Re: [PATCH] mremap: add MREMAP_NOHOLE flag --resend

On Tue, 17 Mar 2015 14:09:39 -0700 Shaohua Li [off-list ref] wrote:

There was a similar patch posted before, but it doesn't get merged. I'd like
to try again if there are more discussions.
http://marc.info/?l=linux-mm&m=141230769431688&w=2

mremap can be used to accelerate realloc. The problem is mremap will
punch a hole in original VMA, which makes specific memory allocator
unable to utilize it. Jemalloc is an example. It manages memory in 4M
chunks. mremap a range of the chunk will punch a hole, which other
mmap() syscall can fill into. The 4M chunk is then fragmented, jemalloc
can't handle it.

Daniel's changelog had additional details regarding the userspace
allocators' behaviour.  It would be best to incorporate that into your
changelog.

Daniel also had microbenchmark testing results for glibc and jemalloc. 
Can you please do this?

I'm not seeing any testing results for tcmalloc and I'm not seeing
confirmation that this patch will be useful for tcmalloc.  Has anyone
tried it, or sought input from tcmalloc developers?

This patch adds a new flag for mremap. With it, mremap will not punch the
hole. page tables of original vma will be zapped in the same way, but
vma is still there. That is original vma will look like a vma without
pagefault. Behavior of new vma isn't changed.

For private vma, accessing original vma will cause
page fault and just like the address of the vma has never been accessed.
So for anonymous, new page/zero page will be fault in. For file mapping,
new page will be allocated with file reading for cow, or pagefault will
use existing page cache.

For shared vma, original and new vma will map to the same file. We can
optimize this without zaping original vma's page table in this case, but
this patch doesn't do it yet.

Since with MREMAP_NOHOLE, original vma still exists. pagefault handler
for special vma might not able to handle pagefault for mremap'd area.
The patch doesn't allow vmas with VM_PFNMAP|VM_MIXEDMAP flags do NOHOLE
mremap.

At some point (preferably an early point) we'd like a manpage update
and a cc: to linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org please.