linux-kernel - Re: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data

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Date:   Mon, 31 Jul 2023 20:40:24 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Baoquan He <bhe@...hat.com>, Jiri Olsa <olsajiri@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Liu Shixin <liushixin2@...wei.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data

On Mon, Jul 31, 2023 at 09:24:50PM +0200, David Hildenbrand wrote:
> On 31.07.23 21:21, Lorenzo Stoakes wrote:
> > On Mon, Jul 24, 2023 at 08:23:55AM +0200, David Hildenbrand wrote:
> > > Hi,
> > >
> > > >
> > > > I met this too when I executed below command to trigger a kcore reading.
> > > > I wanted to do a simple testing during system running and got this.
> > > >
> > > >     makedumpfile --mem-usage /proc/kcore
> > > >
> > > > Later I tried your above objdump testing, it corrupted system too.
> > > >
> > >
> > > What do you mean with "corrupted system too" --  did it not only fail to
> > > dump the system, but also actually harmed the system?
> > >
> > > @Lorenzo do you plan on reproduce + fix, or should we consider reverting
> > > that change?
> > >
> > > --
> > > Cheers,
> > >
> > > David / dhildenb
> > >
> >
> > Apologies I mised this, I have been very busy lately not least with book :)
> >
> > Concerning, I will take a look as I get a chance. I think the whole series
> > would have to be reverted which would be... depressing... as other patches
> > in series eliminates the bounce buffer altogether.
> >
>
> I spotted
>
> https://lkml.kernel.org/r/069dd40aa71e634b414d07039d72467d051fb486.camel@gmx.de
>

Find that slightly confusing, they talk about just reveritng the patch but then
also add a kern_addr_valid()?

I'm also confused about people talking about just reverting the patch, as
4c91c07c93bb drops the bounce buffer altogether... presumably they mean
reverting both?

Clearly this is an arm64 thing (obviously), I have some arm64 hardware let me
see if I can repro...

Baoquan, Jiri - are you reverting more than just the one commit? And does doing
this go from not working -> working? Or from not working (worst case oops) ->
error?

> today, maybe that's related.
>
> --
> Cheers,
>
> David / dhildenb
>