linux-kernel - Re: [PATCH 3/3] mm:fix gup_pud

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Date:   Fri, 20 Sep 2019 21:28:47 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Qiujun Huang <hqjagain@...il.com>, akpm@...ux-foundation.org
Cc:     ira.weiny@...el.com, jgg@...pe.ca, dan.j.williams@...el.com,
        rppt@...ux.ibm.com, jhubbard@...dia.com, keith.busch@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm:fix gup_pud_range

On 9/20/19 9:21 PM, Qiujun Huang wrote:
> __get_user_pages_fast try to walk the page table but the
> hugepage pte is replace by hwpoison swap entry by mca path.
> ...


Can you describe this in more details. I guess you are facing the issue 
with respect PUD level PTE entry that got updated by hwpoison as a swap 
entry. Since we don't specifically check for pud_present(), we walk the 
page table with wrong values and that results in corruption?


> [15798.177437] mce: Uncorrected hardware memory error in
> 				user-access at 224f1761c0
> [15798.180171] MCE 0x224f176: Killing pal_main:6784 due to
> 				hardware memory corruption
> [15798.180176] MCE 0x224f176: Killing qemu-system-x86:167336
> 				due to hardware memory corruption
> ...
> [15798.180206] BUG: unable to handle kernel
> [15798.180226] paging request at ffff891200003000
> [15798.180236] IP: [<ffffffff8106edae>] gup_pud_range+
> 				0x13e/0x1e0
> ...
> 
> We need to skip the hwpoison entry in gup_pud_range.
> 
> Signed-off-by: Qiujun Huang <hqjagain@...il.com>
> ---
>   mm/gup.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 98f13ab..6157ed9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2230,6 +2230,8 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>   		next = pud_addr_end(addr, end);
>   		if (pud_none(pud))
>   			return 0;
> +		if (unlikely(!pud_present(pud)))
> +			return 0;


You should be able to remove that if (pud_none(pud)) check and just keep 
the pud_present() check?

>   		if (unlikely(pud_huge(pud))) {
>   			if (!gup_huge_pud(pud, pudp, addr, next, flags,
>   					  pages, nr))
>