Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration

[Hugh Dickins <hughd@google.com>]

On Fri, 15 Aug 2025, John Hubbard wrote:
> On 8/15/25 3:18 AM, Will Deacon wrote:
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index adffe663594d..656835890f05 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_unpinnable_folios(
> >  			continue;
> >  		}
> >  
> > -		if (!folio_test_lru(folio) && drain_allow) {
> > +		if (drain_allow &&
> > +		   (!folio_test_lru(folio) || folio_test_mlocked(folio))) {
> 
> That should work, yes.
> 
> Alternatively, after thinking about this a bit today, it seems to me that the
> mlock batching is a little too bold, given the presence of gup/pup. And so I'm
> tempted to fix the problem closer to the root cause, like this (below).
> 
> But maybe this is actually *less* wise than what you have proposed...
> 
> I'd like to hear other mm folks' opinion on this approach:
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index a1d93ad33c6d..edecdd32996e 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -278,7 +278,15 @@ void mlock_new_folio(struct folio *folio)
>  
>  	folio_get(folio);
>  	if (!folio_batch_add(fbatch, mlock_new(folio)) ||
> -	    folio_test_large(folio) || lru_cache_disabled())
> +	    folio_test_large(folio) || lru_cache_disabled() ||
> +	/*
> +	 * If this is being called as part of a gup FOLL_LONGTERM operation in
> +	 * CMA/MOVABLE zones with MLOCK_ONFAULT active, then the newly faulted
> +	 * in folio will need to immediately migrate to a pinnable zone.
> +	 * Allowing the mlock operation to batch would break the ability to
> +	 * migrate the folio. Instead, force immediate processing.
> +	 */
> +	 (current->flags & PF_MEMALLOC_PIN))
>  		mlock_folio_batch(fbatch);
>  	local_unlock(&mlock_fbatch.lock);
>  }

It's certainly worth considering this approach: it is consistent with
the lru_cache_disabled() approach (but I'm not a great fan of the
lru_cache_disabled() approach, often wonder how much damage it does).

But I think you've placed this in the wrong function: mlock_new_folio()
should already be satisfactorily handled, it's mlock_folio() that's
the problematic one.

I didn't know of PF_MEMALLOC_PIN at all: as you say,
let's hear other opinions.

Hugh