On Mon, Jan 25, 2021 at 5:15 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 25.01.21 08:41, Muchun Song wrote:
> > On Mon, Jan 25, 2021 at 2:40 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >>
> >> On Mon, Jan 25, 2021 at 8:05 AM David Rientjes <rientjes@xxxxxxxxxx> wrote:
> >>>
> >>>
> >>> On Sun, 17 Jan 2021, Muchun Song wrote:
> >>>
> >>>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> >>>> index ce4be1fa93c2..3b146d5949f3 100644
> >>>> --- a/mm/sparse-vmemmap.c
> >>>> +++ b/mm/sparse-vmemmap.c
> >>>> @@ -29,6 +29,7 @@
> >>>> #include <linux/sched.h>
> >>>> #include <linux/pgtable.h>
> >>>> #include <linux/bootmem_info.h>
> >>>> +#include <linux/delay.h>
> >>>>
> >>>> #include <asm/dma.h>
> >>>> #include <asm/pgalloc.h>
> >>>> @@ -40,7 +41,8 @@
> >>>> * @remap_pte: called for each non-empty PTE (lowest-level) entry.
> >>>> * @reuse_page: the page which is reused for the tail vmemmap pages.
> >>>> * @reuse_addr: the virtual address of the @reuse_page page.
> >>>> - * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> >>>> + * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> >>>> + * or is mapped from.
> >>>> */
> >>>> struct vmemmap_remap_walk {
> >>>> void (*remap_pte)(pte_t *pte, unsigned long addr,
> >>>> @@ -50,6 +52,10 @@ struct vmemmap_remap_walk {
> >>>> struct list_head *vmemmap_pages;
> >>>> };
> >>>>
> >>>> +/* The gfp mask of allocating vmemmap page */
> >>>> +#define GFP_VMEMMAP_PAGE \
> >>>> + (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN | __GFP_THISNODE)
> >>>> +
> >>>
> >>> This is unnecessary, just use the gfp mask directly in allocator.
> >>
> >> Will do. Thanks.
> >>
> >>>
> >>>> static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> >>>> unsigned long end,
> >>>> struct vmemmap_remap_walk *walk)
> >>>> @@ -228,6 +234,75 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> >>>> free_vmemmap_page_list(&vmemmap_pages);
> >>>> }
> >>>>
> >>>> +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> >>>> + struct vmemmap_remap_walk *walk)
> >>>> +{
> >>>> + pgprot_t pgprot = PAGE_KERNEL;
> >>>> + struct page *page;
> >>>> + void *to;
> >>>> +
> >>>> + BUG_ON(pte_page(*pte) != walk->reuse_page);
> >>>> +
> >>>> + page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> >>>> + list_del(&page->lru);
> >>>> + to = page_to_virt(page);
> >>>> + copy_page(to, (void *)walk->reuse_addr);
> >>>> +
> >>>> + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> >>>> +}
> >>>> +
> >>>> +static void alloc_vmemmap_page_list(struct list_head *list,
> >>>> + unsigned long start, unsigned long end)
> >>>> +{
> >>>> + unsigned long addr;
> >>>> +
> >>>> + for (addr = start; addr < end; addr += PAGE_SIZE) {
> >>>> + struct page *page;
> >>>> + int nid = page_to_nid((const void *)addr);
> >>>> +
> >>>> +retry:
> >>>> + page = alloc_pages_node(nid, GFP_VMEMMAP_PAGE, 0);
> >>>> + if (unlikely(!page)) {
> >>>> + msleep(100);
> >>>> + /*
> >>>> + * We should retry infinitely, because we cannot
> >>>> + * handle allocation failures. Once we allocate
> >>>> + * vmemmap pages successfully, then we can free
> >>>> + * a HugeTLB page.
> >>>> + */
> >>>> + goto retry;
> >>>
> >>> Ugh, I don't think this will work, there's no guarantee that we'll ever
> >>> succeed and now we can't free a 2MB hugepage because we cannot allocate a
> >>> 4KB page. We absolutely have to ensure we make forward progress here.
> >>
> >> This can trigger a OOM when there is no memory and kill someone to release
> >> some memory. Right?
> >>
> >>>
> >>> We're going to be freeing the hugetlb page after this succeeeds, can we
> >>> not use part of the hugetlb page that we're freeing for this memory
> >>> instead?
> >>
> >> It seems a good idea. We can try to allocate memory firstly, if successful,
> >> just use the new page to remap (it can reduce memory fragmentation).
> >> If not, we can use part of the hugetlb page to remap. What's your opinion
> >> about this?
> >
> > If the HugeTLB page is a gigantic page which is allocated from
> > CMA. In this case, we cannot use part of the hugetlb page to remap.
> > Right?
>
> Right; and I don't think the "reuse part of a huge page as vmemmap while
> freeing, while that part itself might not have a proper vmemmap yet (or
> might cover itself now)" is particularly straight forward. Maybe I'm
> wrong :)
>
> Also, watch out for huge pages on ZONE_MOVABLE, in that case you also
> shouldn't allocate the vmemmap from there ...
Yeah, you are right. So I tend to trigger OOM to kill other processes to
reclaim some memory when we allocate memory fails.
>
> --
> Thanks,
>
> David / dhildenb
>
