Re: [PATCH v3 1/2] mm: Abstract THP allocation

[David Hildenbrand]

On 11.09.24 15:05, Dev Jain wrote:
> On 9/11/24 18:30, David Hildenbrand wrote:
> > On 11.09.24 14:53, Dev Jain wrote:
> > > On 9/11/24 14:57, David Hildenbrand wrote:
> > > > On 11.09.24 08:55, Dev Jain wrote:
> > > > > In preparation for the second patch, abstract away the THP allocation
> > > > > logic present in the create_huge_pmd() path, which corresponds to the
> > > > > faulting case when no page is present.
> > > > >
> > > > > There should be no functional change as a result of applying
> > > > > this patch.
> > > >
> > > > [...]
> > > >
> > > > > +
> > > > > +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
> > > > > +            struct vm_area_struct *vma, unsigned long haddr)
> > > > > +{
> > > > > +    pmd_t entry;
> > > > > +
> > > > > +    entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > > > > +    entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> > > > > +    folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
> > > > > +    folio_add_lru_vma(folio, vma);
> > > > > +    set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
> > > > > +    update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
> > > >
> > > > It's quite weird to see a mixture of haddr and vmf->address, and
> > > > likely this mixture is wrong or not not required.
> > > >
> > > > Looking at arc's update_mmu_cache_pmd() implementation, I cannot see
> > > > how passing in the unaligned address would do the right thing. But
> > > > maybe arc also doesn't trigger that code path ... who knows :)
> > >
> > > If I am reading correctly, arch/arc/mm/tlb.c: update_mmu_cache_pmd()
> > > calls update_mmu_cache_range() which is already expecting an unaligned
> > > address? But...
> >
> > So update_mmu_cache_pmd() calls
> >
> >      update_mmu_cache_range(NULL, vma, addr, &pte, HPAGE_PMD_NR);
> >
> > But update_mmu_cache_range() only aligns it *to page boundary*:
> >
> >      unsigned long vaddr = vaddr_unaligned & PAGE_MASK;
>
> Ah, totally missed that it was PAGE_MASK. Thanks.
> > We obtain the correct hugepage-aligned physical address from the PTE
> >
> >      phys_addr_t paddr = pte_val(*ptep) & PAGE_MASK_PHYS;
> >
> > Then, we look at the offset in our folio
> >
> >      unsigned long offset = offset_in_folio(folio, paddr);
> >
> > And adjust both vaddr and paddr
> >
> >      paddr -= offset;
> >      vaddr -= offset;
> >
> > To then use that combination with
> >
> >      __inv_icache_pages(paddr, vaddr, nr);
> >
> > If I am not wrong, getting a non-hugepage aligned vaddr messes up
> > things here. But only regarding the icache I think.
>
> Looks like it...

As we are adding a fresh page where there previously wasn't anything 
mapped (no icache invaldiation required?), and because most anon 
mappings are not executable, maybe that's why nobody notices so far.

--
Cheers,

David / dhildenb