linux-kernel - Re: [PATCH v2] xen: Convert kmap() to kmap_local

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Date:   Wed, 20 Apr 2022 15:28:03 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
cc:     Julia Lawall <julia.lawall@...ia.fr>, ira.weiny@...el.com,
        Alaa Mohamed <eng.alaamohamedsoliman.am@...il.com>,
        outreachy@...ts.linux.dev, boris.ostrovsky@...cle.com,
        jgross@...e.com, sstabellini@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] xen:  Convert kmap() to kmap_local_page()



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.

OK, thanks for pointing that out.  That seems like a key point that should
be mentioned in log messages.  It's not even visible in the context lines
around the patch.

>
> 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()?

Sure.  I'll take a look at the precise example Alaa started with and see
if I can come up with something.

thanks,
julia