Re: reply: [RFC] pin_user_pages_fast failure count increased

[Hyesoo Yu <hyesoo.yu@samsung.com>]

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On Thu, May 22, 2025 at 07:52:41PM -0700, John Hubbard wrote:
> On 5/22/25 7:37 PM, 김재원 wrote:
> ...
> > I think this is what you meant, please let me know if you have an idea to make this nicer.
> > We may be to able to prepare the patch next week.
> > 
> >  static long
> >  check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
> >  {
> > +       bool any_unpinnable;
> >         LIST_HEAD(movable_folio_list);
> >  
> > -       collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
> > -       if (list_empty(&movable_folio_list))
> > -               return 0;
> > +       any_unpinnable = collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
> > +       if (list_empty(&movable_folio_list)) {
> > +               if (any_unpinnable)
> > +                       pofs_unpin(pofs);
> 
> I think this is correct, although as I mentioned in the other thread,
> that implies that commit 1aaf8c122918 (which didn't add nor remove 
> any pof unpinning) is probably not the true or only culprit, right?
> 
> > +               return any_unpinnable ? -EAGAIN : 0;
> 
> Ha, the "?" operator almost always does more harm than good.
> 
> Here, for example, it has obscured from you the fact that any_unpinnable
> is being checked twice, when you could have merged those into a single "if".
> 

Hello,

I was wondering if the original problem - an infinite loop when pages allocated by
cma_alloc() in vm_ops->fault are passed to GUP - still remains unresolved.
(To be honest, I'm not quite sure how such pages end up being pinned via GUP.
 Is that the expected behavior, or could it possibly indicate a bug ?)

Would it be make sense to try calling __lru_add_drain_all(false) in collect_longterm_unpinnable_folios ?
We could consider limiting the number of -EAGAIN retries to a fixed count (e.g., 5 attempts) to
avoid an infinite loop. Or perhaps in check_and_migrate_movable_pages_or_folio(),
if the list is empty but any_unpinnable is true, we should consider returning an error
instead of EAGAIN to explicitly prevent longterm pinning of CMA allocated memory.

I'd be interested to hear what others think.

Thanks,
Hyesoo Yu.


> > +       }
> >  
> >         return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);
> >  }
> thanks,
> -- 
> John Hubbard
> 
> 

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