Re: [RESEND 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

[Ira Weiny]

On Fri, Mar 22, 2019 at 03:12:55PM -0700, Dan Williams wrote:
> On Sun, Mar 17, 2019 at 7:36 PM <ira.weiny@xxxxxxxxx> wrote:
> >
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> >
> > DAX pages were previously unprotected from longterm pins when users
> > called get_user_pages_fast().
> >
> > Use the new FOLL_LONGTERM flag to check for DEVMAP pages and fall
> > back to regular GUP processing if a DEVMAP page is encountered.
> >
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > ---
> >  mm/gup.c | 29 +++++++++++++++++++++++++----
> >  1 file changed, 25 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 0684a9536207..173db0c44678 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1600,6 +1600,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >                         goto pte_unmap;
> >
> >                 if (pte_devmap(pte)) {
> > +                       if (unlikely(flags & FOLL_LONGTERM))
> > +                               goto pte_unmap;
> > +
> >                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
> >                         if (unlikely(!pgmap)) {
> >                                 undo_dev_pagemap(nr, nr_start, pages);
> > @@ -1739,8 +1742,11 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> >         if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
> >                 return 0;
> >
> > -       if (pmd_devmap(orig))
> > +       if (pmd_devmap(orig)) {
> > +               if (unlikely(flags & FOLL_LONGTERM))
> > +                       return 0;
> >                 return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
> > +       }
> >
> >         refs = 0;
> >         page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> > @@ -1777,8 +1783,11 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> >         if (!pud_access_permitted(orig, flags & FOLL_WRITE))
> >                 return 0;
> >
> > -       if (pud_devmap(orig))
> > +       if (pud_devmap(orig)) {
> > +               if (unlikely(flags & FOLL_LONGTERM))
> > +                       return 0;
> >                 return __gup_device_huge_pud(orig, pudp, addr, end, pages, nr);
> > +       }
> >
> >         refs = 0;
> >         page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> > @@ -2066,8 +2075,20 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >                 start += nr << PAGE_SHIFT;
> >                 pages += nr;
> >
> > -               ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
> > -                                             gup_flags);
> > +               if (gup_flags & FOLL_LONGTERM) {
> > +                       down_read(&current->mm->mmap_sem);
> > +                       ret = __gup_longterm_locked(current, current->mm,
> > +                                                   start, nr_pages - nr,
> > +                                                   pages, NULL, gup_flags);
> > +                       up_read(&current->mm->mmap_sem);
> > +               } else {
> > +                       /*
> > +                        * retain FAULT_FOLL_ALLOW_RETRY optimization if
> > +                        * possible
> > +                        */
> > +                       ret = get_user_pages_unlocked(start, nr_pages - nr,
> > +                                                     pages, gup_flags);
> 
> I couldn't immediately grok why this path needs to branch on
> FOLL_LONGTERM? Won't get_user_pages_unlocked(..., FOLL_LONGTERM) do
> the right thing?

Unfortunately holding the lock is required to support FOLL_LONGTERM (to check
the VMAs) but we don't want to hold the lock to be optimal (specifically allow
FAULT_FOLL_ALLOW_RETRY).  So I'm maintaining the optimization for *_fast users
who do not specify FOLL_LONGTERM.

Another way to do this would have been to define __gup_longterm_unlocked with
the above logic, but that seemed overkill at this point.

Ira