount(). But with #3, folio_mapcount() for a PTE-mapped 2M page mapped in a single VMA would yield 1 instead of 512 (right?). That's what I mean. #1 has problems wrt smaps and migration (though there were other problems with those anyway that Mike has fixed), and #2 makes MADV_COLLAPSE slow to the point of being unusable for some applications. It seems like the least bad option is #1, but maybe we should have a face-to-face discussion about it? I'm still collecting some more performance numbers. - James
