Re: [PATCH] smaps: Fix the abnormal memory statistics obtained through /proc/pid/smaps

[David Hildenbrand]

> > > Therefore, when obtaining pages through the follow_trans_huge_pmd
> > > interface, add the FOLL_FORCE flag to count the pages corresponding to
> > > PROTNONE to solve the above problem.
> >
> > We really want to avoid the usage of FOLL_FORCE, and ideally limit it
> > to ptrace only.
>
> Fundamentally when removing FOLL_NUMA we did already assumed !FORCE is
> FOLL_NUMA.  It means to me after the removal it's not possible to say in a
> gup walker that "it's not FORCEd, but I don't want to trigger NUMA but just
> get the page".
>
> Is that what we want?  Shall we document that in FOLL_FORCE if we intended
> to enforce numa balancing as long as !FORCE?

That was the idea, yes. I could have sworn we had that at least in some 
patch description.

Back then, I played with special-casing on gup_can_follow_protnone() on 
FOLL_GET | FOLL_PIN. But it's all just best guesses.

Can always be added if deemed necessary and worth it.

Here, it's simply an abuse of that GUP function that I wasn't aware of 
-- otherwise I'd have removed that before hand.

> > > Signed-off-by: liubo <liubo254@xxxxxxxxxx>
> > > Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
> > > ---
> > >   fs/proc/task_mmu.c | 6 ++++--
> > >   1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index c1e6531cb02a..ed08f9b869e2 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -571,8 +571,10 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> > >          bool migration = false;
> > >
> > >          if (pmd_present(*pmd)) {
> > > -               /* FOLL_DUMP will return -EFAULT on huge zero page */
> > > -               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> > > +               /* FOLL_DUMP will return -EFAULT on huge zero page
> > > +                * FOLL_FORCE follow a PROT_NONE mapped page
> > > +                */
> > > +               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP | FOLL_FORCE);
> > >          } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> > >                  swp_entry_t entry = pmd_to_swp_entry(*pmd);
> >
> > Might do as an easy fix. But we really should get rid of that
> > absolutely disgusting usage of follow_trans_huge_pmd().
> >
> > We don't need 99% of what follow_trans_huge_pmd() does here.
> >
> > Would the following also fix your issue?
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 507cd4e59d07..fc744964816e 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -587,8 +587,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> >          bool migration = false;
> >
> >          if (pmd_present(*pmd)) {
> > -               /* FOLL_DUMP will return -EFAULT on huge zero page */
> > -               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> > +               page = vm_normal_page_pmd(vma, addr, *pmd);
> >          } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> >                  swp_entry_t entry = pmd_to_swp_entry(*pmd);
> >
> > It also skips the shared zeropage and pmd_devmap(),
> >
> > Otherwise, a simple pmd_page(*pmd) + is_huge_zero_pmd(*pmd) check will do, but I
> > suspect vm_normal_page_pmd() might be what we actually want to have here.
> >
> > Because smaps_pte_entry() properly checks for vm_normal_page().
>
> There're indeed some very trivial detail in vm_normal_page_pmd() that's
> different, but maybe not so relevant.  E.g.,
>
> 	if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
> 		return -ENOMEM;

Note that we're not even passing FOLL_GET | FOLL_PIN. Because we're not 
actually doing GUP. So the refcount is not that relevant.

> 	if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
> 		return -EREMOTEIO;
>
> I'm not sure whether the p2pdma page would matter in any form here.  E.g.,
> whether it can be mapped privately.

Good point, but I don't think that people messing with GUP even imagined 
that we would call that function from a !GUP place.

This was wrong from the very start. If we're not in GUP, we shouldn't 
call GUP functions.

--
Cheers,

David / dhildenb