linux-kernel - Re: a question of split_huge

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Message-ID: <CAHbLzko9DtqTpamdGAZRtRm9fq8E-FQR_79SHso7SeheUjzEzA@mail.gmail.com>
Date:   Fri, 10 Jul 2020 10:29:27 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Mika Penttilä <mika.penttila@...tfour.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        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

On Fri, Jul 10, 2020 at 2:35 AM Alex Shi <alex.shi@...ux.alibaba.com> wrote:
>
> 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?

I think this is the same problem which has been discussed a couple of
weeks ago. Please refer to:
https://lore.kernel.org/linux-mm/20200619001938.GA135965@carbon.dhcp.thefacebook.com/

I think the conclusion is split_huge_page() can't be used in this path
at all. But we didn't reach a fix yet.

>
> 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);        <==
> >
>