On Wed, Feb 8, 2023 at 8:16 AM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Tue, Feb 07, 2023 at 04:26:02PM -0800, James Houghton wrote: > > On Tue, Feb 7, 2023 at 3:13 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > James, > > > > > > On Tue, Feb 07, 2023 at 02:46:04PM -0800, James Houghton wrote: > > > > > Here is the result: [1] (sorry it took a little while heh). The > > > > > > Thanks. From what I can tell, that number shows that it'll be great we > > > start with your rfcv1 mapcount approach, which mimics what's proposed by > > > Matthew for generic folio. > > > > Do you think the RFC v1 way is better than doing the THP-like way > > *with the additional MMU notifier*? > > What's the additional MMU notifier you're referring? An MMU notifier that informs KVM that a collapse has happened without having to invalidate_range_start() and invalidate_range_end(), the one you're replying to lower down in the email. :) [ see below... ] > > > > > > > > > > > implementation of the "RFC v1" way is pretty horrible[2] (and this > > > > > > Any more information on why it's horrible? :) > > > > I figured the code would speak for itself, heh. It's quite complicated. > > > > I really didn't like: > > 1. The 'inc' business in copy_hugetlb_page_range. > > 2. How/where I call put_page()/folio_put() to keep the refcount and > > mapcount synced up. > > 3. Having to check the page cache in UFFDIO_CONTINUE. > > I think the complexity is one thing which I'm fine with so far. However > when I think again about the things behind that complexity, I noticed there > may be at least one flaw that may not be trivial to work around. > > It's about truncation. The problem is now we use the pgtable entry to > represent the mapcount, but the pgtable entry cannot be zapped easily, > unless vma unmapped or collapsed. > > It means e.g. truncate_inode_folio() may stop working for hugetlb (of > course, with page lock held). The mappings will be removed for real, but > not the mapcount for HGM anymore, because unmap_mapping_folio() only zaps > the pgtable leaves, not the ones that we used to account for mapcounts. > > So the kernel may see weird things, like mapcount>0 after > truncate_inode_folio() being finished completely. > > For HGM to do the right thing, we may want to also remove the non-leaf > entries when truncating or doing similar things like a rmap walk to drop > any mappings for a page/folio. Though that's not doable for now because > the locks that truncate_inode_folio() is weaker than what we need to free > the pgtable non-leaf entries - we'll need mmap write lock for that, the > same as when we unmap or collapse. > > Matthew's design doesn't have such issue if the ptes need to be populated, > because mapcount is still with the leaves; not the case for us here. > > If that's the case, _maybe_ we still need to start with the stupid but > working approach of subpage mapcounts. Good point. I can't immediately think of a solution. I would prefer to go with the subpage mapcount approach to simplify HGM for now; optimizing mapcount for HugeTLB can then be handled separately. If you're ok with this, I'll go ahead and send v2. One way that might be possible: using the PAGE_SPECIAL bit on the hstate-level PTE to indicate if mapcount has been incremented or not (if the PTE is pointing to page tables). As far as I can tell, PAGE_SPECIAL doesn't carry any meaning for HugeTLB PTEs, but we would need to be careful with existing PTE examination code as to not misinterpret these PTEs. > > [...] > > > > > > Matthew is trying to solve the same problem with THPs right now: [3]. > > > > > I haven't figured out how we can apply Matthews's approach to HGM > > > > > right now, but there probably is a way. (If we left the mapcount > > > > > increment bits in the same place, we couldn't just check the > > > > > hstate-level PTE; it would have already been made present.) > > > > > > I'm just worried that (1) this may add yet another dependency to your work > > > which is still during discussion phase, and (2) whether the folio approach > > > is easily applicable here, e.g., we may not want to populate all the ptes > > > for hugetlb HGMs by default. > > > > That's true. I definitely don't want to wait for this either. It seems > > like Matthew's approach won't work very well for us -- when doing a > > lot of high-granularity UFFDIO_CONTINUEs on a 1G page, checking all > > the PTEs to see if any of them are mapped would get really slow. > > I think it'll be a common problem to userfaultfd when it comes, e.g., > userfaultfd by design is PAGE_SIZE based so far. It needs page size > granule on pgtable manipulations, unless we extend the userfaultfd protocol > to support folios, iiuc. > > > > > > > > > > > > > > > > We could: > > > > > - use the THP-like way and tolerate ~1 second collapses > > > > > > > > Another thought here. We don't necessarily *need* to collapse the page > > > > table mappings in between mmu_notifier_invalidate_range_start() and > > > > mmu_notifier_invalidate_range_end(), as the pfns aren't changing, > > > > we aren't punching any holes, and we aren't changing permission bits. > > > > If we had an MMU notifier that simply informed KVM that we collapsed > > > > the page tables *after* we finished collapsing, then it would be ok > > > > for hugetlb_collapse() to be slow. [ from above... ] This MMU notifier. :) > > > > > > That's a great point! It'll definitely apply to either approach. > > > > > > > > > > > If this MMU notifier is something that makes sense, it probably > > > > applies to MADV_COLLAPSE for THPs as well. > > > > > > THPs are definitely different, mmu notifiers should be required there, > > > afaict. Isn't that what the current code does? > > > > > > See collapse_and_free_pmd() for shmem and collapse_huge_page() for anon. > > > > Oh, yes, of course, MADV_COLLAPSE can actually move things around and > > properly make THPs. Thanks. But it would apply if we were only > > collapsing PTE-mapped THPs, I think? > > Yes it applies I think. And if I'm not wrong it's also doing so. :) See > collapse_pte_mapped_thp(). > > While for anon we always allocate a new page, hence not applicable. > > -- > Peter Xu Thanks Peter! - James
