On Tue, Nov 10, 2020 at 02:40:54PM +0800, Muchun Song wrote:
> Only the first HugeTLB page should split the PMD to PTE. The other 63
> HugeTLB pages
> do not need to split. Here I want to make sure we are the first.
I think terminology is loosing me here.
Say you allocate a 2MB HugeTLB page at ffffea0004100000.
The vmemmap range that the represents this is ffffea0004000000 - ffffea0004200000.
That is a 2MB chunk PMD-mapped.
So, in order to free some of those vmemmap pages, we need to break down
that area, remapping it to PTE-based.
I know what you mean, but we are not really splitting hugetlg pages, but
the memmap range they are represented with.
About:
"Only the first HugeTLB page should split the PMD to PTE. The other 63
HugeTLB pages
do not need to split. Here I want to make sure we are the first."
That only refers to gigantic pages, right?
> > > +static void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > > +{
> > > + pmd_t *pmd;
> > > + spinlock_t *ptl;
> > > + LIST_HEAD(free_pages);
> > > +
> > > + if (!free_vmemmap_pages_per_hpage(h))
> > > + return;
> > > +
> > > + pmd = vmemmap_to_pmd(head);
> > > + ptl = vmemmap_pmd_lock(pmd);
> > > + if (vmemmap_pmd_huge(pmd)) {
> > > + VM_BUG_ON(!pgtable_pages_to_prealloc_per_hpage(h));
> >
> > I think that checking for free_vmemmap_pages_per_hpage is enough.
> > In the end, pgtable_pages_to_prealloc_per_hpage uses free_vmemmap_pages_per_hpage.
>
> The free_vmemmap_pages_per_hpage is not enough. See the comments above.
My comment was about the VM_BUG_ON.
--
Oscar Salvador
SUSE L3
