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

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Date:   Wed, 16 Sep 2020 09:21:02 -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 3:57 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>
>
> On Tue, 15 Sep 2020, Mikulas Patocka wrote:
>
> >
> >
> > On Tue, 15 Sep 2020, Mikulas Patocka wrote:
> >
> > > > > - __copy_from_user_inatomic_nocache doesn't flush cache for leading and
> > > > > trailing bytes.
> > > >
> > > > You want copy_user_flushcache(). See how fs/dax.c arranges for
> > > > dax_copy_from_iter() to route to pmem_copy_from_iter().
> > >
> > > Is it something new for the kernel 5.10? I see only __copy_user_flushcache
> > > that is implemented just for x86 and arm64.
> > >
> > > There is __copy_from_user_flushcache implemented for x86, arm64 and power.
> > > It is used in lib/iov_iter.c under
> > > #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE - so should I use this?

Yes, but maybe not directly.

> > >
> > > Mikulas
> >
> > ... and __copy_user_flushcache is not exported for modules. So, I am stuck
> > with __copy_from_user_inatomic_nocache.
> >
> > Mikulas
>
> 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.

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?