On Wed, Nov 05, 2014 at 04:49:41PM +0200, Kirill A. Shutemov wrote:
> We're going to allow mapping of individual 4k pages of THP compound and
> we need a cheap way to find out how many time the compound page is
> mapped with PMD -- compound_mapcount() does this.
>
> page_mapcount() counts both: PTE and PMD mappings of the page.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
...
> @@ -1837,6 +1839,9 @@ static void __split_huge_page_refcount(struct page *page,
> atomic_sub(tail_count, &page->_count);
> BUG_ON(atomic_read(&page->_count) <= 0);
>
> + page->_mapcount = *compound_mapcount_ptr(page);
Is atomic_set() necessary?
> + page[1].mapping = page->mapping;
> +
> __mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES, -1);
>
> ClearPageCompound(page);
...
> @@ -760,6 +763,8 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
> bad += free_pages_check(page + i);
> if (bad)
> return false;
> + if (order)
> + page[1].mapping = NULL;
>
> if (!PageHighMem(page)) {
> debug_check_no_locks_freed(page_address(page),
> @@ -6632,10 +6637,12 @@ static void dump_page_flags(unsigned long flags)
> void dump_page_badflags(struct page *page, const char *reason,
> unsigned long badflags)
> {
> - printk(KERN_ALERT
> - "page:%p count:%d mapcount:%d mapping:%p index:%#lx\n",
> + pr_alert("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
> page, atomic_read(&page->_count), page_mapcount(page),
> page->mapping, page->index);
> + if (PageCompound(page))
> + printk(" compound_mapcount: %d", compound_mapcount(page));
> + printk("\n");
These two printk() should be pr_alert(), too?
> dump_page_flags(page->flags);
> if (reason)
> pr_alert("page dumped because: %s\n", reason);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f706a6af1801..eecc9301847d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -986,9 +986,30 @@ void page_add_anon_rmap(struct page *page,
> void do_page_add_anon_rmap(struct page *page,
> struct vm_area_struct *vma, unsigned long address, int flags)
> {
> - int first = atomic_inc_and_test(&page->_mapcount);
> + bool compound = flags & RMAP_COMPOUND;
> + bool first;
> +
> + VM_BUG_ON_PAGE(!PageLocked(compound_head(page)), page);
> +
> + if (PageTransCompound(page)) {
> + struct page *head_page = compound_head(page);
> +
> + if (compound) {
> + VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> + first = atomic_inc_and_test(compound_mapcount_ptr(page));
Is compound_mapcount_ptr() well-defined for tail pages?
This function seems to access struct page of the page next to a given page,
so if the given page is the last tail page of a thp, page outside the thp
will be accessed. Do you have a prevention from this?
atomic_inc_and_test(compound_mapcount_ptr(head_page)) is what you intended?
> + } else {
> + /* Anon THP always mapped first with PMD */
> + first = 0;
> + VM_BUG_ON_PAGE(!compound_mapcount(head_page),
> + head_page);
> + atomic_inc(&page->_mapcount);
> + }
> + } else {
> + VM_BUG_ON_PAGE(compound, page);
> + first = atomic_inc_and_test(&page->_mapcount);
> + }
> +
> if (first) {
> - bool compound = flags & RMAP_COMPOUND;
> int nr = compound ? hpage_nr_pages(page) : 1;
> /*
> * We use the irq-unsafe __{inc|mod}_zone_page_stat because
...
> @@ -1032,10 +1052,19 @@ void page_add_new_anon_rmap(struct page *page,
>
> VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
> SetPageSwapBacked(page);
> - atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
> if (compound) {
> + atomic_t *compound_mapcount;
> +
> VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> + compound_mapcount = (atomic_t *)&page[1].mapping;
You can use compound_mapcount_ptr() here.
Thanks,
Naoya Horiguchi
> + /* increment count (starts at -1) */
> + atomic_set(compound_mapcount, 0);
> __inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> + } else {
> + /* Anon THP always mapped first with PMD */
> + VM_BUG_ON_PAGE(PageTransCompound(page), page);
> + /* increment count (starts at -1) */
> + atomic_set(&page->_mapcount, 0);
> }
> __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
> __page_set_anon_rmap(page, vma, address, 1);
--
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