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

Mel Gorman
2010-11-26T11:47:07Z

On Thu, Nov 25, 2010 at 05:49:16PM +0100, Andrea Arcangeli wrote:
> > > <SNIP>

The updates are fine.

> > > 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.
>

Now that you've explained what is necessary, it's not worth it as part of the
series. It's sizable enough as it is for review. Consider it as a follow-up
patch maybe.

> > > 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

Much better.

>
> > > 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();
>

This helps. Less guesswork involved :)

> 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();
>

Thanks

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab