Re: [PATCH 3/3] mm: Optimize mremap() by PTE batching

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, May 06, 2025 at 02:49:04PM +0100, Lorenzo Stoakes wrote:
> On Tue, May 06, 2025 at 10:30:56AM +0530, Dev Jain wrote:
> > Use folio_pte_batch() to optimize move_ptes(). Use get_and_clear_full_ptes()
> > so as to elide TLBIs on each contig block, which was previously done by
> > ptep_get_and_clear().
>
> No mention of large folios

Sorry didn't finish my sentence here. I suggest you add more detail
here. Again the 'why' and what is this for etc. etc.

Equally I don't think having code that seemingly randomly invokes batched
functions is a good idea, I think the relevant logic should be separated
into functions that explicit reference large folios or should have comments
explaining what you're doing.

You can see some of how I separated out such things for my
MREMAP_RELOCATE_ANON series at [0] for instance.

[0]: https://lore.kernel.org/linux-mm/8ca7c8219a2a0e67e1c5c277d0c0d070052de401.1746305604.git.lorenzo.stoakes@oracle.com/

Thanks!

>
> >
> > Signed-off-by: Dev Jain <dev.jain@arm.com>
> > ---
> >  mm/mremap.c | 24 +++++++++++++++++++-----
> >  1 file changed, 19 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 1a08a7c3b92f..3621c07d8eea 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -176,7 +176,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >  	struct vm_area_struct *vma = pmc->old;
> >  	bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma);
> >  	struct mm_struct *mm = vma->vm_mm;
> > -	pte_t *old_ptep, *new_ptep, pte;
> > +	pte_t *old_ptep, *new_ptep, old_pte, pte;
>
> Obviously given previous comment you know what I'm going to say here :) let's
> put old_pte, pte in a new decl.
>
> >  	pmd_t dummy_pmdval;
> >  	spinlock_t *old_ptl, *new_ptl;
> >  	bool force_flush = false;
> > @@ -185,6 +185,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >  	unsigned long old_end = old_addr + extent;
> >  	unsigned long len = old_end - old_addr;
> >  	int err = 0;
> > +	int nr;
> >
> >  	/*
> >  	 * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
> > @@ -237,10 +238,14 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >
> >  	for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE,
> >  				   new_ptep++, new_addr += PAGE_SIZE) {
>
> Hm this just seems wrong, even if we're dealing with a large folio we're still
> offsetting by PAGE_SIZE each time and iterating through further sub-pages?
>
> Shouldn't we be doing something like += nr and += PAGE_SIZE * nr?
>
> Then it'd make sense to initialise nr to 1.
>
> Honestly I'd prefer us though to refactor move_ptes() to something like:
>
> 	for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE,
> 				   new_ptep++, new_addr += PAGE_SIZE) {
> 		pte_t old_pte = ptep_get(old_ptep);
>
> 		if (pte_none(old_pte))
> 			continue;
>
> 		move_pte(pmc, vma, old_ptep, old_pte);
> 	}
>
> Declaring this new move_pte() where you can put the rest of the stuff.
>
> I'd much rather we do this than add to the mess as-is.
>
>
>
> > -		if (pte_none(ptep_get(old_ptep)))
> > +		const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > +		int max_nr = (old_end - old_addr) >> PAGE_SHIFT;
> > +
> > +		nr = 1;
> > +		old_pte = ptep_get(old_ptep);
>
> You can declare this in the for loop, no need for us to contaminate whole
> function scope with it.
>
> Same with 'nr' in this implementation (though I'd rather you changed it up, see
> above).
>
> > +		if (pte_none(old_pte))
> >  			continue;
> >
> > -		pte = ptep_get_and_clear(mm, old_addr, old_ptep);
> >  		/*
> >  		 * If we are remapping a valid PTE, make sure
> >  		 * to flush TLB before we drop the PTL for the
> > @@ -252,8 +257,17 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >  		 * the TLB entry for the old mapping has been
> >  		 * flushed.
> >  		 */
> > -		if (pte_present(pte))
> > +		if (pte_present(old_pte)) {
> > +			if ((max_nr != 1) && maybe_contiguous_pte_pfns(old_ptep, old_pte)) {
> > +				struct folio *folio = vm_normal_folio(vma, old_addr, old_pte);
> > +
> > +				if (folio && folio_test_large(folio))
> > +					nr = folio_pte_batch(folio, old_addr, old_ptep,
> > +					old_pte, max_nr, fpb_flags, NULL, NULL, NULL);
>
> Indentation seems completely broken here? I also hate that we're nesting to this
> degree? Can we please find a way not to?
>
> This function is already a bit of a clogged mess, I'd rather we clean up then
> add to it.
>
> (See above again :)
>
>
> > +			}
> >  			force_flush = true;
> > +		}
> > +		pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr, 0);
> >  		pte = move_pte(pte, old_addr, new_addr);
> >  		pte = move_soft_dirty_pte(pte);
> >
> > @@ -266,7 +280,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >  				else if (is_swap_pte(pte))
> >  					pte = pte_swp_clear_uffd_wp(pte);
> >  			}
> > -			set_pte_at(mm, new_addr, new_ptep, pte);
> > +			set_ptes(mm, new_addr, new_ptep, pte, nr);
> >  		}
> >  	}
> >
> > --
> > 2.30.2
> >
>
> Cheers, Lorenzo