Re: [PATCH] mm: __delete_from_page_cache show Bad page if mapped

[Kirill A. Shutemov]

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@xxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov