linux-kernel - Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

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Message-ID: <CAPcyv4gY2SZZwiv9DtjRk4js3gS=vf4YLJvmsMJ196aps4ZHcQ@mail.gmail.com>
Date:	Mon, 30 Nov 2015 14:08:53 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	mauricio.porto@....com, Linux MM <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

On Mon, Nov 23, 2015 at 12:04 PM, Toshi Kani <toshi.kani@....com> wrote:
> The following oops was observed when mmap() with MAP_POPULATE
> pre-faulted pmd mappings of a DAX file.  follow_trans_huge_pmd()
> expects that a target address has a struct page.
>
>   BUG: unable to handle kernel paging request at ffffea0012220000
>   follow_trans_huge_pmd+0xba/0x390
>   follow_page_mask+0x33d/0x420
>   __get_user_pages+0xdc/0x800
>   populate_vma_page_range+0xb5/0xe0
>   __mm_populate+0xc5/0x150
>   vm_mmap_pgoff+0xd5/0xe0
>   SyS_mmap_pgoff+0x1c1/0x290
>   SyS_mmap+0x1b/0x30
>
> Fix it by making the PMD pre-fault handling consistent with PTE.
> After pre-faulted in faultin_page(), follow_page_mask() calls
> follow_trans_huge_pmd(), which is changed to call follow_pfn_pmd()
> for VM_PFNMAP or VM_MIXEDMAP.  follow_pfn_pmd() handles FOLL_TOUCH
> and returns with -EEXIST.
>
> Reported-by: Mauricio Porto <mauricio.porto@....com>
> Signed-off-by: Toshi Kani <toshi.kani@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Matthew Wilcox <willy@...ux.intel.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---

Hey Toshi,

I ended up fixing this differently with follow_pmd_devmap() introduced
in this series:

https://lists.01.org/pipermail/linux-nvdimm/2015-November/003033.html

Does the latest libnvdimm-pending branch [1] pass your test case?

[1]: git://git.kernel.org/pub/scm/linux/kernel/git/djbw/nvdimm libnvdimm-pending
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