Re: [RFC PATCH 1/7] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON

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

On Sun, Mar 23, 2025 at 01:53:02PM +0100, David Hildenbrand wrote:
> On 22.03.25 08:21, Lorenzo Stoakes wrote:
> > On Sat, Mar 22, 2025 at 07:17:05AM +0100, David Hildenbrand wrote:
> > > On 22.03.25 06:33, David Hildenbrand wrote:
> > > > On 22.03.25 01:14, Jann Horn wrote:
> > > > > On Fri, Mar 21, 2025 at 10:54 PM Lorenzo Stoakes
> > > > > <lorenzo.stoakes@oracle.com> wrote:
> > > > > > diff --git a/mm/mremap.c b/mm/mremap.c
> > > > > > index 0865387531ed..bb67562a0114 100644
> > > > > > --- a/mm/mremap.c
> > > > > > +++ b/mm/mremap.c
> > > > > [...]
> > > > > > +/*
> > > > > > + * If the folio mapped at the specified pte entry can have its index and mapping
> > > > > > + * relocated, then do so.
> > > > > > + *
> > > > > > + * Returns the number of pages we have traversed, or 0 if the operation failed.
> > > > > > + */
> > > > > > +static unsigned long relocate_anon(struct pagetable_move_control *pmc,
> > > > > > +               unsigned long old_addr, unsigned long new_addr, pte_t pte,
> > > > > > +               bool undo)
> > > > > > +{
> > > > > > +       struct page *page;
> > > > > > +       struct folio *folio;
> > > > > > +       struct vm_area_struct *old, *new;
> > > > > > +       pgoff_t new_index;
> > > > > > +       unsigned long ret = 1;
> > > > > > +
> > > > > > +       old = pmc->old;
> > > > > > +       new = pmc->new;
> > > > > > +
> > > > > > +       /* Ensure we have truly got an anon folio. */
> > > > > > +       page = vm_normal_page(old, old_addr, pte);
> > > > > > +       if (!page)
> > > > > > +               return ret;
> > > > > > +       folio = page_folio(page);
> > > > > > +       folio_lock(folio);
> > > > > > +
> > > > > > +       /* no-op. */
> > > > > > +       if (!folio_test_anon(folio) || folio_test_ksm(folio))
> > > > > > +               goto out;
> > > > > > +
> > > > > > +       /*
> > > > > > +        * This should not happen as we explicitly disallow this, but check
> > > > > > +        * anyway.
> > > > > > +        */
> > > > > > +       if (folio_test_large(folio)) {
> > > > > > +               ret = 0;
> > > > > > +               goto out;
> > > > > > +       }
> > > > >
> > > > > Do I understand correctly that you assume here that the page is
> > > > > exclusively mapped? Maybe we could at least
> > > > > WARN_ON(folio_mapcount(folio) != 1) or something like that?
> > > > >
> > > > > (I was also wondering if the PageAnonExclusive bit is somehow
> > > > > relevant, but we should probably not look at or touch that here,
> > > > > unless we want to think about cases where we _used to_ have a child
> > > > > from which the page may have been GUP'd...)
> > > >
> > > > UFFDIO_MOVE implements something similar. Right now we keep it simple:
> > > >
> > > > 	if (folio_test_large(src_folio) ||
> > > > 	    folio_maybe_dma_pinned(src_folio) ||
> > > > 	    !PageAnonExclusive(&src_folio->page)) {
> > > > 		err = -EBUSY;
> > > > 		goto out;
> > > > 	}
> > > >
> > > > Whereby we
> > > >
> > > > a) Make sure we cover all PTEs (-> small folio, single PTE). Large
> > > > PTE-mapped folios are split.
> > > >
> > > > b) Make sure there are no GUP pins (maybe not required here?)
> > > >
> > > > c) The folio is exclusive to this process
> > >
> > > On additional note as my memory comes back: if PAE is set, there cannot be
> > > other (inactive) mappings from the swapcache. So whenever we use folio lock
> > > + mapcount data, the possibility of the swapcache (having inactive mappings
> > > from other processes etc.) must be considered.
> >
> > Ack, do you have a feel for how such a check would work?
>
> Likely under folio lock
>
> if (folio_test_swapcache(folio) && !folio_free_swap(folio)) {
> 	/* unable to move. */
> 	folio_unlock(folio)
> 	return -ENOTGOINGTOHAPPEN;
> }

Thanks! Will adapt.

>
> --
> Cheers,
>
> David / dhildenb
>