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Date:   Tue, 12 Sep 2023 21:28:33 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     tglx@linutronix.de, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        =?utf-8?B?QW5kcsOvwr/CvQ==?= Almeida <andrealmeid@igalia.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] futex: Use a folio instead of a page
Message-ID: <nkjcgasa4nq2zh6y2iim5r6qrhn2opwldaok5smf65kmfqu2un@zvazsmqrhjiu>
References: <20230821142207.2537124-1-willy@infradead.org>
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+Cc tglx.

On Mon, 21 Aug 2023, Matthew Wilcox (Oracle) wrote:

>The futex code already handles compound pages correctly, but using a
>folio lets us tell the compiler that we already have the head page and
>it doesn't need to call compound_head() again.
>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

>Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>---
> kernel/futex/core.c | 67 ++++++++++++++++++++++-----------------------
> 1 file changed, 33 insertions(+), 34 deletions(-)
>
>diff --git a/kernel/futex/core.c b/kernel/futex/core.c
>index f10587d1d481..d1d7b3c175a4 100644
>--- a/kernel/futex/core.c
>+++ b/kernel/futex/core.c
>@@ -222,7 +222,8 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
> {
>	unsigned long address = (unsigned long)uaddr;
>	struct mm_struct *mm = current->mm;
>-	struct page *page, *tail;
>+	struct page *page;
>+	struct folio *folio;
>	struct address_space *mapping;
>	int err, ro = 0;
>
>@@ -273,54 +274,52 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
>		err = 0;
>
>	/*
>-	 * The treatment of mapping from this point on is critical. The page
>-	 * lock protects many things but in this context the page lock
>+	 * The treatment of mapping from this point on is critical. The folio
>+	 * lock protects many things but in this context the folio lock
>	 * stabilizes mapping, prevents inode freeing in the shared
>	 * file-backed region case and guards against movement to swap cache.
>	 *
>-	 * Strictly speaking the page lock is not needed in all cases being
>-	 * considered here and page lock forces unnecessarily serialization
>+	 * Strictly speaking the folio lock is not needed in all cases being
>+	 * considered here and folio lock forces unnecessarily serialization.
>	 * From this point on, mapping will be re-verified if necessary and
>-	 * page lock will be acquired only if it is unavoidable
>+	 * folio lock will be acquired only if it is unavoidable
>	 *
>-	 * Mapping checks require the head page for any compound page so the
>-	 * head page and mapping is looked up now. For anonymous pages, it
>-	 * does not matter if the page splits in the future as the key is
>-	 * based on the address. For filesystem-backed pages, the tail is
>-	 * required as the index of the page determines the key. For
>-	 * base pages, there is no tail page and tail == page.
>+	 * Mapping checks require the folio so it is looked up now. For
>+	 * anonymous pages, it does not matter if the folio is split
>+	 * in the future as the key is based on the address. For
>+	 * filesystem-backed pages, the precise page is required as the
>+	 * index of the page determines the key.
>	 */
>-	tail = page;
>-	page = compound_head(page);
>-	mapping = READ_ONCE(page->mapping);
>+	folio = page_folio(page);
>+	mapping = READ_ONCE(folio->mapping);
>
>	/*
>-	 * If page->mapping is NULL, then it cannot be a PageAnon
>+	 * If folio->mapping is NULL, then it cannot be an anonymous
>	 * 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
>	 * found it, but truncated or holepunched or subjected to
>-	 * invalidate_complete_page2 before we got the page lock (also
>+	 * invalidate_complete_page2 before we got the folio lock (also
>	 * cases which we are happy to fail).  And we hold a reference,
>	 * so refcount care in invalidate_inode_page's remove_mapping
>	 * prevents drop_caches from setting mapping to NULL beneath us.
>	 *
>	 * 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->mapping.
>+	 * an unlikely race, but we do need to retry for folio->mapping.
>	 */
>	if (unlikely(!mapping)) {
>		int shmem_swizzled;
>
>		/*
>-		 * Page lock is required to identify which special case above
>-		 * applies. If this is really a shmem page then the page lock
>+		 * Folio lock is required to identify which special case above
>+		 * applies. If this is really a shmem page then the folio lock
>		 * will prevent unexpected transitions.
>		 */
>-		lock_page(page);
>-		shmem_swizzled = PageSwapCache(page) || page->mapping;
>-		unlock_page(page);
>-		put_page(page);
>+		folio_lock(folio);
>+		shmem_swizzled = folio_test_swapcache(folio) || folio->mapping;
>+		folio_unlock(folio);
>+		folio_put(folio);
>
>		if (shmem_swizzled)
>			goto again;
>@@ -331,14 +330,14 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
>	/*
>	 * Private mappings are handled in a simple way.
>	 *
>-	 * If the futex key is stored on an anonymous page, then the associated
>+	 * If the futex key is stored in anonymous memory, then the associated
>	 * object is the mm which is implicitly pinned by the calling process.
>	 *
>	 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
>	 * it's a read-only handle, it's expected that futexes attach to
>	 * the object not the particular process.
>	 */
>-	if (PageAnon(page)) {
>+	if (folio_test_anon(folio)) {
>		/*
>		 * A RO anonymous page will never change and thus doesn't make
>		 * sense for futex operations.
>@@ -357,10 +356,10 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
>
>		/*
>		 * The associated futex object in this case is the inode and
>-		 * the page->mapping must be traversed. Ordinarily this should
>-		 * be stabilised under page lock but it's not strictly
>+		 * the folio->mapping must be traversed. Ordinarily this should
>+		 * be stabilised under folio lock but it's not strictly
>		 * necessary in this case as we just want to pin the inode, not
>-		 * update the radix tree or anything like that.
>+		 * update i_pages or anything like that.
>		 *
>		 * The RCU read lock is taken as the inode is finally freed
>		 * under RCU. If the mapping still matches expectations then the
>@@ -368,9 +367,9 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
>		 */
>		rcu_read_lock();
>
>-		if (READ_ONCE(page->mapping) != mapping) {
>+		if (READ_ONCE(folio->mapping) != mapping) {
>			rcu_read_unlock();
>-			put_page(page);
>+			folio_put(folio);
>
>			goto again;
>		}
>@@ -378,19 +377,19 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
>		inode = READ_ONCE(mapping->host);
>		if (!inode) {
>			rcu_read_unlock();
>-			put_page(page);
>+			folio_put(folio);
>
>			goto again;
>		}
>
>		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
>		key->shared.i_seq = get_inode_sequence_number(inode);
>-		key->shared.pgoff = page_to_pgoff(tail);
>+		key->shared.pgoff = folio->index + folio_page_idx(folio, page);
>		rcu_read_unlock();
>	}
>
> out:
>-	put_page(page);
>+	folio_put(folio);
>	return err;
> }
>
>--
>2.40.1
>