[PATCH 0/3] tty: replace page allocator calls with k[mz]alloc()

[PATCH 0/3] tty: replace page allocator calls with k[mz]alloc()

[Mike Rapoport (Microsoft)]

This is a (small) part of larger work of replacing page allocator calls
with kmalloc.

My initial intention a few month ago was to remove ugly casts [1], but then
willy pointed out that Linus objected to something like this [2] and it
looks like more than a decade old technical debt.

Largely, anything that doesn't need struct page (or a memdesc in the
future) should just use kmalloc() or kvmalloc() to allocate memory.
kmalloc() guarantees alignment, physical contiguity and working
virt_to_phys() and beside nicer API that returns void * on alloc and
doesn't require to know the allocation size on free, kmalloc() provides
better debugging capabilities than page allocator.

Another thing is that touching these allocation sites gives the reviewers
opportunity to see if a PAGE_SIZE buffer is actually needed or maybe
another size is appropriate.

For larger allocations that don't need physically contiguous memory
kvmalloc() can be a better option that __get_free_pages() because under
memory pressure it's is easier to allocate several order-0 pages than a
physically contiguous chunk with the same number of pages.

And last, but not least, removing needless calls to page allocator should
help with memdesc (aka project folio) conversion. There will be way less
places to audit to see if the user was actually using struct page.

The patches are deliberately kept small: each one deals with a single
driver or subsystem and with a single type of allocation, so that every
conversion can be reviewed, and if needed reverted, on its own.

[1] https://lore.kernel.org/all/20251018093002.3660549-1-rppt@kernel.org/
[2] https://lore.kernel.org/all/CA+55aFwp4iy4rtX2gE2WjBGFL=NxMVnoFeHqYa2j1dYOMMGqxg@mail.gmail.com/

---
Mike Rapoport (Microsoft) (3):
      tty: port: replace get_zeroed_page() with kzalloc()
      serial: core: replace get_zeroed_page() with kzalloc()
      tty: hvcs: replace __get_free_page() with kmalloc()

 drivers/tty/hvc/hvcs.c           |  6 +++---
 drivers/tty/serial/serial_core.c | 16 ++++++++--------
 drivers/tty/tty_port.c           |  8 ++++----
 3 files changed, 15 insertions(+), 15 deletions(-)
---
base-commit: 1b78070aaef63512688aebfbc82365ef9d6660f1
change-id: 20260829-tty-84f05b74862a

--
Sincerely yours,
Mike.

[PATCH 1/3] tty: port: replace get_zeroed_page() with kzalloc()

[Mike Rapoport (Microsoft)]

tty_port_alloc_xmit_buf() allocates the transmit buffer of a tty port. The
buffer only backs the port's kfifo, the data being sent is copied in and
out of it.

This buffer can be allocated with kmalloc() as there's nothing special
about it to go directly to the page allocator.

kmalloc() provides a better API that does not require ugly casts and
kfree() does not need to know the size of the freed object.

Performance difference between kmalloc() and __get_free_pages() is not
measurable as both allocators take an object/page from a per-CPU list for
fast path allocations.

For the slow path the performance is anyway determined by the amount of
reclaim involved rather than by what allocator is used.

Replace use of get_zeroed_page() with kzalloc() and free_page() with
kfree().

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
Assisted-by: copilot:claude-opus
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 drivers/tty/tty_port.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 54359310e293..44122921fc45 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -240,13 +240,13 @@ EXPORT_SYMBOL_GPL(tty_port_unregister_device);
 
 int tty_port_alloc_xmit_buf(struct tty_port *port)
 {
-	/* We may sleep in get_zeroed_page() */
+	/* We may sleep in kzalloc() */
 	guard(mutex)(&port->buf_mutex);
 
 	if (port->xmit_buf)
 		return 0;
 
-	port->xmit_buf = (u8 *)get_zeroed_page(GFP_KERNEL);
+	port->xmit_buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
 	if (port->xmit_buf == NULL)
 		return -ENOMEM;
 
@@ -259,7 +259,7 @@ EXPORT_SYMBOL(tty_port_alloc_xmit_buf);
 void tty_port_free_xmit_buf(struct tty_port *port)
 {
 	guard(mutex)(&port->buf_mutex);
-	free_page((unsigned long)port->xmit_buf);
+	kfree(port->xmit_buf);
 	port->xmit_buf = NULL;
 	INIT_KFIFO(port->xmit_fifo);
 }
@@ -288,7 +288,7 @@ static void tty_port_destructor(struct kref *kref)
 	/* check if last port ref was dropped before tty release */
 	if (WARN_ON(port->itty))
 		return;
-	free_page((unsigned long)port->xmit_buf);
+	kfree(port->xmit_buf);
 	tty_port_destroy(port);
 	if (port->ops && port->ops->destruct)
 		port->ops->destruct(port);

-- 
2.53.0

[PATCH 2/3] serial: core: replace get_zeroed_page() with kzalloc()

[Mike Rapoport (Microsoft)]

uart_alloc_xmit_buf() allocates the transmit buffer of a serial port. The
buffer only backs the port's kfifo, the data being sent is copied in and
out of it.

This buffer can be allocated with kmalloc() as there's nothing special
about it to go directly to the page allocator.

kmalloc() provides a better API that does not require ugly casts and
kfree() does not need to know the size of the freed object.

Performance difference between kmalloc() and __get_free_pages() is not
measurable as both allocators take an object/page from a per-CPU list for
fast path allocations.

For the slow path the performance is anyway determined by the amount of
reclaim involved rather than by what allocator is used.

While on it, make the local variable holding the buffer a pointer to get
rid of the casts.

Replace use of get_zeroed_page() with kzalloc() and free_page() with
kfree().

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
Assisted-by: copilot:claude-opus
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 drivers/tty/serial/serial_core.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 95774b0f1484..f12ce7d190fe 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -247,29 +247,29 @@ static int uart_alloc_xmit_buf(struct tty_port *port)
 	struct uart_state *state = container_of(port, struct uart_state, port);
 	struct uart_port *uport;
 	unsigned long flags;
-	unsigned long page;
+	unsigned char *buf;
 
 	/*
 	 * Initialise and allocate the transmit and temporary
 	 * buffer.
 	 */
-	page = get_zeroed_page(GFP_KERNEL);
-	if (!page)
+	buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!buf)
 		return -ENOMEM;
 
 	uport = uart_port_ref_lock(state, &flags);
 	if (!state->port.xmit_buf) {
-		state->port.xmit_buf = (unsigned char *)page;
+		state->port.xmit_buf = buf;
 		kfifo_init(&state->port.xmit_fifo, state->port.xmit_buf,
 				PAGE_SIZE);
 		uart_port_unlock_deref(uport, flags);
 	} else {
 		uart_port_unlock_deref(uport, flags);
 		/*
-		 * Do not free() the page under the port lock, see
+		 * Do not free() the buffer under the port lock, see
 		 * uart_free_xmit_buf().
 		 */
-		free_page(page);
+		kfree(buf);
 	}
 
 	return 0;
@@ -283,7 +283,7 @@ static void uart_free_xmit_buf(struct tty_port *port)
 	char *xmit_buf;
 
 	/*
-	 * Do not free() the transmit buffer page under the port lock since
+	 * Do not free() the transmit buffer under the port lock since
 	 * this can create various circular locking scenarios. For instance,
 	 * console driver may need to allocate/free a debug object, which
 	 * can end up in printk() recursion.
@@ -294,7 +294,7 @@ static void uart_free_xmit_buf(struct tty_port *port)
 	INIT_KFIFO(port->xmit_fifo);
 	uart_port_unlock_deref(uport, flags);
 
-	free_page((unsigned long)xmit_buf);
+	kfree(xmit_buf);
 }
 
 /*

-- 
2.53.0

[PATCH 3/3] tty: hvcs: replace __get_free_page() with kmalloc()

[Mike Rapoport (Microsoft)]

hvcs_initialize() allocates the buffer that receives the partner info
returned by the H_VTERM_PARTNER_INFO hypercall. The buffer is passed to
the hypervisor as a physical address and it must not cross a page
boundary.

kmalloc() guarantees that a power of two sized allocation is aligned to
its size, so a PAGE_SIZE allocation is page aligned as well.

This buffer can be allocated with kmalloc() as there's nothing special
about it to go directly to the page allocator.

kmalloc() provides a better API that does not require ugly casts and
kfree() does not need to know the size of the freed object.

Performance difference between kmalloc() and __get_free_pages() is not
measurable as both allocators take an object/page from a per-CPU list for
fast path allocations.

For the slow path the performance is anyway determined by the amount of
reclaim involved rather than by what allocator is used.

Replace use of __get_free_page() with kmalloc() and free_page() with
kfree().

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
Assisted-by: copilot:claude-opus
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 drivers/tty/hvc/hvcs.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/hvc/hvcs.c b/drivers/tty/hvc/hvcs.c
index d32b1e3c50bf..b94d59fa3627 100644
--- a/drivers/tty/hvc/hvcs.c
+++ b/drivers/tty/hvc/hvcs.c
@@ -1470,7 +1470,7 @@ static int hvcs_initialize(void)
 		goto register_fail;
 	}
 
-	hvcs_pi_buff = (unsigned long *) __get_free_page(GFP_KERNEL);
+	hvcs_pi_buff = kmalloc(PAGE_SIZE, GFP_KERNEL);
 	if (!hvcs_pi_buff) {
 		rc = -ENOMEM;
 		goto buff_alloc_fail;
@@ -1486,7 +1486,7 @@ static int hvcs_initialize(void)
 	return 0;
 
 kthread_fail:
-	free_page((unsigned long)hvcs_pi_buff);
+	kfree(hvcs_pi_buff);
 buff_alloc_fail:
 	tty_unregister_driver(hvcs_tty_driver);
 register_fail:
@@ -1528,7 +1528,7 @@ static void __exit hvcs_module_exit(void)
 	kthread_stop(hvcs_task);
 
 	spin_lock(&hvcs_pi_lock);
-	free_page((unsigned long)hvcs_pi_buff);
+	kfree(hvcs_pi_buff);
 	hvcs_pi_buff = NULL;
 	spin_unlock(&hvcs_pi_lock);
 

-- 
2.53.0