RE: [PATCH] mm: allow page_cache_get_speculative in interrupt context

[Liang, Kan]

> 
> On Tue,  1 Aug 2017 13:39:06 -0400 kan.liang@xxxxxxxxx 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?
>

The code I'm implementing will call __get_user_page_fast from NMI context.
__get_user_page_fast will eventually call page_cache_get_speculative().
 
> 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.

Sorry for the confusion.

According to the comments, __get_user_page_fast should be IRQ-safe. But it's not.
This patch could be used to resolve the inconsistencies between the comments
and implementations for now.
The generic __get_user_page_fast was introduced by the commit
2667f50e8b81457fcb4a3dbe6aff3e81ea009e13
mm: introduce a general RCU get_user_pages_fast()
I think the kernel after the commit should be fixed.

Thanks,
Kan