linux-kernel - Re: [PATCH v2 2/2] mm: speed up mremap by 500x on large regions

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Message-ID: <20181013013540.GA207108@joelaf.mtv.corp.google.com>
Date:   Fri, 12 Oct 2018 18:35:40 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     David Miller <davem@...emloft.net>
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Subject: Re: [PATCH v2 2/2] mm: speed up mremap by 500x on large regions

On Fri, Oct 12, 2018 at 11:18:36AM -0700, David Miller wrote:
> From: Joel Fernandes <joel@...lfernandes.org>
[...]
> > Also, do we not flush the caches from any path when we munmap
> > address space?  We do call do_munmap on the old mapping from mremap
> > after moving to the new one.
> 
> Sparc makes sure that shared mapping have consistent colors.  Therefore
> all that's left are private mappings and those will be initialized by
> block stores to clear the page out or similar.
> 
> Also, when creating new mappings, we flush the D-cache when necessary
> in update_mmu_cache().
> 
> We also maintain a bit in the page struct to track when a page which
> was potentially written to on one cpu ends up mapped into another
> address space and flush as necessary.
> 
> The cache is write-through, which simplifies the preconditions we have
> to maintain.

Makes sense, thanks. For the moment I sent patches to enable this on arm64
and x86. We can enable it on sparc as well at a later time as it sounds it
could be a safe optimization to apply to that architecture as well.

thanks,

 - Joel