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

From: Shaohua Li <shli@fb.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, danielmicay@gmail.com,
	linux-api@vger.kernel.org, Rik van Riel <riel@redhat.com>,
	Hugh Dickins <hughd@google.com>, Mel Gorman <mel@csn.ul.ie>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@suse.cz>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH] mremap: add MREMAP_NOHOLE flag --resend
Date: Wed, 18 Mar 2015 22:08:26 -0700	[thread overview]
Message-ID: <20150319050826.GA1591708@devbig257.prn2.facebook.com> (raw)
In-Reply-To: <20150318153100.5658b741277f3717b52e42d9@linux-foundation.org>

On Wed, Mar 18, 2015 at 03:31:00PM -0700, Andrew Morton wrote:
> On Tue, 17 Mar 2015 14:09:39 -0700 Shaohua Li <shli@fb.com> 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.

I'll extract some from his changelog in next post

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

I run Daniel's microbenchmark too, and not surprise the result is
similar:
glibc: 32.82
jemalloc: 70.35
jemalloc+mremap: 33.01
tcmalloc: 68.81

but tcmalloc doesn't support mremap currently, so I cant test it.

> 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@vger.kernel.org please.

ok, will add in next post.

Thanks,
Shaohua

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