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

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Message-ID: <2940450.687JKscXgg@leap>
Date:   Wed, 20 Apr 2022 15:22:10 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Julia Lawall <julia.lawall@...ia.fr>, ira.weiny@...el.com
Cc:     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 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()?

Thanks,

Fabio