linux-kernel - Re: [PATCH] mm: rectify a page bad reason

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Message-ID: <20201229121353.GB28221@casper.infradead.org>
Date:   Tue, 29 Dec 2020 12:13:53 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Rongwei Wang <rongwei.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: rectify a page bad reason

On Tue, Dec 29, 2020 at 11:11:56AM +0800, Rongwei Wang wrote:
> When I was doing some memory-related projects, it always reported error
> "nonzero mapcount", but its judgment condition was that _mapcount was not equal
> to -1, so I felt the original string was a bit inappropriate, so I tried to
> update it.

But '_mapcount' of -1 _is_ a mapcount of 0.  If we need to improve the
documentation somewhere, that'd be better than changing this message.

I do wonder if we want to add:

	if (unlikely(page_has_type(page))
		bad_reason = "page still typed";

It's covered by the non-zero mapcount case, but is slightly misleading.

>  	if (unlikely(atomic_read(&page->_mapcount) != -1))
> -		bad_reason = "nonzero mapcount";
> +		bad_reason = "non-(-1) _mapcount";
>  	if (unlikely(page->mapping != NULL))
>  		bad_reason = "non-NULL mapping";
>  	if (unlikely(page_ref_count(page) != 0))
> -- 
> 1.8.3.1
> 
>