linux-kernel - Re: a question of split_huge

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Date:   Fri, 10 Jul 2020 10:22:09 +0300
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     Alex Shi <alex.shi@...ux.alibaba.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>
Subject: Re: a question of split_huge_page



On 10.7.2020 10.00, Alex Shi wrote:
>
> 在 2020/7/10 下午1:28, Mika Penttilä 写道:
>>> 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?
>>
> Why you say so? It looks like the tail page returned and be used
> 	pages = __iommu_dma_alloc_pages() in iommu_dma_alloc_remap()
> and still on node's lru. Is this right?
>
> thanks!
IMHO they are new pages coming from alloc_pages_node() so they are not
on lru. And split_huge_page() frees not pinned tail pages again to page
allocator.

Thanks,
Mika




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