linux-kernel - Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache

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Message-ID: <20200504031036.GB16070@bombadil.infradead.org>
Date:   Sun, 3 May 2020 20:10:36 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page
 cache

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?

-	put_page(page);

(a)
+	put_thp(page);

(b)
+	put_page_nr(page, nr);

(c)
+	if (page_ref_sub_return(page, nr) == 0)
+		__put_page(page);