Re: [PATCH v2 1/2] powerpc: Fix virt_addr_valid() check

[Kefeng Wang]

On 2022/1/20 15:31, Christophe Leroy wrote:
> Le 19/01/2022 à 02:15, Kefeng Wang a écrit :
> > On 2022/1/11 14:04, Christophe Leroy wrote:
> > > Le 11/01/2022 à 05:37, Nicholas Piggin a écrit :
> > > > Excerpts from Kefeng Wang's message of January 8, 2022 9:58 pm:
> > > > > Hi PPC maintainers， ping..
> > > > Hmm. I might have confused myself about this. I'm going back and
> > > > trying to work out what I was thinking when I suggested it. This
> > > > works on 64e because vmalloc space is below the kernel linear map,
> > > > right?
> > > >
> > > > On 64s it is the other way around and it is still possible to enable
> > > > flatmem on 64s. Altough we might just not hit the problem there because
> > > > __pa() will not mask away the vmalloc offset for 64s so it will still
> > > > return something that's outside the pfn_valid range for flatmem. That's
> > > > very subtle though.
> > > That's the way it works on PPC32 at least, so for me it's not chocking
> > > to have it work the same way on PPC64s.
> > >
> > > The main issue here is the way __pa() works. On PPC32 __pa = va -
> > > PAGE_OFFSET, so it works correctly for any address.
> > > On PPC64, __pa() works by masking out the 2 top bits instead of
> > > substracting PAGE_OFFSET, so the test must add a verification that we
> > > really have the 2 top bits set at first. This is what (addr >=
> > > PAGE_OFFSET) does. Once this first test is done, we can perfectly rely
> > > on pfn_valid() just like PPC32, I see absolutely no point in an
> > > additionnal test checking the addr is below KERN_VIRT_START.
> >
> > Hi Christophe and Nicholas， for ppc32， I think we need check the upper
> > limit,
> Why ? Have you experimented any problem at all on PPC32 with the way it
> is done at the moment ?
>
> I don't think we have to change PPC32 at all unless we have a real
> reason to do it.

yes, I missed this commit in old kernel(lts5.10), you have fixed the 
upper limit.

commit 602946ec2f90d5bd965857753880db29d2d9a1e9
Author: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Date:   Tue Oct 12 12:40:37 2021 +0200

    powerpc: Set max_mapnr correctly


> > eg,  addr >= PAGE_OFFSET && addr < high_memory
> Isn't it exactly what pfn_valid() already do today ?
> Why change that at all ?
>
> Christophe
>
> > arch/powerpc/mm/mem.c:  high_memory = (void *) __va(max_low_pfn *
> > PAGE_SIZE);
> >
> > for ppc32 max_low_pfn is the upper low memory pfn,  and For ppc64,
> > high_memory is
> >
> > the max memory pfn, it looks good too, correct me if I'm wrong, if the
> > above check
> >
> > is ok, I will send a new v3,  thanks.
> >
> >
> >
> >
> > > > The checks added to __pa actually don't prevent vmalloc memory from
> > > > being passed to it either on 64s, only a more basic test.
> > > That's correct. It is the role of pfn_valid() to check that.
> > >
> > > Christophe
> > >
> > > > I think 64s wants (addr >= PAGE_OFFSET && addr < KERN_VIRT_START) as
> > > > the condition.  Could possibly add that check to __pa as well to
> > > > catch vmalloc addresses.
> > > >
> > > > Thanks,
> > > > Nick
> > > >