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

[Mel Gorman]

On Thu, Aug 31, 2023 at 02:21:06PM +0800, Muchun Song wrote:
> 
> 
> > On Aug 30, 2023, at 18:27, Usama Arif <usama.arif@xxxxxxxxxxxxx> 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!
> 
> Hi Mel,
> 
> Could you help us answer this question? I think you must be the expert of
> CONFIG_DEFERRED_STRUCT_PAGE_INIT. I summarize the context here. As we all know,
> some struct pages are uninnitialized when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
> enabled, if someone allocates a larger memory (e.g. order is 4) via buddy
> allocator and fails to allocate the memory, then we will go into the compacting
> routine, which will traverse all pfns and use pfn_to_page to access its struct
> page, however, those struct pages may be uninnitialized (so it's arbitrary data).
> Our question is how to prevent the compacting routine from accessing those
> uninitialized struct pages? We'll be appreciated if you know the answer.
> 

I didn't check the code but IIRC, the struct pages should be at least
valid and not contain arbitrary data once page_alloc_init_late finishes.

-- 
Mel Gorman
SUSE Labs