linux-kernel - How pte_mkspecial makes zero-page COW on x86 ARCH

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Message-ID: <CAHqbYQtLG0yE98kCHkCaec+oHF7c2Pwiqrsi5C2eHnGfbsR+sQ@mail.gmail.com>
Date:	Wed, 19 Feb 2014 10:28:40 -0500
From:	Hu Yaohui <loki2441@...il.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: How pte_mkspecial makes zero-page COW on x86 ARCH

Hi All,
I read the code in do_anonymous_page of the latest linux kernel.
I am pretty confused how pte_mkspecial to make the zero-page COW on
x86 architecture
<code>
static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                unsigned long address, pte_t *page_table, pmd_t *pmd,
                unsigned int flags)
{
.....
        /* Use the zero-page for reads */
        if (!(flags & FAULT_FLAG_WRITE)) {
                entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
                                                vma->vm_page_prot));
                page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
                if (!pte_none(*page_table))
                        goto unlock;
                goto setpte;
        }
......
}
</code>
In the previous linux kernel, this is done by pte_wrprotect to make page COW.
Thanks for your time!

Best Wishes,
Yaohui
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