linux-kernel - Re: [PATCH] mm: __delete_from_page

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Message-ID: <20160301111656.GA19095@node.shutemov.name>
Date:	Tue, 1 Mar 2016 14:16:56 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Sasha Levin <sasha.levin@...cle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: __delete_from_page_cache show Bad page if mapped

On Mon, Feb 29, 2016 at 10:45:59PM -0800, Hugh Dickins wrote:
> Commit e1534ae95004 ("mm: differentiate page_mapped() from page_mapcount()
> for compound pages") changed the famous BUG_ON(page_mapped(page)) in
> __delete_from_page_cache() to VM_BUG_ON_PAGE(page_mapped(page)): which
> gives us more info when CONFIG_DEBUG_VM=y, but nothing at all when not.
> 
> Although it has not usually been very helpul, being hit long after the
> error in question, we do need to know if it actually happens on users'
> systems; but reinstating a crash there is likely to be opposed :)
> 
> In the non-debug case, pr_alert("BUG: Bad page cache") plus dump_page(),
> dump_stack(), add_taint() - I don't really believe LOCKDEP_NOW_UNRELIABLE,
> but that seems to be the standard procedure now.  Move that, or the
> VM_BUG_ON_PAGE(), up before the deletion from tree: so that the
> unNULLified page->mapping gives a little more information.
> 
> If the inode is being evicted (rather than truncated), it won't have
> any vmas left, so it's safe(ish) to assume that the raised mapcount is
> erroneous, and we can discount it from page_count to avoid leaking the
> page (I'm less worried by leaking the occasional 4kB, than losing a
> potential 2MB page with each 4kB page leaked).
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov