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Date:   Sat, 28 Mar 2020 16:50:31 -0400 (EDT)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Adam Borowski <kilobyte@angband.pl>
cc:     Chen Wandun <chenwandun@huawei.com>, jslaby@suse.com,
        gregkh@linuxfoundation.org, daniel.vetter@ffwll.ch,
        sam@ravnborg.org, b.zolnierkie@samsung.com, lukas@wunner.de,
        ghalat@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH next] vt: fix a warning when kmalloc alloc large memory
In-Reply-To: <20200328031257.GA30454@angband.pl>
Message-ID: <nycvar.YSQ.7.76.2003281641230.2671@knanqh.ubzr>
References: <20200328021340.27315-1-chenwandun@huawei.com> <nycvar.YSQ.7.76.2003272232340.2671@knanqh.ubzr> <20200328031257.GA30454@angband.pl>
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On Sat, 28 Mar 2020, Adam Borowski wrote:

> On Fri, Mar 27, 2020 at 10:55:14PM -0400, Nicolas Pitre wrote:
> > On Sat, 28 Mar 2020, Chen Wandun wrote:
> > 
> > > If the memory size that use kmalloc() to allocate exceed MAX_ORDER pages,
> > > it will hit the WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN)), so add memory
> > > allocation flag __GFP_NOWARN to silence a warning, othervise, it will
> > > cause panic if panic_on_warn is enable.
> > 
> > Wow! How do you manage that? You need something like a 1024x1024 text 
> > screen to get such a big memory allocation.
> 
> ioctl(VT_RESIZE) allows up to 32767x32767, unprivileged for a local user.
> That's 4GB per console.

In fact that's not exactly true. The code has this protection:

        if (new_screen_size > (4 << 20))
                return -EINVAL;

The problem is with the unicode screen content whose buffer is larger 
than the legacy glyph buffer. Still, the above test is a bit iffy as it 
depends on the default MAX_ORDER value which is configurable.


Nicolas