Re: [PATCH] riscv: mm: remove the BUG_ON check of mapping the last 4K bytes of memory

[Alexandre Ghiti]

Hi Palmer,

On 2/15/22 00:41, Palmer Dabbelt wrote:
> On Tue, 25 Jan 2022 08:10:41 PST (-0800), alex@xxxxxxxx wrote:
> > On 1/25/22 16:55, Jisheng Zhang wrote:
> > > remove the BUG_ON check of mapping the last 4K bytes of the addressable
> > > memory since "this is true for every kernel actually" as pointed out
> > > by Alexandre.
> > >
> > > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > > Reviewed-by: Alexandre Ghiti <alex@xxxxxxxx>
> > > ---
> > >   arch/riscv/mm/init.c | 8 --------
> > >   1 file changed, 8 deletions(-)
> > >
> > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > > index cf4d018b7d66..8347d0fda8cd 100644
> > > --- a/arch/riscv/mm/init.c
> > > +++ b/arch/riscv/mm/init.c
> > > @@ -811,14 +811,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> > >       BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
> > >       BUG_ON((kernel_map.phys_addr % PMD_SIZE) != 0);
> > >
> > > -#ifdef CONFIG_64BIT
> > > -    /*
> > > -     * The last 4K bytes of the addressable memory can not be 
> > > mapped because
> > > -     * of IS_ERR_VALUE macro.
> > > -     */
> > > -    BUG_ON((kernel_map.virt_addr + kernel_map.size) > 
> > > ADDRESS_SPACE_END - SZ_4K);
> > > -#endif
> >
> >
> > This BUG_ON seems pretty legit to me: I re-read the exchanges we had,
> > and I see that I didn't notice that in your v2, you actually removed the
> > BUG_ON. So that's my bad, what I meant in the first place was that the
> > BUG_ON is true for 32-bit and 64-bit kernels actually.
>
> There's actually an ifndef 64BIT above that sort of handles this case 
> (though I didn't check to see if we're getting the limits correct, so 
> it may not work properly).  That's shrinking the memory, rather than 
> just firing a BUG, and it's not really any more code so we should go 
> that way for both.  I could see leaving a BUG in there, maybe just 
> explicitly using IS_ERR_VALUE as that's really what we're checking for 
> (though if that's not 4K a bunch of stuff will break, so maybe it just 
> doesn't matter).


I think you're talking about:

memory_limit = KERN_VIRT_SIZE - (IS_ENABLED(CONFIG_64BIT) ? SZ_4G : 0);

If yes, that's shrinking the available physical memory we can map in the 
linear mapping, whereas the BUG_ON here is triggered if the kernel 
mapping is very (very) large and overlaps the last *4K* of the address 
space, so I still think the BUG_ON is legit although very unlikely.

However, I'm wondering if the shrinking still holds with the recent move 
of KASAN, I may be back with a fix for that.

Alex


> > Sorry my RB was not right on this one :(
> >
> > Alex
> >
> >
> > > -
> > >       pt_ops_set_early();
> > >
> > >       /* Setup early PGD for fixmap */