Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sat, 20 Jan 2024 14:13:14 +0800 | Subject | Re: [PATCH v10 0/4] RISC-V: mm: Make SV48 the default address space | From | Yangyu Chen <> |
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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 >
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