On 10.7.2020 7.51, Alex Shi wrote:
>
> 在 2020/7/10 上午12:07, Kirill A. Shutemov 写道:
>> On Thu, Jul 09, 2020 at 04:50:02PM +0100, Matthew Wilcox wrote:
>>> On Thu, Jul 09, 2020 at 11:11:11PM +0800, Alex Shi wrote:
>>>> Hi Kirill & Matthew,
>>>>
>>>> In the func call chain, from split_huge_page() to lru_add_page_tail(),
>>>> Seems tail pages are added to lru list at line 963, but in this scenario
>>>> the head page has no lru bit and isn't set the bit later. Why we do this?
>>>> or do I miss sth?
>>> I don't understand how we get to split_huge_page() with a page that's
>>> not on an LRU list. Both anonymous and page cache pages should be on
>>> an LRU list. What am I missing?>
>
> Thanks a lot for quick reply!
> What I am confusing is the call chain: __iommu_dma_alloc_pages()
> to split_huge_page(), in the func, splited page,
> page = alloc_pages_node(nid, alloc_flags, order);
> And if the pages were added into lru, they maybe reclaimed and lost,
> that would be a panic bug. But in fact, this never happened for long time.
> Also I put a BUG() at the line, it's nevre triggered in ltp, and run_vmtests
In __iommu_dma_alloc_pages, after split_huge_page(), who is taking a
reference on tail pages? Seems tail pages are freed and the function
errornously returns them in pages[] array for use?
> in kselftest.
>
>> Right, and it's never got removed from LRU during the split. The tail
>> pages have to be added to LRU because they now separate from the tail
>> page.
>>
> According to the explaination, looks like we could remove the code path,
> since it's never got into. (base on my v15 patchset). Any comments?
>
> Thanks
> Alex
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 7c52c5228aab..c28409509ad3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2357,17 +2357,6 @@ static void lru_add_page_tail(struct page *head, struct page *page_tail,
> if (!list)
> SetPageLRU(page_tail);
>
> if (likely(PageLRU(head)))
> list_add_tail(&page_tail->lru, &head->lru);
> else if (list) {
> /* page reclaim is reclaiming a huge page */
> get_page(page_tail);
> list_add_tail(&page_tail->lru, list);
> - } else {
> - /*
> - * Head page has not yet been counted, as an hpage,
> - * so we must account for each subpage individually.
> - *
> - * Put page_tail on the list at the correct position
> - * so they all end up in order.
> - */
> - VM_BUG_ON_PAGE(1, head);
> - add_page_to_lru_list_tail(page_tail, lruvec,
> - page_lru(page_tail));
> }
> }
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