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Date:   Wed, 20 Apr 2022 15:57:14 +0200 (CEST)
From:   Julia Lawall <julia.lawall@inria.fr>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
cc:     Julia Lawall <julia.lawall@inria.fr>, ira.weiny@intel.com,
        Alaa Mohamed <eng.alaamohamedsoliman.am@gmail.com>,
        outreachy@lists.linux.dev, boris.ostrovsky@oracle.com,
        jgross@suse.com, sstabellini@kernel.org,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] xen:  Convert kmap() to kmap_local_page()
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On Wed, 20 Apr 2022, Fabio M. De Francesco wrote:

> On mercoledì 20 aprile 2022 15:40:10 CEST Julia Lawall wrote:
> >
> > On Wed, 20 Apr 2022, Fabio M. De Francesco wrote:
> >
> > > On mercoledì 20 aprile 2022 08:03:05 CEST Julia Lawall wrote:
> > > >
> > > > On Wed, 20 Apr 2022, Alaa Mohamed wrote:
> > > >
> > > > > kmap() is being deprecated and these usages are all local to the
> thread
> > > > > so there is no reason kmap_local_page() can't be used.
> > > > >
> > > > > Replace kmap() calls with kmap_local_page().
> > > >
> > > > OK, so from a Coccinelle point of view, could we do
> > > >
> > > > @@
> > > > expression e1,e2,x,f;
> > > > @@
> > > >
> > > > e1 =
> > > > - kmap
> > > > + kmap_local_page
> > > >     (e2)
> > > > ... when != x = e1 // not stored in any location and not passed to
> > > another function
> > > >     when != f(...,e1,...)
> > > >     when != x = e2
> > > >     when != f(...,e2,...)
> > > > -kunmap(e2)
> > > > +kunmap_local(e1)
> > > >
> > > > julia
> > > >
> > >
> > > I've never spent sufficient time to understand properly the syntax and
> > > semantics of expressions of Coccinelle. However, thanks Julia, this
> code
> > > looks good and can be very helpful.
> > >
> > > Only a minor objection... it doesn't tell when 'e2' has been allocated
> > > within the same function where the kmap() call is.
> > >
> > > In the particular case that I cite above, I'd prefer to remove the
> > > allocation of the page (say with alloc_page()) and convert kmap() /
> kunmap()
> > > to use kmalloc() / kfree().
> > >
> > > Fox example, this is done in the following patch:
> > >
> > > commit 633b0616cfe0 ("x86/sgx: Remove unnecessary kmap() from
> > > sgx_ioc_enclave_init()") from Ira Weiny.
> > >
> > > Can Coccinelle catch also those special cases where a page that is
> passed
> > > to kmap() is allocated within that same function (vs. being passed as
> > > argument to this function) and, if so, propose a replacement with
> > > kmalloc()?
> >
> > It looks complex in this case, because the allocation is in another
> > function, and it is passed to another function.
>
> This is not the special case I was talking about. In this case your code
> for Coccinelle tells the right proposal and it is exactly what Alaa did in
> her patch (which is good!).
>
> I'm talking about other special cases like the one I pointed to with the
> link I provided. I'm sorry if my bad English made you think that Alaa's
> patch was one of those cases where the page is allocated within the same
> function where kmap() is.
>
> I hope that now I've been clearer :)

Ah, sorry for the misunderstanding.  If you have an example, I can take a
look and propose something for this special case.

julia
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