On Wed, Apr 10, 2013 at 11:59:29AM +0530, Aneesh Kumar K.V wrote: > David Gibson <d...@au1.ibm.com> writes: > > On Thu, Apr 04, 2013 at 11:27:44AM +0530, Aneesh Kumar K.V wrote: [snip] > >> @@ -97,13 +100,45 @@ void __destroy_context(int context_id) > >> } > >> EXPORT_SYMBOL_GPL(__destroy_context); > >> > >> +#ifdef CONFIG_PPC_64K_PAGES > >> +static void destroy_pagetable_page(struct mm_struct *mm) > >> +{ > >> + int count; > >> + struct page *page; > >> + > >> + page = mm->context.pgtable_page; > >> + if (!page) > >> + return; > >> + > >> + /* drop all the pending references */ > >> + count = atomic_read(&page->_mapcount) + 1; > >> + /* We allow PTE_FRAG_NR(16) fragments from a PTE page */ > >> + count = atomic_sub_return(16 - count, &page->_count); > > > > You should really move PTE_FRAG_NR to a header so you can actually use > > it here rather than hard coding 16. > > > > It took me a fair while to convince myself that there is no race here > > with something altering mapcount and count between the atomic_read() > > and the atomic_sub_return(). It could do with a comment to explain > > why that is safe. > > > > Re-using the mapcount field for your index also seems odd, and it took > > me a while to convince myself that that's safe too. Wouldn't it be > > simpler to store a pointer to the next sub-page in the mm_context > > instead? You can get from that to the struct page easily enough with a > > shift and pfn_to_page(). > > I found using _mapcount simpler in this case. I was looking at it not > as an index, but rather how may fragments are mapped/used already.
Except that it's actually (#fragments - 1). Using subpage pointer makes the fragments calculation (very slightly) harder, but the calculation of the table address easier. More importantly it avoids adding effectively an extra variable - which is then shoehorned into a structure not really designed to hold it. > Using > subpage pointer in mm->context.xyz means, we have to calculate the > number of fragments used/mapped via the pointer. We need the fragment > count so that we can drop page reference count correctly here. > > > > > >> + if (!count) { > >> + pgtable_page_dtor(page); > >> + reset_page_mapcount(page); > >> + free_hot_cold_page(page, 0); > > > > It would be nice to use put_page() somehow instead of duplicating its > > logic, though I realise the sparc code you've based this on does the > > same thing. > > That is not exactly put_page. We can avoid lots of check in this > specific case. [snip] > >> +static pte_t *__alloc_for_cache(struct mm_struct *mm, int kernel) > >> +{ > >> + pte_t *ret = NULL; > >> + struct page *page = alloc_page(GFP_KERNEL | __GFP_NOTRACK | > >> + __GFP_REPEAT | __GFP_ZERO); > >> + if (!page) > >> + return NULL; > >> + > >> + spin_lock(&mm->page_table_lock); > >> + /* > >> + * If we find pgtable_page set, we return > >> + * the allocated page with single fragement > >> + * count. > >> + */ > >> + if (likely(!mm->context.pgtable_page)) { > >> + atomic_set(&page->_count, PTE_FRAG_NR); > >> + atomic_set(&page->_mapcount, 0); > >> + mm->context.pgtable_page = page; > >> + } > > > > .. and in the unlikely case where there *is* a pgtable_page already > > set, what then? Seems like you should BUG_ON, or at least return NULL > > - as it is you will return the first sub-page of that page again, > > which is very likely in use. > > > As explained in the comment above, we return with the allocated page > with fragment count set to 1. So we end up having only one fragment. The > other option I had was to to free the allocated page and do a > get_from_cache under the page_table_lock. But since we already allocated > the page, why not use that ?. It also keep the code similar to > sparc. My point is that I can't see any circumstance under which we should ever hit this case. Which means if we do something is badly messed up and we should BUG() (or at least WARN()). -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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