Re: [PATCH v3 04/14] mm/rmap: Break COW PTE in rmap walking

[Barry Song <baohua@kernel.org>]

On Mon, Dec 26, 2022 at 11:56 PM Chih-En Lin <shiyn.lin@gmail.com> wrote:
>
> On Mon, Dec 26, 2022 at 10:40:49PM +1300, Barry Song wrote:
> > On Tue, Dec 20, 2022 at 8:25 PM Chih-En Lin <shiyn.lin@gmail.com> wrote:
> > >
> > > Some of the features (unmap, migrate, device exclusive, mkclean, etc)
> > > might modify the pte entry via rmap. Add a new page vma mapped walk
> > > flag, PVMW_BREAK_COW_PTE, to indicate the rmap walking to break COW PTE.
> > >
> > > Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
> > > ---
> > >  include/linux/rmap.h |  2 ++
> > >  mm/migrate.c         |  3 ++-
> > >  mm/page_vma_mapped.c |  2 ++
> > >  mm/rmap.c            | 12 +++++++-----
> > >  mm/vmscan.c          |  7 ++++++-
> > >  5 files changed, 19 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > index bd3504d11b155..d0f07e5519736 100644
> > > --- a/include/linux/rmap.h
> > > +++ b/include/linux/rmap.h
> > > @@ -368,6 +368,8 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
> > >  #define PVMW_SYNC              (1 << 0)
> > >  /* Look for migration entries rather than present PTEs */
> > >  #define PVMW_MIGRATION         (1 << 1)
> > > +/* Break COW-ed PTE during walking */
> > > +#define PVMW_BREAK_COW_PTE     (1 << 2)
> > >
> > >  struct page_vma_mapped_walk {
> > >         unsigned long pfn;
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index dff333593a8ae..a4be7e04c9b09 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -174,7 +174,8 @@ void putback_movable_pages(struct list_head *l)
> > >  static bool remove_migration_pte(struct folio *folio,
> > >                 struct vm_area_struct *vma, unsigned long addr, void *old)
> > >  {
> > > -       DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> > > +       DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr,
> > > +                             PVMW_SYNC | PVMW_MIGRATION | PVMW_BREAK_COW_PTE);
> > >
> > >         while (page_vma_mapped_walk(&pvmw)) {
> > >                 rmap_t rmap_flags = RMAP_NONE;
> > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > > index 93e13fc17d3cb..5dfc9236dc505 100644
> > > --- a/mm/page_vma_mapped.c
> > > +++ b/mm/page_vma_mapped.c
> > > @@ -251,6 +251,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> > >                         step_forward(pvmw, PMD_SIZE);
> > >                         continue;
> > >                 }
> > > +               if (pvmw->flags & PVMW_BREAK_COW_PTE)
> > > +                       break_cow_pte(vma, pvmw->pmd, pvmw->address);
> > >                 if (!map_pte(pvmw))
> > >                         goto next_pte;
> > >  this_pte:
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 2ec925e5fa6a9..b1b7dcbd498be 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -807,7 +807,8 @@ static bool folio_referenced_one(struct folio *folio,
> > >                 struct vm_area_struct *vma, unsigned long address, void *arg)
> > >  {
> > >         struct folio_referenced_arg *pra = arg;
> > > -       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> > > +       /* it will clear the entry, so we should break COW PTE. */
> > > +       DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE);
> >
> > what do you mean by breaking cow pte? in memory reclamation case, we are only
> > checking and clearing page referenced bit in pte, do we really need to
> > break cow?
>
> Since we might clear page referenced bit, it will modify the write
> protection shared page table (COW-ed PTE). We should duplicate it.
>
> Actually, I didn’t break COW at first because it will conditionally
> modify the table and only clear the referenced bit.
> So, if clearing page referenced bit is fine to the COW-ed PTE table
> and the break COW PTE is unnecessary here, we can remove it.

if a page is mapped by 100 processes and anyone of these 100 processes
access this page, we will get a reference bit in the PTE. Otherwise, we will
have to scan 100 PTEs to figure out if a page is accessed and should be
kept in LRU.
i don't see the fundamental necessity to duplicate PTE only because of clearing
the reference bit. as keeping the pte shared will help save a lot of cost for
memory reclamation for those CPUs which have hardware reference bits
in PTE.

>
> Thanks,
> Chih-En Lin

Thanks
barry