Re: [External] Re: [PATCH v14 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Fri, Feb 5, 2021 at 7:54 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Feb 04, 2021 at 11:50:39AM +0800, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we should allocate the
> > vmemmap pages associated with it. But we may cannot allocate vmemmap pages
> > when the system is under memory pressure, in this case, we just refuse to
> > free the HugeTLB page instead of looping forever trying to allocate the
> > pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> [...]
>
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 4cfca27c6d32..5518283aa667 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1397,16 +1397,26 @@ static void __free_huge_page(struct page *page)
> >               h->resv_huge_pages++;
> >
> >       if (HPageTemporary(page)) {
> > -             list_del(&page->lru);
> >               ClearHPageTemporary(page);
> > +
> > +             if (alloc_huge_page_vmemmap(h, page, GFP_ATOMIC)) {
> > +                     h->surplus_huge_pages++;
> > +                     h->surplus_huge_pages_node[nid]++;
> > +                     goto enqueue;
> > +             }
> > +             list_del(&page->lru);
> >               update_and_free_page(h, page);
> >       } else if (h->surplus_huge_pages_node[nid]) {
> > +             if (alloc_huge_page_vmemmap(h, page, GFP_ATOMIC))
> > +                     goto enqueue;
> > +
> >               /* remove the page from active list */
> >               list_del(&page->lru);
> >               update_and_free_page(h, page);
> >               h->surplus_huge_pages--;
> >               h->surplus_huge_pages_node[nid]--;
> >       } else {
> > +enqueue:
> >               arch_clear_hugepage_flags(page);
> >               enqueue_huge_page(h, page);
>
> Ok, we just keep them in the pool in case we fail to allocate.
>
>
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index ddd872ab6180..0bd6b8d7282d 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -169,6 +169,8 @@
> >   * (last) level. So this type of HugeTLB page can be optimized only when its
> >   * size of the struct page structs is greater than 2 pages.
>
> [...]
>
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head, gfp_t gfp_mask)
> > +{
> > +     int ret;
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +     unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > +     if (!free_vmemmap_pages_per_hpage(h))
> > +             return 0;
> > +
> > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +
> > +     /*
> > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > +      * discarded vmemmap pages must be allocated and remapping.
> > +      */
> > +     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > +                               gfp_mask | __GFP_NOWARN | __GFP_THISNODE);
>
> Why don't you set all the GFP flags here?

Originally, I wanted to let the caller know the GFP flag which they
used. But setting all the GFP flags here also makes sense to me.
And we can remove the @gfp_mask parameter of the
alloc_huge_page_vmemmap. It is simple.

>
> vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, GFP_ATOMIC|
>                     __GFP_NOWARN | __GFP_THISNODE) ?

I will use this.

>
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index 50c1dc00b686..277eb43aebd5 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
>
> [...]
>
> > +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> > +                                gfp_t gfp_mask, struct list_head *list)
>
> I think it would make more sense for this function to get the nid and the
> nr_pages to allocate directly.

Just like alloc_pages(), right? If so, make sense to me.

>
> > +{
> > +     unsigned long addr;
> > +     int nid = page_to_nid((const void *)start);
>
> Uh, that void is a bit ugly. page_to_nid(struct page *)start).
> Do not need the const either.

OK. Will do. Thanks.

>
> > +     struct page *page, *next;
> > +
> > +     for (addr = start; addr < end; addr += PAGE_SIZE) {
> > +             page = alloc_pages_node(nid, gfp_mask, 0);
> > +             if (!page)
> > +                     goto out;
> > +             list_add_tail(&page->lru, list);
> > +     }
>
> and replace this by while(--nr_pages) etc.

OK. Will do.

>
> I did not really go in depth, but looks good to me, and much more simply
> overall.

Yeah. The series only has 8 patches now. It is simpler.

>
> The only thing I am not sure about is the use of GFP_ATOMIC.
> It has been raised before than when we are close to OOM, the user might want
> to try to free up some memory by dissolving free_huge_pages, and so we might
> want to dip in the reserves.
>
> Given the fact that we are prepared to fail, and that we do not retry, I would
> rather use GFP_KERNEL than to have X pages atomically allocated and then realize
> we need to drop them on the ground because we cannot go further at some point.
> I think those reserves would be better off used by someone else in that
> situation.
>
> But this is just my thoughs, and given the fact that there seems to be a consensus
> of susing GFP_ATOMIC.
>
> --
> Oscar Salvador
> SUSE L3