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SubjectRe: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache
On Sun, May 03, 2020 at 08:10:36PM -0700, Matthew Wilcox wrote:
> On Wed, Apr 29, 2020 at 06:36:50AM -0700, Matthew Wilcox wrote:
> > @@ -886,7 +906,7 @@ static int __add_to_page_cache_locked(struct page *page,
> > /* Leave page->index set: truncation relies upon it */
> > if (!huge)
> > mem_cgroup_cancel_charge(page, memcg, false);
> > - put_page(page);
> > + page_ref_sub(page, nr);
> > return xas_error(&xas);
> > }
> > ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
>
> This is wrong. page_ref_sub() will not call __put_page() if the refcount
> gets to zero. What do people prefer?

*sigh*. It's not wrong. The caller holds a reference on the page
already, so calling page_ref_sub() will never reduce the refcount to 0.
The latest version looks like this:

+ page_ref_sub(page, nr);
+ VM_BUG_ON_PAGE(page_count(page) <= 0, page);

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