On Mon, Jan 07, 2019 at 03:09:04PM -0800, Andrew Morton wrote:
> On Mon, 7 Jan 2019 14:39:35 -0800 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Jan 07, 2019 at 02:33:19PM -0800, Andrew Morton wrote:
> > > On Mon, 7 Jan 2019 12:02:24 -0800 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > > After we establish a reference on the page, we check the pointer continues
> > > > to be in the correct position in i_pages. There's no need to check the
> > > > page->mapping or page->index afterwards; if those can change after we've
> > > > got the reference, they can change after we return the page to the caller.
> > >
> > > But that isn't what the comment says.
> >
> > Right. That patch from Nick moved the check from before taking the
> > ref to after taking the ref. It was racy to have it before. But it's
> > unnecessary to have it afterwards -- pages can't move once there's a
> > ref on them. Or if they can move, they can move after the ref is taken.
>
> So Nick's patch was never necessary? I wonder what inspired it.
It was necessary to not check before the pin; that was clearly correct.
Checking after the pin, even with the code the way it was in 2006, was
unnecessary. Look with a bit more context:
- if (page->mapping == NULL || page->index != index)
- break;
-
if (!page_cache_get_speculative(page))
goto repeat;
/* Has the page moved? */
if (unlikely(page != *((void **)pages[i]))) {
page_cache_release(page);
goto repeat;
}
+ /*
+ * must check mapping and index after taking the ref.
+ * otherwise we can get both false positives and false
+ * negatives, which is just confusing to the caller.
+ */
+ if (page->mapping == NULL || page->index != index) {
+ page_cache_release(page);
+ break;
+ }
+
It's not immediately obvious that those added lines merely re-check the
condition checked by the 'page != *((void **)pages[i])', but if you think
about it, if page->index changes, then page must necessarily move within
the radix tree / xarray.
> Would it be excessively cautious to put a WARN_ON_ONCE() in there for a
> while?
I think it would ... it'd get in the way of a subsequent patch to store
only head pages in the page cache.
