Hi, all, On Tue, Aug 17, 2021 at 5:45 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > 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: > > > On Tue, Aug 17, 2021 at 3:07 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > > On X86, it returns 0; on MIPS64 without patch, it hangs in kernel; on > > > MIPS64 with this patch, it returns -1. > > > > As expected. > > > > > 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. If we must fix it in arch code, there are two methods: 1, don't use write-only map (modify protection_map as Liu Lichao did); 2, override arch_vma_access_permitted() to do extra checks. Thomas, which is better? Huacai
