linux-kernel - Re: [dm-devel] [PATCH v5] fault-injection: introduce kvmalloc fallback options

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Date:   Fri, 27 Apr 2018 01:21:52 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     John Stoffel <john@...ffel.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Michal@...ffel.org, eric.dumazet@...il.com, netdev@...r.kernel.org,
        jasowang@...hat.com, Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        dm-devel@...hat.com, Vlastimil Babka <vbabka@...e.cz>,
        Andrew@...ffel.org, David Rientjes <rientjes@...gle.com>,
        Morton <akpm@...ux-foundation.org>,
        virtualization@...ts.linux-foundation.org,
        David Miller <davem@...emloft.net>, edumazet@...gle.com
Subject: Re: [dm-devel] [PATCH v5] fault-injection: introduce kvmalloc
 fallback options

On Thu, Apr 26, 2018 at 05:50:20PM -0400, Mikulas Patocka wrote:
> How is the user or developer supposed to learn about this option, if 
> he gets no crash at all?

Look in /sys/kernel/debug/fail* ? That actually lets you
filter by module, process etc.

I think this patch conflates two things:

1. Make kvmalloc use the vmalloc path.
    This seems a bit narrow.
    What is special about kvmalloc? IMHO nothing - it's yet another user
    of __GFP_NORETRY or __GFP_RETRY_MAYFAIL. As any such
    user, it either recovers correctly or not.
    So IMHO it's just a case of
    making __GFP_NORETRY, __GFP_RETRY_MAYFAIL, or both
    fail once in a while.
    Seems like a better extension to me than focusing on vmalloc.
    I think you will find more bugs this way.

2. Ability to control this from a separate config
   option.

   It's still not that clear to me why is this such a
   hard requirement.  If a distro wants to force specific
   boot time options, why isn't CONFIG_CMDLINE sufficient?

   But assuming it's important to control this kind of
   fault injection to be controlled from
   a dedicated menuconfig option, why not the rest of
   faults?

IMHO if you split 1/2 up, and generalize, the path upstream
will be much smoother.

Hope this helps.

-- 
MST