Re: System freezes after OOM

[Michal Hocko]

On Wed 13-07-16 10:18:35, Mikulas Patocka wrote:
> 
> 
> On Wed, 13 Jul 2016, Michal Hocko wrote:
> 
> > [CC David]
> > 
> > > > It is caused by the commit f9054c70d28bc214b2857cf8db8269f4f45a5e23. 
> > > > Prior to this commit, mempool allocations set __GFP_NOMEMALLOC, so 
> > > > they never exhausted reserved memory. With this commit, mempool 
> > > > allocations drop __GFP_NOMEMALLOC, so they can dig deeper (if the 
> > > > process has PF_MEMALLOC, they can bypass all limits).
> > > 
> > > I wonder whether commit f9054c70d28bc214 ("mm, mempool: only set 
> > > __GFP_NOMEMALLOC if there are free elements") is doing correct thing. 
> > > It says
> > > 
> > >     If an oom killed thread calls mempool_alloc(), it is possible that 
> > > it'll
> > >     loop forever if there are no elements on the freelist since
> > >     __GFP_NOMEMALLOC prevents it from accessing needed memory reserves in
> > >     oom conditions.
> > 
> > I haven't studied the patch very deeply so I might be missing something
> > but from a quick look the patch does exactly what the above says.
> > 
> > mempool_alloc used to inhibit ALLOC_NO_WATERMARKS by default. David has
> > only changed that to allow ALLOC_NO_WATERMARKS if there are no objects
> > in the pool and so we have no fallback for the default __GFP_NORETRY
> > request.
> 
> The swapper core sets the flag PF_MEMALLOC and calls generic_make_request 
> to submit the swapping bio to the block driver. The device mapper driver 
> uses mempools for all its I/O processing.

OK, this is the part I have missed. I didn't realize that the swapout
path, which is indeed PF_MEMALLOC, can get down to blk code which uses
mempools. A quick code travers shows that at least
	make_request_fn = blk_queue_bio
	blk_queue_bio
	  get_request
	    __get_request

might do that. And in that case I agree that the above mentioned patch
has unintentional side effects and should be re-evaluated. David, what
do you think? An obvious fixup would be considering TIF_MEMDIE in
mempool_alloc explicitly.

-- 
Michal Hocko
SUSE Labs