Messages in this thread | | | Subject | Re: [patch 1/8] fix race in clear_page_dirty_for_io() | From | Miklos Szeredi <> | Date | Wed, 07 Mar 2007 11:31:59 +0100 |
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> (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.
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