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

From: Ira Weiny
Date: Mon Mar 25 2019 - 13:24:31 EST

On Mon, Mar 25, 2019 at 01:47:13PM -0300, Jason Gunthorpe wrote:
> 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.

I agree WRT to the API. But I think callers of get_user_pages_unlocked() are
not going to get the behavior they want if they specify FOLL_LONGTERM.

What I could do is BUG_ON (or just WARN_ON) if unlocked is called with
FOLL_LONGTERM similar to the code in get_user_pages_locked() which does not
allow locked and vmas to be passed together:

if (locked) {
/* if VM_FAULT_RETRY can be returned, vmas become invalid */
BUG_ON(vmas);
/* check caller initialized locked */
BUG_ON(*locked != 1);
}

Combining Dan's comment and yours; I could do something like below? (not even
compile tested) Coded with WARN_ON because technically I _think_ the
FOLL_LONGTERM would "work" but not be optimal. I'm just not 100% sure. A
BUG_ON would be most protective IMO.


diff --git a/mm/gup.c b/mm/gup.c
index 173db0c44678..8e31411f485f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1014,6 +1014,29 @@ long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
}
EXPORT_SYMBOL(get_user_pages_locked);

+long __gup_longterm_unlocked(unsigned long start, unsigned long nr_pages,
+ struct page **pages, unsigned int gup_flags)
+{
+ struct mm_struct *mm = current->mm;
+ int locked = 1;
+ long ret;
+
+ down_read(&mm->mmap_sem);
+ if (gup_flags & FOLL_LONGTERM) {
+ ret = __gup_longterm_locked(current, mm,
+ start, nr_pages - nr,
+ pages, NULL, gup_flags);
+ } else {
+ ret = __get_user_pages_locked(current, mm, start, nr_pages,
+ pages, NULL, &locked,
+ gup_flags | FOLL_TOUCH);
+ }
+ if (locked)
+ up_read(&mm->mmap_sem);
+
+ return ret;
+}
+
/*
* get_user_pages_unlocked() is suitable to replace the form:
*
@@ -1032,16 +1055,9 @@ EXPORT_SYMBOL(get_user_pages_locked);
long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
struct page **pages, unsigned int gup_flags)
{
- struct mm_struct *mm = current->mm;
- int locked = 1;
- long ret;
+ WARN_ON(gup_flags & FOLL_LONGTERM);

- down_read(&mm->mmap_sem);
- ret = __get_user_pages_locked(current, mm, start, nr_pages, pages, NULL,
- &locked, gup_flags | FOLL_TOUCH);
- if (locked)
- up_read(&mm->mmap_sem);
- return ret;
+ __gup_longterm_unlocked(start, nr_pages, pages, gup_flags);
}
EXPORT_SYMBOL(get_user_pages_unlocked);

@@ -2075,20 +2091,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
start += nr << PAGE_SHIFT;
pages += nr;

- 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);
- }
+ __gup_longterm_unlocked();

/* Have to be a bit careful with return values */
if (nr > 0) {