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Date: Fri, 30 Aug 2019 09:55:33 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Gao Xiang <gaoxiang25@huawei.com>
Subject: Re: [PATCH v8 20/24] erofs: introduce generic decompression backend
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On Sat, Aug 31, 2019 at 12:52:17AM +0800, Gao Xiang wrote:
> Hi Christoph,
> 
> On Fri, Aug 30, 2019 at 09:35:34AM -0700, Christoph Hellwig wrote:
> > On Thu, Aug 15, 2019 at 12:41:51PM +0800, Gao Xiang wrote:
> > > +static bool use_vmap;
> > > +module_param(use_vmap, bool, 0444);
> > > +MODULE_PARM_DESC(use_vmap, "Use vmap() instead of vm_map_ram() (default 0)");
> > 
> > And how would anyone know which to pick?
> 
> It has significant FIO benchmark difference on sequential read least on arm64...
> I have no idea whether all platform vm_map_ram() behaves better than vmap(),
> so I leave an option for users here...

vm_map_ram is supposed to generally behave better.  So if it doesn't
please report that that to the arch maintainer and linux-mm so that
they can look into the issue.  Having user make choices of deep down
kernel internals is just a horrible interface.

Please talk to maintainers of other bits of the kernel if you see issues
and / or need enhancements.