On Sat, Apr 17, 2021 at 7:56 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > In the subsequent patch, we should allocate the vmemmap pages when
> > freeing a HugeTLB page. But update_and_free_page() can be called
> > under any context, so we cannot use GFP_KERNEL to allocate vmemmap
> > pages. However, we can defer the actual freeing in a kworker to
> > prevent from using GFP_ATOMIC to allocate the vmemmap pages.
>
> Thanks! I knew we would need to introduce a kworker for this when I
> removed the kworker previously used in free_huge_page.
Yeah, but another choice is using GFP_ATOMIC to allocate vmemmap
pages when we are in an atomic context. If not atomic context, just
use GFP_KERNEL. In this case, we can drop kworker.
>
> > The __update_and_free_page() is where the call to allocate vmemmmap
> > pages will be inserted.
>
> This patch adds the functionality required for __update_and_free_page
> to potentially sleep and fail. More questions will come up in the
> subsequent patch when code must deal with the failures.
Right. More questions are welcome.
>
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> > mm/hugetlb.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++----
> > mm/hugetlb_vmemmap.c | 12 ---------
> > mm/hugetlb_vmemmap.h | 17 ++++++++++++
> > 3 files changed, 85 insertions(+), 17 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 923d05e2806b..eeb8f5480170 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1376,7 +1376,7 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
> > h->nr_huge_pages_node[nid]--;
> > }
> >
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +static void __update_and_free_page(struct hstate *h, struct page *page)
> > {
> > int i;
> > struct page *subpage = page;
> > @@ -1399,12 +1399,73 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> > }
> > }
> >
> > +/*
> > + * As update_and_free_page() can be called under any context, so we cannot
> > + * use GFP_KERNEL to allocate vmemmap pages. However, we can defer the
> > + * actual freeing in a workqueue to prevent from using GFP_ATOMIC to allocate
> > + * the vmemmap pages.
> > + *
> > + * free_hpage_workfn() locklessly retrieves the linked list of pages to be
> > + * freed and frees them one-by-one. As the page->mapping pointer is going
> > + * to be cleared in free_hpage_workfn() anyway, it is reused as the llist_node
> > + * structure of a lockless linked list of huge pages to be freed.
> > + */
> > +static LLIST_HEAD(hpage_freelist);
> > +
> > +static void free_hpage_workfn(struct work_struct *work)
> > +{
> > + struct llist_node *node;
> > +
> > + node = llist_del_all(&hpage_freelist);
> > +
> > + while (node) {
> > + struct page *page;
> > + struct hstate *h;
> > +
> > + page = container_of((struct address_space **)node,
> > + struct page, mapping);
> > + node = node->next;
> > + page->mapping = NULL;
> > + h = page_hstate(page);
>
> The VM_BUG_ON_PAGE(!PageHuge(page), page) in page_hstate is going to
> trigger because a previous call to remove_hugetlb_page() will
> set_compound_page_dtor(page, NULL_COMPOUND_DTOR)
Sorry, I did not realise that. Thanks for your reminder.
>
> Note how h(hstate) is grabbed before calling update_and_free_page in
> existing code.
>
> We could potentially drop the !PageHuge(page) in page_hstate. Or,
> perhaps just use 'size_to_hstate(page_size(page))' in free_hpage_workfn.
I prefer not to change the behavior of page_hstate(). So I
should use 'size_to_hstate(page_size(page))' directly.
Thanks Mike.
> --
> Mike Kravetz
