On Wed, 2 Feb 2005, Nick Piggin wrote:
> Well yeah, but the interesting case is when that isn't a lock ;)
>
> I'm not saying what you've got is no good. I'm sure it would be fine
> for testing. And if it happens that we can do the "page_count doesn't
> mean anything after it has reached zero and been freed. Nor will it
> necessarily be zero when a new page is allocated" thing without many
> problems, then this may be a fine way to do it.
>
> I was just pointing out this could be a problem without putting a
> lot of thought into it...
Surely we need to do this the right way. Do we really need to
use page_cache_get()? Is anything relying on page_count == 2 of
the old_page?
I mean we could just speculatively copy, risk copying crap and
discard that later when we find that the pte has changed. This would
simplify the function:
Index: linux-2.6.10/mm/memory.c
===================================================================
--- linux-2.6.10.orig/mm/memory.c 2005-02-01 18:10:46.000000000 -0800
+++ linux-2.6.10/mm/memory.c 2005-02-01 18:43:08.000000000 -0800
@@ -1323,9 +1323,6 @@ static int do_wp_page(struct mm_struct *
/*
* Ok, we need to copy. Oh, well..
*/
- if (!PageReserved(old_page))
- page_cache_get(old_page);
-
if (unlikely(anon_vma_prepare(vma)))
goto no_new_page;
if (old_page == ZERO_PAGE(address)) {
@@ -1336,6 +1333,10 @@ static int do_wp_page(struct mm_struct *
new_page = alloc_page_vma(GFP_HIGHUSER, vma, address);
if (!new_page)
goto no_new_page;
+ /*
+ * No page_cache_get so we may copy some crap
+ * that is later discarded if the pte has changed
+ */
copy_user_highpage(new_page, old_page, address);
}
/*
@@ -1352,7 +1353,6 @@ static int do_wp_page(struct mm_struct *
acct_update_integrals();
update_mem_hiwater();
} else
-
page_remove_rmap(old_page);
break_cow(vma, new_page, address, page_table);
lru_cache_add_active(new_page);
@@ -1363,7 +1363,6 @@ static int do_wp_page(struct mm_struct *
}
pte_unmap(page_table);
page_cache_release(new_page);
- page_cache_release(old_page);
spin_unlock(&mm->page_table_lock);
return VM_FAULT_MINOR;
-
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