On Fri, Jun 11, 2021 at 6:35 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 6/9/21 5:13 AM, Muchun Song wrote:
> > If the vmemmap is huge PMD mapped, we should split the huge PMD firstly
> > and then we can change the PTE page table entry. In this patch, we add
> > the ability of splitting the huge PMD mapping of vmemmap pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> > include/linux/mm.h | 2 +-
> > mm/hugetlb.c | 42 ++++++++++++++++++++++++++++++++++--
> > mm/hugetlb_vmemmap.c | 3 ++-
> > mm/sparse-vmemmap.c | 61 +++++++++++++++++++++++++++++++++++++++++++++-------
> > 4 files changed, 96 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index cadc8cc2c715..b97e1486c5c1 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3056,7 +3056,7 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> > #endif
> >
> > void vmemmap_remap_free(unsigned long start, unsigned long end,
> > - unsigned long reuse);
> > + unsigned long reuse, struct list_head *pgtables);
> > int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > unsigned long reuse, gfp_t gfp_mask);
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index c3b2a8a494d6..3137c72d9cc7 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1609,6 +1609,13 @@ static void __prep_account_new_huge_page(struct hstate *h, int nid)
> > static void __prep_new_huge_page(struct hstate *h, struct page *page)
> > {
> > free_huge_page_vmemmap(h, page);
> > + /*
> > + * Because we store preallocated pages on @page->lru,
> > + * vmemmap_pgtable_free() must be called before the
> > + * initialization of @page->lru in INIT_LIST_HEAD().
> > + */
> > + vmemmap_pgtable_free(&page->lru);
> > +
> > INIT_LIST_HEAD(&page->lru);
> > set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > hugetlb_set_page_subpool(page, NULL);
> > @@ -1775,14 +1782,29 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
> > nodemask_t *node_alloc_noretry)
> > {
> > struct page *page;
> > + LIST_HEAD(pgtables);
> > +
> > + if (vmemmap_pgtable_prealloc(h, &pgtables))
> > + return NULL;
>
> In the previous two patches I asked:
> - Can we wait until later to prealloc vmemmap pages for gigantic pages
> allocated from bootmem?
> - Should we fail to add a hugetlb page to the pool if we can not do
> vmemmap optimization?
>
>
> Depending on the answers to those questions, we may be able to eliminate
> these vmemmap_pgtable_prealloc/vmemmap_pgtable_free calls in hugetlb.c.
> What about adding the calls to free_huge_page_vmemmap?
> At the beginning of free_huge_page_vmemmap, allocate any vmemmap pgtable
> pages. If it fails, skip optimization. We can free any pages before
> returning to the caller.
You are right because we've introduced HPageVmemmapOptimized flag.
It can be useful here. If failing to optimize vmemmap is allowed, we can
eliminate allocating/freeing page table helpers. Thanks for your reminder.
>
> Since we also know the page/address in the page table can we check to see
> if it is already PTE mapped. If so, can we then skip allocation?
Good point. We need to allocate 512 page tables when splitting
1 GB huge page. If we fail to allocate page tables in the middle
of processing of remapping, we should restore the previous
mapping. I just want to clarify something for myself.
Thanks, Mike. I'll try in the next version.
> --
> Mike Kravetz
