Re: [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

[David Hildenbrand]

On 18.06.25 19:23, David Hildenbrand wrote:
> On 18.06.25 19:10, Lorenzo Stoakes wrote:
> > On Wed, Jun 18, 2025 at 06:14:22PM +0200, David Hildenbrand wrote:
> > > On 18.06.25 12:26, Dev Jain wrote:
> > > > +
> > > >     			/*
> > > >     			 * ptl mostly unnecessary, but preempt has to
> > > >     			 * be disabled to update the per-cpu stats
> > > >     			 * inside folio_remove_rmap_pte().
> > > >     			 */
> > > >     			spin_lock(ptl);
> > >
> > > Existing code: The PTL locking should just be moved outside of the loop.
> >
> > Do we really want to hold the PTL for the duration of the loop? Are we sure
> > it's safe to do so? Are there any locks taken in other functions that might
> > sleep that'd mean holding a spinlock would be a problem?
>
> It's a very weird thing to not hold the PTL while walking page tables,
> and then only grabbing it for clearing entries just to make selected
> functions happy ...
>
> I mostly spotted the release_pte_folio(), which I think should be fine
> with a spinlock held. I missed the free_folio_and_swap_cache(), not sure
> if that is problematic.
>
> Interestingly, release_pte_folio() does a
>
> a) node_stat_mod_folio
> b) folio_unlock
> c) folio_putback_lru
>
> ... and folio_putback_lru() is documented to "lru_lock must not be held,
> interrupts must be enabled". Hmmmm. I suspect that doc is wrong.

It's late ... doc is correct. Was reading "must be held".

So yeah, let's leave it as is.

--
Cheers,

David / dhildenb