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Subject: Re: a question of split_huge_page
To:     Alex Shi <alex.shi@linux.alibaba.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Linux-MM <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Hugh Dickins <hughd@google.com>
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From:   =?UTF-8?Q?Mika_Penttil=c3=a4?= <mika.penttila@nextfour.com>
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On 10.7.2020 10.00, Alex Shi wrote:
>
> =E5=9C=A8 2020/7/10 =E4=B8=8B=E5=8D=881:28, Mika Penttil=C3=A4 =E5=86=99=
=E9=81=93:
>>> 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 =3D 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 ti=
me.
>>> Also I put a BUG() at the line, it's nevre triggered in ltp, and run_vm=
tests
>> In=C2=A0 __iommu_dma_alloc_pages, after split_huge_page(),=C2=A0 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 =3D __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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