Re: [External] Re: [v3 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO

[Mike Rapoport]

On Wed, Aug 30, 2023 at 11:27:42AM +0100, Usama Arif wrote:
> 
> On 28/08/2023 12:33, Muchun Song wrote:
> > 
> > 
> > > On Aug 25, 2023, at 19:18, Usama Arif <usama.arif@xxxxxxxxxxxxx> 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, HVO is attempted. If its successful, only the first
> > > HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
> > > after the head struct page are initialized. If it is not successful,
> > > then all of the tail struct pages are initialized.
> > > 
> > > Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
> > 
> > This edition is simpler than before ever, thanks for your work.
> > 
> > There is premise that other subsystems do not access vmemmap pages
> > before the initialization of vmemmap pages associated withe HugeTLB
> > pages allocated from bootmem for your optimization. However, IIUC, the
> > compacting path could access arbitrary struct page when memory fails
> > to be allocated via buddy allocator. So we should make sure that
> > those struct pages are not referenced in this routine. And I know
> > if CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, it will encounter
> > the same issue, but I don't find any code to prevent this from
> > happening. I need more time to confirm this, if someone already knows,
> > please let me know, thanks. So I think HugeTLB should adopt the similar
> > way to prevent this.
> > 
> > Thanks.
> > 
> 
> Thanks for the reviews.
> 
> So if I understand it correctly, the uninitialized pages due to the
> optimization in this patch and due to DEFERRED_STRUCT_PAGE_INIT should be
> treated in the same way during compaction. I see that in isolate_freepages
> during compaction there is a check to see if PageBuddy flag is set and also
> there are calls like __pageblock_pfn_to_page to check if the pageblock is
> valid.
> 
> But if the struct page is uninitialized then they would contain random data
> and these checks could pass if certain bits were set?
> 
> Compaction is done on free list. I think the uninitialized struct pages
> atleast from DEFERRED_STRUCT_PAGE_INIT would be part of freelist, so I think
> their pfn would be considered for compaction.
> 
> Could someone more familiar with DEFERRED_STRUCT_PAGE_INIT and compaction
> confirm how the uninitialized struct pages are handled when compaction
> happens? Thanks!

I'm not familiar with compaction enough to confirm it only touches pages on
the free lists, but DEFERRED_STRUCT_PAGE_INIT makes sure the struct page is
initialized before it's put on a free list.
 
> Usama

-- 
Sincerely yours,
Mike.