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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Mon, 31 Jul 2023 22:34:21 +0200
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     David Hildenbrand <david@redhat.com>, Baoquan He <bhe@redhat.com>,
        Jiri Olsa <olsajiri@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Uladzislau Rezki <urezki@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Liu Shixin <liushixin2@huawei.com>,
        Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data
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On Mon, Jul 31, 2023 at 08:40:24PM +0100, Lorenzo Stoakes wrote:
> 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...

I see the issue on x86

> 
> 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?

yes, I used to revert all 4 patches

I did quick check and had to revert 2 more patches to get clean revert

38b138abc355 Revert "fs/proc/kcore: avoid bounce buffer for ktext data"
e2c3b418d365 Revert "fs/proc/kcore: convert read_kcore() to read_kcore_iter()"
d8bc432cb314 Revert "iov_iter: add copy_page_to_iter_nofault()"
bf2c6799f68c Revert "iov_iter: Kill ITER_PIPE"
ccf4b2c5c5ce Revert "mm: vmalloc: convert vread() to vread_iter()"
de400d383a7e Revert "mm/vmalloc: replace the ternary conditional operator with min()"

jirka