linux-kernel - Re: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user

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Date:   Wed, 16 Sep 2020 10:40:13 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Kani, Toshi" <toshi.kani@....com>,
        "Norton, Scott J" <scott.norton@....com>,
        "Tadakamadla, Rajesh (DCIG/CDI/HPS Perf)" 
        <rajesh.tadakamadla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] pmem: export the symbols __copy_user_flushcache and __copy_from_user_flushcache

On Wed, Sep 16, 2020 at 10:24 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>
>
> On Wed, 16 Sep 2020, Dan Williams wrote:
>
> > On Wed, Sep 16, 2020 at 3:57 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
> > >
> > >
> > >
> > > I'm submitting this patch that adds the required exports (so that we could
> > > use __copy_from_user_flushcache on x86, arm64 and powerpc). Please, queue
> > > it for the next merge window.
> >
> > Why? This should go with the first user, and it's not clear that it
> > needs to be relative to the current dax_operations export scheme.
>
> Before nvfs gets included in the kernel, I need to distribute it as a
> module. So, it would make my maintenance easier. But if you don't want to
> export it now, no problem, I can just copy __copy_user_flushcache from the
> kernel to the module.

That sounds a better plan than exporting symbols with no in-kernel consumer.

> > My first question about nvfs is how it compares to a daxfs with
> > executables and other binaries configured to use page cache with the
> > new per-file dax facility?
>
> nvfs is faster than dax-based filesystems on metadata-heavy operations
> because it doesn't have the overhead of the buffer cache and bios. See
> this: http://people.redhat.com/~mpatocka/nvfs/BENCHMARKS

...and that metadata problem is intractable upstream? Christoph poked
at bypassing the block layer for xfs metadata operations [1], I just
have not had time to carry that further.

[1]: "xfs: use dax_direct_access for log writes", although it seems
he's dropped that branch from his xfs.git