Re: [LSF/MM TOPIC] proposals for topics

[Michal Hocko <mhocko@kernel.org>]

On Fri 29-01-16 07:55:25, Dave Chinner wrote:
> On Tue, Jan 26, 2016 at 10:50:23AM +0100, Michal Hocko wrote:
[...]
> > There have been patches posted during the year to fortify those places
> > which cannot cope with allocation failures for ext[34] and testing
> > has shown that ext* resp. xfs are quite ready to see NOFS allocation
> > failures.
> 
> The XFS situation is compeletely unchanged from last year, and the
> fact that you say it handles NOFS allocation failures just fine
> makes me seriously question your testing methodology.

I am certainly open to suggestions there. My testing managed to identify
some weaker points in ext[34] which led to RO remounts. __GFP_NOFAIL as
the current band aid worked for them. I wasn't able to hit this with
xfs.

> In XFS, *any* memory allocation failure during a transaction will
> either cause a panic through null point deference (because we don't
> check for allocation failure in most cases) or a filesystem
> shutdown (in the cases where we do check). If you haven't seen these
> behaviours, then you haven't been failing memory allocations during
> filesystem modifications.
> 
> We need to fundamentally change error handling in transactions in
> XFS to allow arbitrary memory allocation to fail. That is, we need
> to implement a full transaction rollback capability so we can back
> out changes made during the transaction before the error occurred.
> That's a major amount of work, and I'm probably not going to do
> anything on this in the next year as it's low priority because what
> we have now works.

I am quite confused now. I remember you were the one who complained
about the silent nofail behavior of the allocator because that means
you cannot implement an appropriate fallback strategy. Please also
note that I am talking solely about GFP_NOFS allocation here. The
allocator really cannot do much other than hoplessly retrying and
relying on somebody _else_ to make a forward progress.

That being said, I do understand that allowing GFP_NOFS allocation to
fail is not an easy task and nothing to be done tomorrow or in few
months, but I believe that a discussion with FS people about what
can/should be done in order to make this happen is valuable.

Thanks!
-- 
Michal Hocko
SUSE Labs

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