Re: kernel BUG at mm/shmem.c:LINE!

[Matthew Wilcox]

On Mon, Jul 23, 2018 at 12:14:41PM -0700, Hugh Dickins wrote:
> On Mon, 23 Jul 2018, Matthew Wilcox wrote:
> > On Sun, Jul 22, 2018 at 07:28:01PM -0700, Hugh Dickins wrote:
> > > Whether or not that fixed syzbot's kernel BUG at mm/shmem.c:815!
> > > I don't know, but I'm afraid it has not fixed linux-next breakage of
> > > huge tmpfs: I get a similar page_to_pgoff BUG at mm/filemap.c:1466!
> > > 
> > > Please try something like
> > > mount -o remount,huge=always /dev/shm
> > > cp /dev/zero /dev/shm
> > > 
> > > Writing soon crashes in find_lock_entry(), looking up offset 0x201
> > > but getting the page for offset 0x3c1 instead.
> > 
> > Hmm.  I don't see a crash while running that command,
> 
> Thanks for looking.
> 
> It is the VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page)
> in find_lock_entry(). Perhaps you didn't have CONFIG_DEBUG_VM=y
> on this occasion? Or you don't think of an oops as a kernel crash,
> and didn't notice it in dmesg? I see now that I've arranged for oops
> to crash, since I don't like to miss them myself; but it is a very
> clean oops, no locks held, so can just kill the process and continue.

Usually I run with that turned on, but somehow in my recent messing
with my test system, that got turned off.  Once I turned it back on,
it spots the bug instantly.

> Or is there something more mysterious stopping it from showing up for
> you? It's repeatable for me. When not crashing, that "cp" should fill
> up about half of RAM before it hits the implicit tmpfs volume limit;
> but I am assuming a not entirely fragmented machine - it does need
> to allocate two 2MB pages before hitting the VM_BUG_ON_PAGE().

I tried that too, before noticing that DEBUG_VM was off; raised my test
VM's memory from 2GB to 8GB.

> Are you sure that those pages are free, rather than most of them tails
> of one of the two compound pages involved? I think it's the same in your
> rewrite of struct page, the compound_head field (lru.next), with its low
> bit set, were how to recognize a tail page.

Yes, PageTail was set, and so was TAIL_MAPPING (0xdead0000000000400).
What was going on was the first 2MB page was being stored at indices
0-511, then the second 2MB page was being stored at indices 64-575
instead of 512-1023.

I figured out a fix and pushed it to the 'ida' branch in
git://git.infradead.org/users/willy/linux-dax.git

It won't be in linux-next tomorrow because the nvdimm people have
just dumped a pile of patches into their tree that conflict with
the XArray-DAX rewrite, so Stephen has pulled the XArray tree out
of linux-next temporarily.  I didn't have time to sort out the merge
conflict today because I judged your bug report more important.