On Tue, 22 Jan 2008, Anton Salikhmetov wrote:
>
> /*
> + * Scan the PTEs for pages belonging to the VMA and mark them read-only.
> + * It will force a pagefault on the next write access.
> + */
> +static void vma_wrprotect(struct vm_area_struct *vma)
> +{
> + unsigned long addr;
> +
> + for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
> + spinlock_t *ptl;
> + pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
> + pud_t *pud = pud_offset(pgd, addr);
> + pmd_t *pmd = pmd_offset(pud, addr);
> + pte_t *pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
This is extremely expensive over bigger areas, especially sparsely mapped
ones (it does all the lookups for all four levels over and over and over
again for eachg page).
I think Peter Zijlstra posted a version that uses the regular kind of
nested loop (with inline functions to keep the thing nice and clean),
which gets rid of that.
[ The sad/funny part is that this is all how we *used* to do msync(), back
in the days: we're literally going back to the "pre-cleanup" logic. See
commit 204ec841fbea3e5138168edbc3a76d46747cc987: "mm: msync() cleanup"
for details ]
Quite frankly, I really think you might be better off just doing a
git revert 204ec841fbea3e5138168edbc3a76d46747cc987
and working from there! I just checked, and it still reverts cleanly, and
you'd end up with a nice code-base that (a) has gotten years of testing
and (b) already has the looping-over-the-pagetables code.
Linus
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