Re: [patch 1/8] fix race in clear_page_dirty_for_io()

Re: [patch 1/8] fix race in clear_page_dirty_for_io()

[Andrew Morton]

(cc's reinstated)

On Wed, 07 Mar 2007 09:09:50 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:

> There's a race in clear_page_dirty_for_io() that allows a page to have
> cleared PG_dirty, while being mapped read-write into the page table(s).

I assume you refer to this:

		 * FIXME! We still have a race here: if somebody
		 * adds the page back to the page tables in
		 * between the "page_mkclean()" and the "TestClearPageDirty()",
		 * we might have it mapped without the dirty bit set.
		 */
		if (page_mkclean(page))
			set_page_dirty(page);
		if (TestClearPageDirty(page)) {
			dec_zone_page_state(page, NR_FILE_DIRTY);
			return 1;
		}

I guess the comment actually refers to a writefault after the
set_page_dirty() and before the TestClearPageDirty().  The fault handler
will run set_page_dirty() and will return to userspace to rerun the write. 
The page then gets set pte-dirty but this thread of control will now make
the page !PageDirty() and will write it out.

With Nick's proposed lock-the-page-in-pagefaults patches, we have
lock_page() synchronisation between pagefaults and
clear_page_dirty_for_io() which I think will fix this.

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Re: [patch 1/8] fix race in clear_page_dirty_for_io()

[Miklos Szeredi]

> (cc's reinstated)
> 
> On Wed, 07 Mar 2007 09:09:50 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:
> 
> > There's a race in clear_page_dirty_for_io() that allows a page to have
> > cleared PG_dirty, while being mapped read-write into the page table(s).
> 
> I assume you refer to this:
> 
> 		 * FIXME! We still have a race here: if somebody
> 		 * adds the page back to the page tables in
> 		 * between the "page_mkclean()" and the "TestClearPageDirty()",
> 		 * we might have it mapped without the dirty bit set.
> 		 */
> 		if (page_mkclean(page))
> 			set_page_dirty(page);
> 		if (TestClearPageDirty(page)) {
> 			dec_zone_page_state(page, NR_FILE_DIRTY);
> 			return 1;
> 		}
> 

Yes.

> I guess the comment actually refers to a writefault after the
> set_page_dirty() and before the TestClearPageDirty().  The fault handler
> will run set_page_dirty() and will return to userspace to rerun the write. 
> The page then gets set pte-dirty but this thread of control will now make
> the page !PageDirty() and will write it out.

Yes.

> With Nick's proposed lock-the-page-in-pagefaults patches, we have
> lock_page() synchronisation between pagefaults and
> clear_page_dirty_for_io() which I think will fix this.

After a quick look, I don't think it does.  It locks the page in
do_no_page(), but not for the whole fault.  In particular do_wp_page()
is not affected.  But I haven't yet looked closely at that patch, so I
could be wrong.

Miklos

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