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

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Date:   Tue, 1 Aug 2017 23:51:57 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "axboe@...com" <axboe@...com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [PATCH] mm: allow page_cache_get_speculative in interrupt context

On Tue, Aug 01, 2017 at 08:31:14PM +0000, Liang, Kan wrote:
> > 
> > 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?
> >
> 
> 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.

I don't think so.

It's 2+ year in and nobody stepped onto this until you with your patchset.
There is no real reason to get it backported.

-- 
 Kirill A. Shutemov