Re: [PATCH] mm: __delete_from_page_cache WARN_ON(page_mapped)

[Hugh Dickins]

On Mon, 29 Feb 2016, Kirill A. Shutemov wrote:
> On Sun, Feb 28, 2016 at 08:49:10PM -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, 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.
> 
> So you put here TAINT_WARN plus TAINT_BAD_PAGE. I guess just the second
> would be enough.

You're right, I hadn't thought about the over-tainting at all:
one's enough, yes.

> 
> We can replace WARN_ON() with plain page_mapped(page), plus dump_stack()
> below add_taint().

Okay, I'll post another now, but it does remind me why I used WARN_ON():
that was an easy way of printing a standard format, without having to
think too much.  Now I'm adding a "BUG: Bad page cache" header line to
make it fit in with the "Bad page map" and "Bad page state" messages.

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

Thank you, but since I haven't done it exactly the way you suggest,
I won't assume your Ack carries over to the "Bad page cache" version.

> 
> > ---
> > 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.

And thanks to Joonsoo for taking it on board.

Hugh