Andrew Morton wrote:
> On Fri, 20 Feb 2015 22:20:00 -0500 "Theodore Ts'o" <tytso@xxxxxxx> wrote:
> > +akpm
> I was hoping not to have to read this thread ;)
Sorry for getting so complicated.
> What I'm not really understanding is why the pre-3.19 implementation
> actually worked. We've exhausted the free pages, we're not succeeding
> at reclaiming anything, we aren't able to oom-kill anyone. Yet it
> *does* work - we eventually find that memory and everything proceeds.
> How come? Where did that memory come from?
Even without __GFP_NOFAIL, GFP_NOFS / GFP_NOIO allocations retried forever
(without invoking the OOM killer) if order <= PAGE_ALLOC_COSTLY_ORDER and
TIF_MEMDIE is not set. Somebody else volunteered that memory while retrying.
This implies silent hang-up forever if nobody volunteers memory.
> And yes, I agree that sites such as xfs's kmem_alloc() should be
> passing __GFP_NOFAIL to tell the page allocator what's going on. I
> don't think it matters a lot whether kmem_alloc() retains its retry
> loop. If __GFP_NOFAIL is working correctly then it will never loop
Commit 9879de7373fc ("mm: page_alloc: embed OOM killing naturally into
allocation slowpath") inadvertently changed GFP_NOFS / GFP_NOIO allocations
not to retry unless __GFP_NOFAIL is specified. Therefore, either applying
Johannes's akpm-doesnt-know-why-it-works patch or passing __GFP_NOFAIL
will restore the pre-3.19 behavior (with possibility of silent hang-up).