linux-kernel - Re: a question of split_huge

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Date:   Fri, 10 Jul 2020 12:51:58 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Linux-MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: a question of split_huge_page



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 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));
        }
 }