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 Vbgt8mp4 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Howlett wrote: > > > * Yang Shi [240911 18:16]: > > >> On Wed, Sep 11, 2024 at 12:49=E2=80=AFPM Liam R. Howlett > > >> wrote: > > >>> > > >>> * Helge Deller [240911 15:20]: > > >>>> This is a RFC to change the behaviour of mmap(MAP_STACK) to be > > >>>> sufficient to map memory for usage as stack on all architectures. > > >>>> Currently MAP_STACK is a no-op on Linux, and instead MAP_GROWSDOWN > > >>>> has to be used. > > >>>> To clarify, here is the relevant info from the mmap() man page: > > >>>> > > >>>> MAP_GROWSDOWN > > >>>> This flag is used for stacks. It indicates to the kernel virtu= al > > >>>> memory system that the mapping should extend downward in memor= y. The > > >>>> return address is one page lower than the memory area that is > > >>>> actually created in the process's virtual address space. Touc= hing an > > >>>> address in the "guard" page below the mapping will cause the m= apping > > >>>> to grow by a page. This growth can be repeated until the mappi= ng > > >>>> grows to within a page of the high end of the next lower mappi= ng, > > >>>> at which point touching the "guard" page will result in a SIGS= EGV > > >>>> signal. > > >>>> > > >>>> MAP_STACK (since Linux 2.6.27) > > >>>> Allocate the mapping at an address suitable for a process or t= hread > > >>>> stack. > > >>>> > > >>>> This flag is currently a no-op on Linux. However, by employing= this > > >>>> flag, applications can ensure that they transparently obtain s= upport > > >>>> if the flag is implemented in the future. Thus, it is used in = the > > >>>> glibc threading implementation to allow for the fact that > > >>>> some architectures may (later) require special treatment for > > >>>> stack allocations. A further reason to employ this flag is > > >>>> portability: MAP_STACK exists (and has an effect) on some > > >>>> other systems (e.g., some of the BSDs). > > >>>> > > >>>> The reason to suggest this change is, that on the parisc architect= ure the > > >>>> stack grows upwards. As such, using solely the MAP_GROWSDOWN flag = will not > > >>>> work. Note that there exists no MAP_GROWSUP flag. > > >>>> By changing the behaviour of MAP_STACK to mark the memory area wit= h the > > >>>> VM_STACK bit (which is VM_GROWSUP or VM_GROWSDOWN depending on the > > >>>> architecture) the MAP_STACK flag does exactly what people would ex= pect on > > >>>> all platforms. > > >>>> > > >>>> This change should have no negative side-effect, as all code which > > >>>> used mmap(MAP_GROWSDOWN | MAP_STACK) still work as before. > > >>>> > > >>>> Signed-off-by: Helge Deller > > >>>> > > >>>> diff --git a/include/linux/mman.h b/include/linux/mman.h > > >>>> index bcb201ab7a41..66bc72a0cb19 100644 > > >>>> --- a/include/linux/mman.h > > >>>> +++ b/include/linux/mman.h > > >>>> @@ -156,6 +156,7 @@ calc_vm_flag_bits(unsigned long flags) > > >>>> return _calc_vm_trans(flags, MAP_GROWSDOWN, VM_GROWSDOWN )= | > > >>>> _calc_vm_trans(flags, MAP_LOCKED, VM_LOCKED )= | > > >>>> _calc_vm_trans(flags, MAP_SYNC, VM_SYNC )= | > > >>>> + _calc_vm_trans(flags, MAP_STACK, VM_STACK ) = | > > >>> > > >>> Right now MAP_STACK can be used to set VM_NOHUGEPAGE, but this will > > >>> change the user interface to create a vma that will grow. I'm not > > >>> entirely sure this is okay? > > >> > > >> AFAICT, I don't see this is a problem. Currently huge page also skip= s > > >> the VMAs with VM_GROWS* flags set. See vma_is_temporary_stack(). > > >> __thp_vma_allowable_orders() returns 0 if the vma is a temporary > > >> stack. > > > > > > If someone is using MAP_STACK to avoid having a huge page, they will > > > also get a mapping that grows - which is different than what happens > > > today. > > > > > > I'm not saying that's right, but someone could be abusing the existin= g > > > flag and this will change the behaviour. > > > > Wouldn't a plain mmap() followed by madvise(MADV_NOHUGEPAGE) do exactly= that? > > Why abusing MAP_STACK for that? >=20 > Different sources and reports showed having huge pages for stack > mapping hurts performance. A lot of applications, for example, pthread > lib, allocate stack with MAP_STACK and they don't call MADV_NOHUGEPAGE > on stack mapping. >=20 It makes sense to have a stack with NOHUGEPAGE, but does anyone use MAP_STACK to avoid the extra syscall to madv to set it on mappings that are NOT stacks which would now become stack-like with this change? ... > > >>>> _calc_vm_trans(flags, MAP_STACK, VM_NOHUGEPAGE)= | > > >>>> arch_calc_vm_flag_bits(flags); > > >>>> } > >