Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Jan 14, 2021 at 7:52 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Jan 14, 2021 at 06:54:30PM +0800, Muchun Song wrote:
> > I think this approach may be only suitable for generic huge page only.
> > So we can implement it only for huge page.
> >
> > Hi Oscar,
> >
> > What's your opinion about this?
>
> I tried something like:
>
> static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
>                               unsigned long end,
>                               struct vmemmap_remap_walk *walk)
> {
>         pte_t *pte;
>
>         pte = pte_offset_kernel(pmd, addr);
>
>         if (!walk->reuse_page) {
>                 BUG_ON(pte_none(*pte));
>
>                 walk->reuse_page = pte_page(*pte++);
>                 addr = walk->remap_start;
>         }
>
>         for (; addr != end; addr += PAGE_SIZE, pte++) {
>                 BUG_ON(pte_none(*pte));
>
>                 walk->remap_pte(pte, addr, walk);
>         }
> }
>
> void vmemmap_remap_free(unsigned long start, unsigned long end,
>                         unsigned long reuse)
> {
>         LIST_HEAD(vmemmap_pages);
>         struct vmemmap_remap_walk walk = {
>                 .remap_pte      = vmemmap_remap_pte,
>                 .reuse_addr     = reuse,
>                 .remap_start = start,
>                 .vmemmap_pages  = &vmemmap_pages,
>         };
>
>         BUG_ON(start != reuse + PAGE_SIZE);
>
>         vmemmap_remap_range(reuse, end, &walk);
>         free_vmemmap_page_list(&vmemmap_pages);
> }
>
> but it might overcomplicate things and I am not sure it is any better.
> So I am fine with keeping it as is.
> Should another user come in the future, we can always revisit.
> Maybe just add a little comment in vmemmap_pte_range(), explaining while we
> are "+= PAGE_SIZE" for address and I would like to see a comment in
> vmemmap_remap_free why the BUG_ON and more important what it is checking.

OK, I will add some comments to explain why we do this in
vmemmap_remap_free and vmemmap_remap_free. Thanks.

>
> --
> Oscar Salvador
> SUSE L3