Re: [LSF/MM TOPIC] few MM topics

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jan 24, 2018 at 10:26:49AM +0100, Michal Hocko wrote:
> Hi,
> I would like to propose the following few topics for further discussion
> at LSF/MM this year. MM track would be the most appropriate one but
> there is some overlap with FS and NVDIM
> - memcg OOM behavior has changed around 3.12 as a result of OOM
>   deadlocks when the memcg OOM killer was triggered from the charge
>   path. We simply fail the charge and unroll to a safe place to
>   trigger the OOM killer. This is only done from the #PF path and any
>   g-u-p or kmem accounted allocation can just fail in that case leading
>   to unexpected ENOMEM to userspace. I believe we can return to the
>   original OOM handling now that we have the oom reaper and guranteed
>   forward progress of the OOM path.
>   Discussion http://lkml.kernel.org/r/20171010142434.bpiqmsbb7gttrlcb@dhcp22.suse.cz
> - It seems there is some demand for large (> MAX_ORDER) allocations.
>   We have that alloc_contig_range which was originally used for CMA and
>   later (ab)used for Giga hugetlb pages. The API is less than optimal
>   and we should probably think about how to make it more generic.
> - we have grown a new get_user_pages_longterm. It is an ugly API and
>   I think we really need to have a decent page pinning one with the
>   accounting and limiting.
> - memory hotplug has seen quite some surgery last year and it seems that
>   DAX/nvdim and HMM have some interest in using it as well. I am mostly
>   interested in struct page self hosting which is already done for NVDIM
>   AFAIU. It would be great if we can unify that for the regular mem
>   hotplug as well.
> - I would be very interested to talk about memory softofflining
>   (HWPoison) with somebody familiar with this area because I find the
>   development in that area as more or less random without any design in
>   mind. The resulting code is chaotic and stuffed to "random" places.
> - I would also love to talk to some FS people and convince them to move
>   away from GFP_NOFS in favor of the new scope API. I know this just
>   means to send patches but the existing code is quite complex and it
>   really requires somebody familiar with the specific FS to do that
>   work.

Hm, are you talking about setting PF_MEMALLOC_NOFS instead of passing
*_NOFS to allocation functions and whatnot?  Right now XFS will set it
on any thread which has a transaction open, but that doesn't help for
fs operations that don't have transactions (e.g. reading metadata,
opening files).  I suppose we could just set the flag any time someone
stumbles into the fs code from userspace, though you're right that seems
daunting.

--D

> -- 
> Michal Hocko
> SUSE Labs

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