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SubjectRe: [PATCHv5 14/28] futex, thp: remove special case for THP in get_futex_key
On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> With new THP refcounting, we don't need tricks to stabilize huge page.
> If we've got reference to tail page, it can't split under us.
>
> This patch effectively reverts a5b338f2b0b1.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>
> ---
> kernel/futex.c | 61 ++++++++++++----------------------------------------------
> 1 file changed, 12 insertions(+), 49 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index f4d8a85641ed..cf0192e60ef9 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -399,7 +399,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
> {
> unsigned long address = (unsigned long)uaddr;
> struct mm_struct *mm = current->mm;
> - struct page *page, *page_head;
> + struct page *page;
> int err, ro = 0;
>
> /*
> @@ -442,46 +442,9 @@ again:
> else
> err = 0;
>
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> - page_head = page;
> - if (unlikely(PageTail(page))) {
> - put_page(page);
> - /* serialize against __split_huge_page_splitting() */
> - local_irq_disable();
> - if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
> - page_head = compound_head(page);
> - /*
> - * page_head is valid pointer but we must pin
> - * it before taking the PG_lock and/or
> - * PG_compound_lock. The moment we re-enable
> - * irqs __split_huge_page_splitting() can
> - * return and the head page can be freed from
> - * under us. We can't take the PG_lock and/or
> - * PG_compound_lock on a page that could be
> - * freed from under us.
> - */
> - if (page != page_head) {
> - get_page(page_head);
> - put_page(page);
> - }
> - local_irq_enable();
> - } else {
> - local_irq_enable();
> - goto again;
> - }
> - }
> -#else
> - page_head = compound_head(page);
> - if (page != page_head) {
> - get_page(page_head);
> - put_page(page);
> - }

Hmm, any idea why this was there? Without THP, it was already sure that
get/put_page() on tail page operates on the head page's _count, no?

> -#endif
> -
> - lock_page(page_head);
> -
> + lock_page(page);
> /*
> - * If page_head->mapping is NULL, then it cannot be a PageAnon
> + * If page->mapping is NULL, then it cannot be a PageAnon
> * page; but it might be the ZERO_PAGE or in the gate area or
> * in a special mapping (all cases which we are happy to fail);
> * or it may have been a good file page when get_user_pages_fast
> @@ -493,12 +456,12 @@ again:
> *
> * The case we do have to guard against is when memory pressure made
> * shmem_writepage move it from filecache to swapcache beneath us:
> - * an unlikely race, but we do need to retry for page_head->mapping.
> + * an unlikely race, but we do need to retry for page->mapping.
> */
> - if (!page_head->mapping) {
> - int shmem_swizzled = PageSwapCache(page_head);
> - unlock_page(page_head);
> - put_page(page_head);
> + if (!page->mapping) {
> + int shmem_swizzled = PageSwapCache(page);
> + unlock_page(page);
> + put_page(page);
> if (shmem_swizzled)
> goto again;
> return -EFAULT;
> @@ -511,7 +474,7 @@ again:
> * it's a read-only handle, it's expected that futexes attach to
> * the object not the particular process.
> */
> - if (PageAnon(page_head)) {
> + if (PageAnon(page)) {
> /*
> * A RO anonymous page will never change and thus doesn't make
> * sense for futex operations.
> @@ -526,15 +489,15 @@ again:
> key->private.address = address;
> } else {
> key->both.offset |= FUT_OFF_INODE; /* inode-based key */
> - key->shared.inode = page_head->mapping->host;
> + key->shared.inode = page->mapping->host;
> key->shared.pgoff = basepage_index(page);
> }
>
> get_futex_key_refs(key); /* implies MB (B) */
>
> out:
> - unlock_page(page_head);
> - put_page(page_head);
> + unlock_page(page);
> + put_page(page);
> return err;
> }
>
>



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