On Tue, Aug 17, 2021 at 11:05:15AM +0200, Thomas Gleixner wrote:

Huacai,

On Tue, Aug 17 2021 at 15:38, Huacai Chen wrote:

quoted

On Tue, Aug 17, 2021 at 3:07 PM Thomas Gleixner [off-list ref] wrote:
On X86, it returns 0; on MIPS64 without patch, it hangs in kernel; on
MIPS64 with this patch, it returns -1.

As expected.

quoted

Then, I want to know, on "W implies R" archs (such as X86), should it
return 0? Maybe return -1 is more reasonable? (because the VMA is
marked as write-only). If this program should return -1, then I don't
think this is a MIPS-specific problem.

No. mmap(.., PROT_WRITE...) is simply impossible on x86 and implies
PROT_READ as documented in mmap(2).

So why should this fail and only fail in the fault case, but succeed
when the PTE is already established?

I wouldn't actually mind if it failed on fault -- it's the 'best' we can
do on x86. Doing a RmW op on PROT_WRITE is silly and deserves all the
wreckage it can get.