Re: [PATCH v2 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[Michal Hocko]

On Sat 27-11-21 16:00:43, Andrew Morton wrote:
> On Fri, 26 Nov 2021 11:48:46 +0100 Michal Hocko <mhocko@xxxxxxxx> wrote:
> 
> > On Mon 22-11-21 16:32:31, Michal Hocko wrote:
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > 
> > > Dave Chinner has mentioned that some of the xfs code would benefit from
> > > kvmalloc support for __GFP_NOFAIL because they have allocations that
> > > cannot fail and they do not fit into a single page.
> > > 
> > > The large part of the vmalloc implementation already complies with the
> > > given gfp flags so there is no work for those to be done. The area
> > > and page table allocations are an exception to that. Implement a retry
> > > loop for those.
> > > 
> > > Add a short sleep before retrying. 1 jiffy is a completely random
> > > timeout. Ideally the retry would wait for an explicit event - e.g.
> > > a change to the vmalloc space change if the failure was caused by
> > > the space fragmentation or depletion. But there are multiple different
> > > reasons to retry and this could become much more complex. Keep the retry
> > > simple for now and just sleep to prevent from hogging CPUs.
> > > 
> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > Are there still any concerns around this patch or the approach in
> > general?
> 
> Well.  Like GFP_NOFAIL, every use is a sin.  But I don't think I've
> ever seen a real-world report of anyone hitting GFP_NOFAIL's
> theoretical issues.

I am not sure what you mean here. If you are missing real GFP_NOFAIL use
cases then some have been mentioned in the discussion

> I assume there will be a v3?

Currently I do not have any follow up changes on top of neither of the
patch except for acks and review tags. I can repost with those if you
prefer.

-- 
Michal Hocko
SUSE Labs