Re: [PATCH v10 0/4] RISC-V: mm: Make SV48 the default address space

[Yangyu Chen]

Thanks for your reply.

On 1/20/24 09:34, Charlie Jenkins wrote:
> On Sun, Jan 14, 2024 at 01:26:57AM +0800, Yangyu Chen wrote:
> > Hi, Charlie
> >
> > Although this patchset has been merged I still have some questions about
> > this patchset. Because it breaks regular mmap if address >= 38 bits on
> > sv48 / sv57 capable systems like qemu. For example, If a userspace program
> > wants to mmap an anonymous page to addr=(1<<45) on an sv48 capable system,
> > it will fail and kernel will mmaped to another sv39 address since it does
>
> Thank you for raising this concern. To make sure I am understanding
> correctly, you are passing a hint address of (1<<45) and expecting mmap
> to return 1<<45 and if it returns a different address you are describing
> mmap as failing? If you want an address that is in the sv48 space you
> can pass in an address that is greater than 1<<47.
>
> > not meet the requirement to use sv48 as you wrote:
> >
> > > 	else if ((((_addr) >= VA_USER_SV48)) && (VA_BITS >= VA_BITS_SV48)) \
> > > 		mmap_end = VA_USER_SV48;			\
> > > 	else							\
> > > 		mmap_end = VA_USER_SV39;			\
> >
> > Then, How can a userspace program create a mmap with a hint if the address
> > > = (1<<38) after your patch without MAP_FIXED? The only way to do this is
> > to pass a hint >= (1<<47) on mmap syscall then kernel will return a random
> > address in sv48 address space but the hint address gets lost. I think this
>
> In order to force mmap to return the address provided you must use
> MAP_FIXED. Otherwise, the address is a "hint" and has no guarantees. The
> hint address on riscv is used to mean "don't give me an address that
> uses more bits than this". This behavior is not unique to riscv, arm64
> and powerpc use a similar scheme. In arch/arm64/include/asm/processor.h
> there is the following code:
>
> #define arch_get_mmap_base(addr, base) ((addr > DEFAULT_MAP_WINDOW) ? \
> 					base + TASK_SIZE - DEFAULT_MAP_WINDOW :\
> 					base)
>
> arm64/powerpc are only concerned with a single boundary so the code is simpler.

As you say, this code in arm64/powerpc will not meet the issue I 
address. For example, If the addr here is (1<<50) on arm64, the 
arch_get_mmap_base will return base+TASK_SIZE-DEFAULT_MAP_WINDOW which 
is (1<<vabits_actual). And this behavior on arm64/powerpc/x86 does not 
break anything since we will use a larger address space if the hint 
address is specified on the address > DEFAULT_MAP_WINDOW. The 
corresponding behavior on RISC-V should be if the hint address > BIT(47) 
then use Sv57 address space and use Sv48 when the hint address > BIT(38) 
if we want Sv39 by default.

However, your patch needs the address >= BIT(47) rather than BIT(38) to 
use Sv48 and address >= BIT(56) to use Sv57, thus breaking existing 
userspace software to create mapping on the hint address without 
MAP_FIXED set.

> > violate the principle of mmap syscall as kernel should take the hint and
> > attempt to create the mapping there.
>
> Although the man page for mmap does say "on Linux, the kernel will pick
> a nearby page boundary" it is still a hint address so there is no strict
> requirement (and the precedent has already been set by arm64/powerpc).

Yeah. There is no strict requirement. But currently x86/arm64/powerpc 
works in this situation well. The hint address on these ISAs is not used 
as the upper bound to allocating the address. However, on RISC-V, you 
treat this as the upper bound.

> > I don't think patching in this way is right. However, if we only revert
> > this patch, some programs relying on mmap to return address with effective
> > bits <= 48 will still be an issue and it might expand to other ISAs if
> > they implement larger virtual address space like RISC-V sv57. A better way
> > to solve this might be adding a MAP_48BIT flag to mmap like MAP_32BIT has
> > been introduced for decades.
> >
> > Thanks,
> > Yangyu Chen
>
> - Charlie