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

[Jason Gunthorpe]

On Mon, Mar 25, 2019 at 01:42:26AM -0700, Ira Weiny wrote:
> 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
> > > +++ 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.

get_user_pages_unlocked() is an exported symbol, shouldn't it work
with the FOLL_LONGTERM flag?

I think it should even though we have no user..

Otherwise the GUP API just gets more confusing.

Jason