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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>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org, devel@driverdev.osuosl.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Theodore Ts'o <tytso@mit.edu>, Pavel Machek <pavel@denx.de>,
        David Sterba <dsterba@suse.cz>,
        Amir Goldstein <amir73il@gmail.com>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        Dave Chinner <david@fromorbit.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Jan Kara <jack@suse.cz>,
        Richard Weinberger <richard@nod.at>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-erofs@lists.ozlabs.org, Chao Yu <yuchao0@huawei.com>,
        Miao Xie <miaoxie@huawei.com>,
        Li Guifu <bluce.liguifu@huawei.com>,
        Fang Wei <fangwei1@huawei.com>
Subject: Re: [PATCH v8 20/24] erofs: introduce generic decompression backend
Message-ID: <20190830165533.GA10909@infradead.org>
References: <20190815044155.88483-1-gaoxiang25@huawei.com>
 <20190815044155.88483-21-gaoxiang25@huawei.com>
 <20190830163534.GA29603@infradead.org>
 <20190830165217.GB107220@architecture4>
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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.