On 2/22/21 8:37 PM, Dan Williams wrote: > On Mon, Feb 22, 2021 at 3:24 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: >> On 2/20/21 1:43 AM, Dan Williams wrote: >>> On Tue, Dec 8, 2020 at 9:59 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: >>>> On 12/8/20 9:28 AM, Joao Martins wrote: >> One thing to point out about altmap is that the degradation (in pinning and >> unpining) we observed with struct page's in device memory, is no longer observed >> once 1) we batch ref count updates as we move to compound pages 2) reusing >> tail pages seems to lead to these struct pages staying more likely in cache >> which perhaps contributes to dirtying a lot less cachelines. > > True, it makes it more palatable to survive 'struct page' in PMEM, I want to retract for now what I said above wrt to the no degradation with struct page in device comment. I was fooled by a bug on a patch later down this series. Particular because I accidentally cleared PGMAP_ALTMAP_VALID when unilaterally setting PGMAP_COMPOUND, which consequently lead to always allocating struct pages from memory. No wonder the numbers were just as fast. I am still confident that it's going to be faster and observe less degradation in pinning/init. Init for now is worst-case 2x faster. But to be *as fast* struct pages in memory might still be early to say. The broken masking of the PGMAP_ALTMAP_VALID bit did hide one flaw, where we don't support altmap for basepages on x86/mm and it apparently depends on architectures to implement it (and a couple other issues). The vmemmap allocation isn't the problem, so the previous comment in this thread that altmap doesn't change much in the vmemmap_populate_compound_pages() is still accurate. The problem though resides on the freeing of vmemmap pagetables with basepages *with altmap* (e.g. at dax-device teardown) which require arch support. Doing it properly would mean making the altmap reserve smaller (given fewer pages are allocated), and the ability for the altmap pfn allocator to track references per pfn. But I think it deserves its own separate patch series (probably almost just as big). Perhaps for this set I can stick without altmap as you suggested, and use hugepage vmemmap population (which wouldn't lead to device memory savings) instead of reusing base pages . I would still leave the compound page support logic as metadata representation for > 4K @align, as I think that's the right thing to do. And then a separate series onto improving altmap to leverage the metadata reduction support as done with non-device struct pages. Thoughts? Joao
