From: Joel Fernandes <joel@joelfernandes•org>
To: Balbir Singh <bsingharora@gmail•com>
Cc: linux-mips@linux-mips•org, Rich Felker <dalias@libc•org>,
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"David S. Miller" <davem@davemloft•net>
Subject: Re: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2)
Date: Sat, 27 Oct 2018 12:39:17 -0700 [thread overview]
Message-ID: <20181027193917.GA51131@joelaf.mtv.corp.google.com> (raw)
In-Reply-To: <20181027102102.GO8537@350D>
Hi Balbir,
On Sat, Oct 27, 2018 at 09:21:02PM +1100, Balbir Singh wrote:
> On Wed, Oct 24, 2018 at 07:13:50PM -0700, Joel Fernandes wrote:
> > On Wed, Oct 24, 2018 at 10:57:33PM +1100, Balbir Singh wrote:
> > [...]
> > > > > + pmd_t pmd;
> > > > > +
> > > > > + new_ptl = pmd_lockptr(mm, new_pmd);
> > >
> > >
> > > Looks like this is largely inspired by move_huge_pmd(), I guess a lot of
> > > the code applies, why not just reuse as much as possible? The same comments
> > > w.r.t mmap_sem helping protect against lock order issues applies as well.
> >
> > I thought about this and when I looked into it, it seemed there are subtle
> > differences that make such sharing not worth it (or not possible).
> >
>
> Could you elaborate on them?
The move_huge_page function is defined only for CONFIG_TRANSPARENT_HUGEPAGE
so we cannot reuse it to begin with, since we have it disabled on our
systems. I am not sure if it is a good idea to split that out and refactor it
for reuse especially since our case is quite simple compared to huge pages.
There are also a couple of subtle differences between the move_normal_pmd and
the move_huge_pmd. Atleast 2 of them are:
1. We don't concern ourself with the PMD dirty bit, since the pages being
moved are normal pages and at the soft-dirty bit accounting is at the PTE
level, since we are not moving PTEs, we don't need to do that.
2. The locking is simpler as Kirill pointed, pmd_lock cannot fail however
__pmd_trans_huge_lock can.
I feel it is not super useful to refactor move_huge_pmd to support our case
especially since move_normal_pmd is quite small, so IMHO the benefit of code
reuse isn't there very much.
Do let me know your thoughts and thanks for your interest in this.
thanks,
- Joel
next prev parent reply other threads:[~2018-10-28 0:25 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-13 1:31 [PATCH 0/4] Add support for fast mremap Joel Fernandes (Google)
2018-10-13 1:31 ` [PATCH 1/4] treewide: remove unused address argument from pte_alloc functions (v2) Joel Fernandes (Google)
2018-10-24 8:37 ` Peter Zijlstra
2018-10-25 2:21 ` Joel Fernandes
2018-10-26 8:52 ` Peter Zijlstra
2018-10-25 10:47 ` Kirill A. Shutemov
2018-10-26 8:50 ` Peter Zijlstra
2018-10-13 1:31 ` [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2) Joel Fernandes (Google)
2018-10-15 9:42 ` Christoph Hellwig
2018-10-15 22:33 ` Joel Fernandes
2018-10-16 11:29 ` Vlastimil Babka
2018-10-16 19:43 ` Joel Fernandes
2018-10-17 7:38 ` Vlastimil Babka
2018-10-24 10:12 ` Kirill A. Shutemov
2018-10-24 11:57 ` Balbir Singh
2018-10-24 12:57 ` Kirill A. Shutemov
2018-10-25 2:09 ` Joel Fernandes
2018-10-25 10:19 ` Kirill A. Shutemov
2018-10-26 21:11 ` Joel Fernandes
2018-10-29 10:28 ` Will Deacon
2018-10-25 2:13 ` Joel Fernandes
2018-10-27 10:21 ` Balbir Singh
2018-10-27 19:39 ` Joel Fernandes [this message]
2018-10-28 22:40 ` Balbir Singh
2018-10-13 1:31 ` [PATCH 3/4] arm64: select HAVE_MOVE_PMD for faster mremap (v1) Joel Fernandes (Google)
2018-10-13 1:32 ` [PATCH 4/4] x86: " Joel Fernandes (Google)
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