* Re: a question of split_huge_page
2020-07-10 9:34 ` a question of split_huge_page Alex Shi
@ 2020-07-10 12:56 ` Joerg Roedel
2020-07-10 17:29 ` Yang Shi
1 sibling, 0 replies; 3+ messages in thread
From: Joerg Roedel @ 2020-07-10 12:56 UTC (permalink / raw)
To: Alex Shi, Robin Murphy
Cc: Hugh Dickins, linux-kernel, Matthew Wilcox, Linux-MM, iommu,
Mika Penttilä,
Johannes Weiner, Kirill A. Shutemov
Adding Robin.
On Fri, Jul 10, 2020 at 05:34:52PM +0800, Alex Shi 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?
>
> 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); <==
> >
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^ permalink raw reply [flat|nested] 3+ messages in thread* Re: a question of split_huge_page
2020-07-10 9:34 ` a question of split_huge_page Alex Shi
2020-07-10 12:56 ` Joerg Roedel
@ 2020-07-10 17:29 ` Yang Shi
1 sibling, 0 replies; 3+ messages in thread
From: Yang Shi @ 2020-07-10 17:29 UTC (permalink / raw)
To: Alex Shi
Cc: Hugh Dickins, linux-kernel, Matthew Wilcox, Linux-MM, iommu,
Mika Penttilä,
Johannes Weiner, Kirill A. Shutemov
On Fri, Jul 10, 2020 at 2:35 AM Alex Shi <alex.shi@linux.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); <==
> >
>
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^ permalink raw reply [flat|nested] 3+ messages in thread