Date:	Sat, 13 Feb 2016 12:54:18 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Toshi Kani <toshi.kani@....com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	henning.schild@...mens.com, linux-nvdimm@...ts.01.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/mm/vmfault: Make vmalloc_fault() handle large
 pages

On Thu, Feb 11, 2016 at 05:27:16PM -0700, Toshi Kani wrote:
> The following oops was observed when a read syscall was made to
> a pmem device after a huge amount (>512GB) of vmalloc ranges was
> allocated by ioremap() on a x86_64 system.
> 
>  BUG: unable to handle kernel paging request at ffff880840000ff8
>  IP: [<ffffffff810664ae>] vmalloc_fault+0x1be/0x300
>  PGD c7f03a067 PUD 0
>  Oops: 0000 [#1] SM
>    :
>  Call Trace:
>  [<ffffffff81067335>] __do_page_fault+0x285/0x3e0
>  [<ffffffff810674bf>] do_page_fault+0x2f/0x80
>  [<ffffffff810d6d85>] ? put_prev_entity+0x35/0x7a0
>  [<ffffffff817a6888>] page_fault+0x28/0x30
>  [<ffffffff813bb976>] ? memcpy_erms+0x6/0x10
>  [<ffffffff817a0845>] ? schedule+0x35/0x80
>  [<ffffffffa006350a>] ? pmem_rw_bytes+0x6a/0x190 [nd_pmem]
>  [<ffffffff817a3713>] ? schedule_timeout+0x183/0x240
>  [<ffffffffa028d2b3>] btt_log_read+0x63/0x140 [nd_btt]
>    :
>  [<ffffffff811201d0>] ? __symbol_put+0x60/0x60
>  [<ffffffff8122dc60>] ? kernel_read+0x50/0x80
>  [<ffffffff81124489>] SyS_finit_module+0xb9/0xf0
>  [<ffffffff817a4632>] entry_SYSCALL_64_fastpath+0x1a/0xa4

Please remove those virtual addresses and offsets here as they're
meaningless and leave only the callstack.

> Since 4.1, ioremap() supports large page (pud/pmd) mappings in
> x86_64 and PAE.  vmalloc_fault() however assumes that the vmalloc
> range is limited to pte mappings.
> 
> vmalloc faults do not normally happen in ioremap'd ranges since
> ioremap() sets up the kernel page tables, which are shared by
> user processes.  pgd_ctor() sets the kernel's pgd entries to
> user's during fork().  When allocation of the vmalloc ranges
> crosses a 512GB boundary, ioremap() allocates a new pud table
> and updates the kernel pgd entry to point it.  If user process's
> pgd entry does not have this update yet, a read/write syscall
> to the range will cause a vmalloc fault, which hits the Oops
> above as it does not handle a large page properly.
> 
> Following changes are made to vmalloc_fault().
> 
> 64-bit:
> - No change for the pgd sync operation as it handles large
>   pages already.
> - Add pud_huge() and pmd_huge() to the validation code to
>   handle large pages.
> - Change pud_page_vaddr() to pud_pfn() since an ioremap range
>   is not directly mapped (while the if-statement still works
>   with a bogus addr).
> - Change pmd_page() to pmd_pfn() since an ioremap range is not
>   backed by struct page (while the if-statement still works
>   with a bogus addr).
> 
> 32-bit:
> - No change for the sync operation since the index3 pgd entry
>   covers the entire vmalloc range, which is always valid.
>   (A separate change to sync pgd entry is necessary if this
>    memory layout is changed regardless of the page size.)
> - Add pmd_huge() to the validation code to handle large pages.
>   This is for completeness since vmalloc_fault() won't happen
>   in ioremap'd ranges as its pgd entry is always valid.
> 
> Reported-by: Henning Schild <henning.schild@...mens.com>
> Signed-off-by: Toshi Kani <toshi.kani@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Borislav Petkov <bp@...en8.de>
> ---
> When this patch is accepted, please copy to stable up to 4.1.

You can do that yourself when submitting by adding this to the CC-list
above.

Cc: <stable@...r.kernel.org> # 4.1..

Rest looks ok to me.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

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