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

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Message-ID: <ZMgjqJycJFsgvWOD@murray>
Date:   Mon, 31 Jul 2023 22:12:08 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     David Hildenbrand <david@...hat.com>, Baoquan He <bhe@...hat.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 10:34:21PM +0200, Jiri Olsa wrote:
> 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

Ummmm what? I can't! What repro are you seeing on x86, exactly?

>
> >
> > 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

That's quite a few more reverts and obviously not an acceptable solution here.

Looking at
https://lore.kernel.org/all/CAA5enKaUYehLZGL3abv4rsS7caoUG-pN9wF3R+qek-DGNZufbA@mail.gmail.com
a parallel thread on this, it looks like the issue is that we are no longer
using a no-fault kernel copy in KCORE_TEXT path and arm64 doesn't map everything
in the text range.

Solution would be to reinstate the bounce buffer in this case (ugh). Longer term
solution I think would be to create some iterator helper that does no fault
copies from the kernel.

I will try to come up with a semi-revert that keeps the iterator stuff but
keeps a hideous bounce buffer for the KCORE_TEXT bit with a comment
explaining why...