Re: v4.6 kernel BUG at mm/rmap.c:1101!

[Andrea Arcangeli]

On Mon, May 23, 2016 at 05:06:38PM +0300, Mika Westerberg wrote:
> Hi,
> 
> After upgrading kernel of my desktop system from v4.6-rc7 to v4.6, I've
> started seeing following:
> 
> [176611.093747] page:ffffea0000360000 count:1 mapcount:0 mapping:ffff880034d2e0a1 index:0x1f9b06600 compound_mapcount: 0
> [176611.093751] flags: 0x3fff8000044079(locked|uptodate|dirty|lru|active|head|swapbacked)
> [176611.093752] page dumped because: VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address))
> [176611.093753] page->mem_cgroup:ffff88049e81b800
> [176611.093765] ------------[ cut here ]------------

This is a splitted pmd tail that is triggering a COW, but it's still a
compound page because the physical split didn't happen yet.

So like Kirill correctly pointed out, in such case we've to do
compound_head because the page->mapping that has to be refiled to the
local anon_vma is in the head.

It's just a false positive VM_BUG_ON, the code itself is correct.

Production kernels should be built with CONFIG_DEBUG_VM=n so this is
not going to affect them and there's no bug for the production builds.

Can you test this to shut off the false positive?