Re: [PATCH v5 1/5] mm,page_alloc: Bail out earlier on -ENOMEM in alloc_contig_migrate_range

[Michal Hocko <mhocko@suse.com>]

On Wed 17-03-21 15:42:43, David Hildenbrand wrote:
> On 17.03.21 15:05, Michal Hocko wrote:
> > On Wed 17-03-21 12:12:47, Oscar Salvador wrote:
> > > Currently, __alloc_contig_migrate_range can generate -EINTR, -ENOMEM or -EBUSY,
> > > and report them down the chain.
> > > The problem is that when migrate_pages() reports -ENOMEM, we keep going till we
> > > exhaust all the try-attempts (5 at the moment) instead of bailing out.
> > > 
> > > migrate_pages() bails out right away on -ENOMEM because it is considered a fatal
> > > error. Do the same here instead of keep going and retrying.
> > 
> > I suspect this is not really a real life problem, right? The allocation
> > would be more costly in the end but this is to be expected under a heavy
> > memory pressure.
> > 
> > That being said, bailing out early makes sense to me. But now that
> > you've made me look into the migrate_pages excellent error state reporting
> > I suspect we have a bug here. Note the
> > "Returns the number of pages that were not migrated, or an error code."
> > 
> > but I do not see putback_movable_pages for ret > 0 so it seems we might
> > leak some pages.
> 
> At least in __alloc_contig_migrate_range() we seem to always leave the loop
> with ret <= 0 and do a putback_movable_pages() with ret < 0.
> 
> Which code are you referring to?

OK, my bad. I have managed to confuse myself around the retry bailout
which indeed overrides the return value. So there is no bug. Sorry about
the noise but I still believe making migrate_pages less tricky with
error handling would be an improvement.

Thanks!
-- 
Michal Hocko
SUSE Labs