Re: [RFC] mremap: add MREMAP_NOHOLE flag

[Shaohua Li <shli@kernel.org>]

On Tue, Feb 03, 2015 at 11:19:12AM -0800, Shaohua Li 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.
> 
> 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.

Any comments on this? There are real requirements on this feature.
jemalloc/tcmalloc are good examples here.

Thanks,
Shaohua

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