Re: [v5 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO

[Muchun Song <muchun.song@linux.dev>]

> On Sep 13, 2023, at 18:54, Usama Arif <usama.arif@bytedance.com> wrote:
> 
> The new boot flow when it comes to initialization of gigantic pages
> is as follows:
> - At boot time, for a gigantic page during __alloc_bootmem_hugepage,
> the region after the first struct page is marked as noinit.
> - This results in only the first struct page to be
> initialized in reserve_bootmem_region. As the tail struct pages are
> not initialized at this point, there can be a significant saving
> in boot time if HVO succeeds later on.
> - Later on in the boot, the head page is prepped and the first
> HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
> are initialized.
> - HVO is attempted. If it is not successful, then the rest of the
> tail struct pages are initialized. If it is successful, no more
> tail struct pages need to be initialized saving significant boot time.
> 
> The WARN_ON for increased ref count in gather_bootmem_prealloc was changed
> to a VM_BUG_ON.  This is OK as there should be no speculative references
> this early in boot process. The VM_BUG_ON's are there just in case such code
> is introduced.
> 
> Signed-off-by: Usama Arif <usama.arif@bytedance.com>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks for your work.