On Wed, 7 Aug 2013 19:54:45 -0400 Ed Cashin <[email protected]> wrote:
> > +bio_pageinc(struct bio *bio)
> > +{
> > + struct bio_vec *bv;
> > + struct page *page;
> > + int i;
> > +
> > + bio_for_each_segment(bv, bio, i) {
> > + page = bv->bv_page;
> > + /* Non-zero page count for non-head members of
> > + * compound pages is no longer allowed by the kernel,
> > + * but this has never been seen here.
> > + */
> > + if (unlikely(PageCompound(page)))
> > + if (compound_trans_head(page) != page) {
> > + pr_crit("page tail used for block I/O\n");
> > + BUG();
> > + }
> >
> > But get_page/put_page against a tail page of a compound page should
> > Just Work. The core MM will hunt down the head page and manipulate its
> > refcount.
> >
> > Perhaps the problem here is that AOE is diving under the covers and is
> > using low-level page refcount alterations:
> >
> > + atomic_inc(&page->_count);
> >
> > Why does AOE do this? It would be better if it were to use the
> > official published MM interfaces. If those interfaces need
> > alteration/augmentation then we can do that.
>
> The get_page cannot be used when the _count is 0 (which is exactly when we
> need to increment it), because it will VM_BUG_ON.
>
> /*
> * Getting a normal page or the head of a compound page
> * requires to already have an elevated page->_count.
> */
> VM_BUG_ON(atomic_read(&page->_count) <= 0);
>
But we shouldn't get that far:
static inline void get_page(struct page *page)
{
if (unlikely(PageTail(page)))
if (likely(__get_page_tail(page)))
return;
/*
* Getting a normal page or the head of a compound page
* requires to already have an elevated page->_count.
*/
VM_BUG_ON(atomic_read(&page->_count) <= 0);
atomic_inc(&page->_count);
}
This is a tail page, so we should be using __get_page_tail().
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