linux-kernel - Re: [PATCH] mm: allow page_cache_get

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Date:   Tue, 1 Aug 2017 12:49:18 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     kan.liang@...el.com
Cc:     linux-kernel@...r.kernel.org, axboe@...com,
        viro@...iv.linux.org.uk, kirill.shutemov@...ux.intel.com,
        ying.huang@...el.com, npiggin@...il.com, mingo@...nel.org
Subject: Re: [PATCH] mm: allow page_cache_get_speculative in interrupt
 context

On Tue,  1 Aug 2017 13:39:06 -0400 kan.liang@...el.com wrote:

> Kernel panic when calling the IRQ-safe __get_user_pages_fast in NMI
> handler.
> 
> The bug was introduced by commit:
> 
>   2947ba054a4d ("x86/mm/gup: Switch GUP to the generic
> get_user_page_fast() implementation")
> 
> The original x86 __get_user_page_fast used plain get_page() or
> page_ref_add(). However, the generic __get_user_page_fast uses
> page_cache_get_speculative(), which has VM_BUG_ON(in_interrupt()).
> 
> There is no reason to prevent page_cache_get_speculative from using in
> interrupt context. According to the author, putting a BUG_ON there is
> just because the code is not verifying correctness of interrupt races.
> I did some tests in interrupt context. There is no issue found.
> Removing VM_BUG_ON(in_interrupt()) for page_cache_get_speculative().

What code calls page_cache_get_speculative() from NMI context?  

I'm trying to work out which kernel versions need this fix, but there
isn't enough info in the changelog for this.  Please don't do that.