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.
