Re: [libc/string] State of PAGE_COPY_FWD / PAGE_COPY_THRESHOLD

[Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>]

> On Nov 1, 2016, at 5:59 PM, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> 
...
> $ cat sysdeps/x86/pagecopy.h
> 
> #define PAGE_SIZE           4096
> #define PAGE_COPY_THRESHOLD PAGE_SIZE
> 
> #define PAGE_COPY_FWD(dstp, srcp, nbytes_left, nbytes)  /* Implement it */
> 
> It should work on any other architecture as well.  Now the question
> is whether this actually does make sense for Linux.  Hurd/mach provided
> a syscall (?) to actually copy the pages (vm_copy) which seems to apply
> some tricks to avoid full copy pages. By 'linux zero page sharing' are 
> you referring to KSM (Kernel Samepage Merging)? 
> 
> If so, on a system without a provided kernel interface to work directed 
> with underlying memory mapping (such as for mach), mem{cpy,set} will 
> actually need to touch the pages and it will be up to kernel page fault 
> mechanism to actually handle it (by identifying common pages and adjusting
> vma mapping accordingly). And AFAIK this are only enabled on KSM if you 
> actually madavise the page explicit. So I am not grasping the need to
> actually implement page copying on Linux.

Linux kernel has a reserved page filled with zeroes, so it there /were/ a syscall to tell kernel to map N consecutive pages starting at address PTR to that zero page, we could use that in GLIBC for really big memset(0).

A quick investigation shows that there is no such syscall provided by the Linux kernel.  Doesn't mean we can't ask for / implement one.

--
Maxim Kuvyrkov
www.linaro.org