linux-kernel - Re: a question of split_huge

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Date:   Fri, 10 Jul 2020 17:34:52 +0800
From:   Alex Shi <alex.shi@...ux.alibaba.com>
To:     Mika Penttilä <mika.penttila@...tfour.com>,
        "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>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org
Subject: Re: a question of split_huge_page

在 2020/7/10 下午1:28, Mika Penttilä 写道:
> 
> 
> 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?
> 

CC Joerg and iommu list,

That's a good question. seems the split_huge_page was never triggered here,
since the func would check the PageLock first. and have page->mapping and PageAnon
check, any of them couldn't be matched for the alloced page.

Hi Joerg,
would you like look into this? do we still need the split_huge_page() here?

Thanks
Alex

int split_huge_page_to_list(struct page *page, struct list_head *list)
{
        struct page *head = compound_head(page);
        struct deferred_split *ds_queue = get_deferred_split_queue(head);
        struct anon_vma *anon_vma = NULL;
        struct address_space *mapping = NULL;
        int count, mapcount, extra_pins, ret;
        pgoff_t end;

        VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
        VM_BUG_ON_PAGE(!PageLocked(head), head);	<==
>