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

[Muchun Song]

> 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.

Thanks.