Re: [patch 019/154] mm: make madvise(MADV_WILLNEED) support swap file prefetch

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Sasha Levin wrote:
> Hi Andrea,
> 
> On 12/16/2013 03:52 PM, Andrea Arcangeli wrote:
> > Is the bug reproducible? If yes the simplest is probably to add some
> > allocation tracking to the page, so if page->ptl is null we can simply
> > print a stack trace of who allocated the page (and later forgot to
> > initialize the ptl).
> 
> Yes, it's easy to reproduce.

I'm trying to reproduce it with trinity. No luck so far. Any suggestions?
Kernel config? VM setup? Do you have swap enabled? How do you run trinity?

> I've done as suggested and here's the trace from
> the allocation:
> 
> [  184.139519]  [<ffffffff8107de0f>] save_stack_trace+0x2f/0x50
> [  184.140706]  [<ffffffff81257769>] get_page_from_freelist+0x759/0x7a0
> [  184.141605]  [<ffffffff81258438>] __alloc_pages_nodemask+0x3b8/0x520
> [  184.142810]  [<ffffffff812a4baf>] alloc_pages_vma+0x1df/0x220
> [  184.143631]  [<ffffffff812bcd58>] do_huge_pmd_wp_page+0x2d8/0x730
> [  184.144526]  [<ffffffff81280e01>] __handle_mm_fault+0x2b1/0x3d0
> [  184.145361]  [<ffffffff81281053>] handle_mm_fault+0x133/0x1c0
> [  184.146129]  [<ffffffff812815f8>] __get_user_pages+0x448/0x640
> [  184.147055]  [<ffffffff812827a4>] __mlock_vma_pages_range+0xd4/0xe0
> [  184.147980]  [<ffffffff812828c0>] __mm_populate+0x110/0x190
> [  184.148933]  [<ffffffff812839b2>] SyS_mlock+0xf2/0x130
> [  184.149689]  [<ffffffff843c5e50>] tracesys+0xdd/0xe2

It's trace from huge page allocation, not from page table allocation we
are interested in.

In our case we need to know who allocated pmd_page(*pmd) when 

orig_pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);

crashes. Note: we usually allocate page tables with __GFP_NOTRACK. It
probably need to be changed for this experiment.

> > Agree with Kirill that it would help to verify the bug goes away by
> > disabling USE_SPLIT_PTE_PTLOCKS.
> 
> It seems that the bug is gone without USE_SPLIT_PTE_PTLOCKS.

What about PMD sibling: USE_SPLIT_PMD_PTLOCKS?
I mean USE_SPLIT_PTE_PTLOCKS == 1, USE_SPLIT_PMD_PTLOCKS == 0.

-- 
 Kirill A. Shutemov

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