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

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

> On Aug 31, 2023, at 17:58, Mel Gorman <mgorman@techsingularity.net> wrote:
> 
> On Thu, Aug 31, 2023 at 02:21:06PM +0800, Muchun Song wrote:
>> 
>> 
>>> On Aug 30, 2023, at 18:27, Usama Arif <usama.arif@bytedance.com> wrote:
>>> On 28/08/2023 12:33, Muchun Song wrote:
>>>>> On Aug 25, 2023, at 19:18, 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, 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@bytedance.com>
>>>> 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.

However, the buddy allocator is ready before page_alloc_init_late(), so it
may access arbitrary data in compacting routine, right?

> 
> -- 
> Mel Gorman
> SUSE Labs