linux-kernel - Re: [PATCH V3 2/2] mm/mmap: Drop generic protection

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Message-ID: <Yqw0pxSNLZWPyPIG@infradead.org>
Date:   Fri, 17 Jun 2022 01:00:39 -0700
From:   "hch@...radead.org" <hch@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V3 2/2] mm/mmap: Drop generic protection_map[] array

On Fri, Jun 17, 2022 at 05:48:11AM +0000, Christophe Leroy wrote:
> Is it a good idea to duplicate vm_get_page_prot() in each architecture ? 

It is a completely trivial array index.  And I really like the idea
of not having the protection_map in common code - it really is an
implementation detail.  But what we could do is something like

#define DECLARE_VM_GET_PAGE_PROT				\
pgprot_t vm_get_page_prot(unsigned long vm_flags)		\
{								\
        return protection_map[vm_flags &			\
		(VM_READ | VM_WRITE | VM_EXEC | VM_SHARED)];	\
}								\
EXPORT_SYMBOL(vm_get_page_prot);

as a helper for the architectures.

> Maybe it is, but it will also mean changing common code like 
> mm/debug_vm_pgtable.c which accesses protection_map[] directly as of today.

That's already gone thanks to the good work from Anshuman.