Re: [PATCH 06 of 66] alter compound get_page/put_page

Andrea Arcangeli
2010-11-25T16:51:09Z

On Thu, Nov 18, 2010 at 12:37:05PM +0000, Mel Gorman wrote:
> On Wed, Nov 03, 2010 at 04:27:41PM +0100, Andrea Arcangeli wrote:
> > From: Andrea Arcangeli <aarcange@redhat.com>
> >
> > Alter compound get_page/put_page to keep references on subpages too, in order
> > to allow __split_huge_page_refcount to split an hugepage even while subpages
> > have been pinned by one of the get_user_pages() variants.
> >
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > Acked-by: Rik van Riel <riel@redhat.com>
> > ---
> >
> > diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
> > --- a/arch/powerpc/mm/gup.c
> > +++ b/arch/powerpc/mm/gup.c
> > @@ -16,6 +16,16 @@
> >
> > #ifdef __HAVE_ARCH_PTE_SPECIAL
> >
> > +static inline void pin_huge_page_tail(struct page *page)
> > +{
>
> Minor nit, but get_huge_page_tail?
>
> Even though "pin" is what it does, pin isn't used elsewhere in naming.

Agreed.

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -16,7 +16,7 @@

#ifdef __HAVE_ARCH_PTE_SPECIAL

-static inline void pin_huge_page_tail(struct page *page)
+static inline void get_huge_page_tail(struct page *page)
{
/*
* __split_huge_page_refcount() cannot run
@@ -58,7 +58,7 @@ static noinline int gup_pte_range(pmd_t
return 0;
}
if (PageTail(page))
- pin_huge_page_tail(page);
+ get_huge_page_tail(page);
pages[*nr] = page;
(*nr)++;

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -105,7 +105,7 @@ static inline void get_head_page_multipl
atomic_add(nr, &page->_count);
}

-static inline void pin_huge_page_tail(struct page *page)
+static inline void get_huge_page_tail(struct page *page)
{
/*
* __split_huge_page_refcount() cannot run
@@ -139,7 +139,7 @@ static noinline int gup_huge_pmd(pmd_t p
VM_BUG_ON(compound_head(page) != head);
pages[*nr] = page;
if (PageTail(page))
- pin_huge_page_tail(page);
+ get_huge_page_tail(page);
(*nr)++;
page++;
refs++;

> > diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> > --- a/arch/x86/mm/gup.c
> > +++ b/arch/x86/mm/gup.c
> > @@ -105,6 +105,16 @@ static inline void get_head_page_multipl
> > atomic_add(nr, &page->_count);
> > }
> >
> > +static inline void pin_huge_page_tail(struct page *page)
> > +{
> > + /*
> > + * __split_huge_page_refcount() cannot run
> > + * from under us.
> > + */
> > + VM_BUG_ON(atomic_read(&page->_count) < 0);
> > + atomic_inc(&page->_count);
> > +}
> > +
>
> This is identical to the x86 implementation. Any possibility they can be
> shared?

There is no place for me today to put gup_fast "equal" bits so I'd
need to create it and just doing it for a single inline function
sounds overkill. I could add a asm-generic/gup_fast.h add move that
function there and include the asm-generic/gup_fast.h from
asm*/include/gup_fast.h, is that what we want just for one function?
With all #ifdefs included I would end up writing more code than the
function itself. No problem with me in doing it though.

> > static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
> > unsigned long end, int write, struct page **pages, int *nr)
> > {
> > @@ -128,6 +138,8 @@ static noinline int gup_huge_pmd(pmd_t p
> > do {
> > VM_BUG_ON(compound_head(page) != head);
> > pages[*nr] = page;
> > + if (PageTail(page))
> > + pin_huge_page_tail(page);
> > (*nr)++;
> > page++;
> > refs++;
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -351,9 +351,17 @@ static inline int page_count(struct page
> >
> > static inline void get_page(struct page *page)
> > {
> > - page = compound_head(page);
> > - VM_BUG_ON(atomic_read(&page->_count) == 0);
> > + VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
>
> Oof, this might need a comment. It's saying that getting a normal page or the
> head of a compound page must already have an elevated reference count. If
> we are getting a tail page, the reference count is stored in both the head
> and the tail so the BUG check does not apply.

Ok.

diff --git a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -353,8 +353,20 @@ static inline int page_count(struct page

static inline void get_page(struct page *page)
{
+ /*
+ * Getting a normal page or the head of a compound page
+ * requires to already have an elevated page->_count. Only if
+ * we're getting a tail page, the elevated page->_count is
+ * required only in the head page, so for tail pages the
+ * bugcheck only verifies that the page->_count isn't
+ * negative.
+ */
VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
atomic_inc(&page->_count);
+ /*
+ * Getting a tail page will elevate both the head and tail
+ * page->_count(s).
+ */
if (unlikely(PageTail(page))) {
/*
* This is safe only because

> > diff --git a/mm/swap.c b/mm/swap.c
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -56,17 +56,83 @@ static void __page_cache_release(struct
> > del_page_from_lru(zone, page);
> > spin_unlock_irqrestore(&zone->lru_lock, flags);
> > }
> > +}
> > +
> > +static void __put_single_page(struct page *page)
> > +{
> > + __page_cache_release(page);
> > free_hot_cold_page(page, 0);
> > }
> >
> > +static void __put_compound_page(struct page *page)
> > +{
> > + compound_page_dtor *dtor;
> > +
> > + __page_cache_release(page);
> > + dtor = get_compound_page_dtor(page);
> > + (*dtor)(page);
> > +}
> > +
> > static void put_compound_page(struct page *page)
> > {
> > - page = compound_head(page);
> > - if (put_page_testzero(page)) {
> > - compound_page_dtor *dtor;
> > -
> > - dtor = get_compound_page_dtor(page);
> > - (*dtor)(page);
> > + if (unlikely(PageTail(page))) {
> > + /* __split_huge_page_refcount can run under us */
>
> So what? The fact you check PageTail twice is a hint as to what is
> happening and that we are depending on the order of when the head and
> tails bits get cleared but it's hard to be certain of that.

This is correct, we depend on the split_huge_page_refcount to clear
PageTail before overwriting first_page.

+ /*
+ * 1) clear PageTail before overwriting first_page
+ * 2) clear PageTail before clearing PageHead for VM_BUG_ON
+ */
+ smp_wmb();

The first PageTail check is just to define the fast path and skip
reading page->first_page and doing smb_rmb() for the much more common
head pages (we're in unlikely branch).

So then we read first_page, we smb_rmb() and if pagetail is still set
after that, we can be sure first_page isn't a dangling pointer.

>
> > + struct page *page_head = page->first_page;
> > + smp_rmb();
> > + if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
> > + unsigned long flags;
> > + if (unlikely(!PageHead(page_head))) {
> > + /* PageHead is cleared after PageTail */
> > + smp_rmb();
> > + VM_BUG_ON(PageTail(page));
> > + goto out_put_head;
> > + }
> > + /*
> > + * Only run compound_lock on a valid PageHead,
> > + * after having it pinned with
> > + * get_page_unless_zero() above.
> > + */
> > + smp_mb();
> > + /* page_head wasn't a dangling pointer */
> > + compound_lock_irqsave(page_head, &flags);
> > + if (unlikely(!PageTail(page))) {
> > + /* __split_huge_page_refcount run before us */
> > + compound_unlock_irqrestore(page_head, flags);
> > + VM_BUG_ON(PageHead(page_head));
> > + out_put_head:
> > + if (put_page_testzero(page_head))
> > + __put_single_page(page_head);
> > + out_put_single:
> > + if (put_page_testzero(page))
> > + __put_single_page(page);
> > + return;
> > + }
> > + VM_BUG_ON(page_head != page->first_page);
> > + /*
> > + * We can release the refcount taken by
> > + * get_page_unless_zero now that
> > + * split_huge_page_refcount is blocked on the
> > + * compound_lock.
> > + */
> > + if (put_page_testzero(page_head))
> > + VM_BUG_ON(1);
> > + /* __split_huge_page_refcount will wait now */
> > + VM_BUG_ON(atomic_read(&page->_count) <= 0);
> > + atomic_dec(&page->_count);
> > + VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
> > + compound_unlock_irqrestore(page_head, flags);
> > + if (put_page_testzero(page_head))
> > + __put_compound_page(page_head);
> > + } else {
> > + /* page_head is a dangling pointer */
> > + VM_BUG_ON(PageTail(page));
> > + goto out_put_single;
> > + }
> > + } else if (put_page_testzero(page)) {
> > + if (PageHead(page))
> > + __put_compound_page(page);
> > + else
> > + __put_single_page(page);
> > }
> > }
> >
> > @@ -75,7 +141,7 @@ void put_page(struct page *page)
> > if (unlikely(PageCompound(page)))
> > put_compound_page(page);
> > else if (put_page_testzero(page))
> > - __page_cache_release(page);
> > + __put_single_page(page);
> > }
> > EXPORT_SYMBOL(put_page);
> >
>
> Functionally, I don't see a problem so
>
> Acked-by: Mel Gorman <mel@csn.ul.ie>
>
> but some expansion on the leader and the comment, even if done as a
> follow-on patch, would be nice.

diff --git a/mm/swap.c b/mm/swap.c
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -79,8 +79,18 @@ static void put_compound_page(struct pag
/* __split_huge_page_refcount can run under us */
struct page *page_head = page->first_page;
smp_rmb();
+ /*
+ * If PageTail is still set after smp_rmb() we can be sure
+ * that the page->first_page we read wasn't a dangling pointer.
+ * See __split_huge_page_refcount() smp_wmb().
+ */
if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
unsigned long flags;
+ /*
+ * Verify that our page_head wasn't converted
+ * to a a regular page before we got a
+ * reference on it.
+ */
if (unlikely(!PageHead(page_head))) {
/* PageHead is cleared after PageTail */
smp_rmb();