Re: [GIT PULL] Please pull NFS client fixes for 4.12

[Michal Hocko <mhocko@kernel.org>]

On Thu 11-05-17 12:45:00, Trond Myklebust wrote:
> On Thu, 2017-05-11 at 14:26 +0200, Michal Hocko wrote:
> > On Thu 11-05-17 12:16:37, Trond Myklebust wrote:
> > > On Thu, 2017-05-11 at 09:59 +0200, Michal Hocko wrote:
> > > > On Thu 11-05-17 10:53:27, Nikolay Borisov wrote:
> > > > > 
> > > > > 
> > > > > On 10.05.2017 19:47, Trond Myklebust wrote:
> > > > 
> > > > [...]
> > > > > > - Cleanup and removal of some memory failure paths now that
> > > > > >   GFP_NOFS is guaranteed to never fail.
> > > > > 
> > > > > What guarantees that? Since if this is the case then this can
> > > > > result in
> > > > > a lot of opportunities for cleanup across the whole kernel
> > > > > tree.
> > > > > After
> > > > > discussing with mhocko (cc'ed) it seems that in practice
> > > > > everything
> > > > > below COSTLY_ORDER which are not GFP_NORETRY will never fail.
> > > > > But
> > > > > this
> > > > > semantic is not the same as GFP_NOFAIL. E.g. nothing guarantees
> > > > > that
> > > > > this will stay like that in the future?
> > > > 
> > > > In practice it is hard to change the semantic of small
> > > > allocations
> > > > never
> > > > fail _practically_. But this is absolutely not guaranteed! They
> > > > can
> > > > fail
> > > > e.g. when the allocation context is the oom victim. Removing
> > > > error
> > > > paths
> > > > for allocation failures is just wrong.
> > > 
> > > OK, this makes no fucking sense at all.
> > > 
> > > Either allocations can fail or they can't.
> > > 1) If they can't fail, then we don't need the checks.
> > > 2) If they can fail, then we do need them, and this hand wringing
> > > in
> > > the MM community about GFP_* semantics and how we need to prevent
> > > failure is fucking pointless.
> > 
> > everything which is not __GFP_NOFAIL might fail. We try hard not to
> > fail
> > small allocations requests as much as we can in general but you
> > _have_ to
> > check for failures. There is simply no way to guarantee "never fail"
> > semantic for all allocation requests. This has been like that
> > basically
> > since years. And even this try-to-be-nofailing for small allocations
> > has
> > been PITA for some corner cases.
> 
> I'll take that as a vote for (2), then.
> 
> I know that failures could occur in the past. That's why those code
> paths were there. The problem is that the MM community has been making
> lots of noise on mailing lists, conferences and LWN articles about how
> we must not fail small allocations because the MM community believes
> that nobody expects it. This is confusing everyone...

It was exactly other way around. We would like to _get_rid_of_ this do
not fail behavior because it is causing a major headaches in out of
memory corner cases. Just take GFP_NOFS as an example. It is a weak
reclaim context because we cannot reclaim fs metadata and that might be
a lot of memory so we cannot trigger the OOM killer and have to rely on
a different allocation context or kswapd to make a progress on our
behalf. We would really like to fail those requests instead. I've tried
that in the past but it was deemed to dangerous because _all_ kernel
paths would have to be checked for a sane failure behavior. So we are
keeping status quo instead.

> It confused Neil Brown, who contributed these patches, and it confused
> me and all the other reviewers of these patches on the linux-nfs
> mailing list.
> 
> So if indeed (2) is correct, then please can we have a clear statement
> _when discussing improvements to memory allocation semantics_ that
> GFP_* still can fail, still will fail, and that callers should assume
> it will fail and should test their code paths assuming the failure
> case.

I do not see any explicit documentation which would encourage users to
not check for the allocation failure. Only __GFP_NOFAIL is documented it
_must_ retry for ever. Of course I am open for any documentation
improvements.
-- 
Michal Hocko
SUSE Labs