Re: [PATCH RFC] mm: allow __GFP_NOFAIL allocation up to BLK_MAX_BLOCK_SIZE to support LBS

[Michal Hocko <mhocko@suse.com>]

On Fri 31-10-25 16:55:44, Matthew Wilcox wrote:
> On Fri, Oct 31, 2025 at 09:46:17AM -0700, Shakeel Butt wrote:
> > Now for the interface to allow NOFS+NOFAIL+higher_order, I think a new
> > (FS specific) gfp is fine but will require some maintenance to avoid
> > abuse.
> 
> I don't think a new GFP flag is the answer.  GFP_TRUST_ME_BRO just
> doesn't feel right.

Yeah, as usual a new gfp flag seems convenient except history has taught 
us this rarely works.

> > I am more interested in how to codify "you can reclaim one I've already
> > allocated". I have a different scenario where network stack keep
> > stealing memory from direct reclaimers and keeping them in reclaim for
> > long time. If we have some mechanism to allow reclaimers to get the
> > memory they have reclaimed (at least for some cases), I think that can
> > be used in both cases.
> 
> The only thing that comes to mind is putting pages freed by reclaim on
> a list in task_struct instead of sending them back to the allocator.
> Then the task can allocate from there and free up anything else it's
> reclaimed at some later point.  I don't think this is a good idea,
> but it's the only idea that comes to mind.

I have played with that idea years ago. Mostly to deal with direct
reclaim unfairness when some reclaimers were doing a lot of work on
behalf of everybody else. IIRC I have hit into different problems, like
reclaim throttling and over-reclaim.

Anyway, page allocator does respect GFP_NOFAIL even for high order
requests. The oom killer will be disabled for order-4 but as these will
likely be GFP_NOFS anyway then the order doesn't make much of a
difference. So these requests could really take long time to succeed but
I guess this will be generally understood. As the vmalloc fallback
doesn't seem to be a feasible option short (maybe even mid) term then
this is the only choice we have other than failing allocations and
seeing a lot of fs failures.

That being said I would much rather go and drop the order warning than
trying to invent some fine tuning based on usecase. We might need to
invent some OOM protection for order-3 nofail requests as OOM killer
could just make too much harm killing tasks without much of chance to
defragment memory. Let's deal with that once we see that happening.

-- 
Michal Hocko
SUSE Labs