linux-kernel - Re: [PATCH] mm: __delete_from_page_cache WARN_ON(page

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Date:	Mon, 29 Feb 2016 16:44:59 +0900
From:	Joonsoo Kim <js1304@...il.com>
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 Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: __delete_from_page_cache WARN_ON(page_mapped)

2016-02-29 13:49 GMT+09:00 Hugh Dickins <hughd@...gle.com>:
> 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, use WARN_ON() plus dump_page() and 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>
> ---
> I think this should go into v4.5, so I've written it with an atomic_sub
> on page->_count; but Joonsoo will probably want some page_ref thingy.

Okay. I will do it after this patch is merged.

Thanks for notification.

Thanks.