On Tue, 18 Apr 2006 11:21:25 -0700 (PDT)
Christoph Lameter <[email protected]> wrote:
> Note that this is again only a partial solution. mprotect() also has the
> potential of changing the write status to read.
yes. in change_pte_range().
Note:
fork() and mprotect() both requires mm->mmap_sem.
So both of them is not problem when migration holds mm->mmap_sem.
If we does lazy migration or memory hot removing or allows migration from
another process, this will be problem.
> Are there any additional occurrences? Would you check and fix this one as well?
>
pte_modify() looks to be called only by mprotect(). I checked all mk_pte() but
not found no occurrences now. But I'll have to do more.
> If we cannot get to all the locations or if these fixes get too extensive
> then I think we better drop the preservation of write permissions and
> tolerate the occurrence of some useless COW after migration.
>
yes. I agree.
-Kame
>
>
> Migration entries with write permission must become SWP_MIGRATION_READ
> entries if a COW mapping is processed. The migration entries from which
> the copy is being made must also become SWP_MIGRATION_READ. This mimicks
> the copying of pte for an anonymous page.
>
> Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
> Signed-off-by: Christoph Lameter <[email protected]>
>
> Index: linux-2.6.17-rc1-mm3/mm/memory.c
> ===================================================================
> --- linux-2.6.17-rc1-mm3.orig/mm/memory.c 2006-04-18 10:58:33.000000000 -0700
> +++ linux-2.6.17-rc1-mm3/mm/memory.c 2006-04-18 11:09:23.000000000 -0700
> @@ -434,7 +434,9 @@ copy_one_pte(struct mm_struct *dst_mm, s
> /* pte contains position in swap or file, so copy. */
> if (unlikely(!pte_present(pte))) {
> if (!pte_file(pte)) {
> - swap_duplicate(pte_to_swp_entry(pte));
> + swp_entry_t entry = pte_to_swp_entry(pte);
> +
> + swap_duplicate(entry);
> /* make sure dst_mm is on swapoff's mmlist. */
> if (unlikely(list_empty(&dst_mm->mmlist))) {
> spin_lock(&mmlist_lock);
> @@ -443,6 +445,19 @@ copy_one_pte(struct mm_struct *dst_mm, s
> &src_mm->mmlist);
> spin_unlock(&mmlist_lock);
> }
> + if (is_migration_entry(entry) &&
> + is_cow_mapping(vm_flags)) {
> + page = migration_entry_to_page(entry);
> +
> + /*
> + * COW mappings require pages in both parent
> + * and child to be set to read.
> + */
> + entry = make_migration_entry(page,
> + ` SWP_MIGRATION_READ);
> + pte = swp_entry_to_pte(entry);
> + set_pte_at(src_mm, addr, src_pte, pte);
> + }
> }
> goto out_set_pte;
> }
>
-
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