4.19-stable review patch. If anyone has any objections, please let me know.
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From: Carlos Llamas cmllamas@google.com
commit c6d05e0762ab276102246d24affd1e116a46aa0c upstream.
Each transaction is associated with a 'struct binder_buffer' that stores the metadata about its buffer area. Since commit 74310e06be4d ("android: binder: Move buffer out of area shared with user space") this struct is no longer embedded within the buffer itself but is instead allocated on the heap to prevent userspace access to this driver-exclusive info.
Unfortunately, the space of this struct is still being accounted for in the total buffer size calculation, specifically for async transactions. This results in an additional 104 bytes added to every async buffer request, and this area is never used.
This wasted space can be substantial. If we consider the maximum mmap buffer space of SZ_4M, the driver will reserve half of it for async transactions, or 0x200000. This area should, in theory, accommodate up to 262,144 buffers of the minimum 8-byte size. However, after adding the extra 'sizeof(struct binder_buffer)', the total number of buffers drops to only 18,724, which is a sad 7.14% of the actual capacity.
This patch fixes the buffer size calculation to enable the utilization of the entire async buffer space. This is expected to reduce the number of -ENOSPC errors that are seen on the field.
Fixes: 74310e06be4d ("android: binder: Move buffer out of area shared with user space") Signed-off-by: Carlos Llamas cmllamas@google.com Reviewed-by: Alice Ryhl aliceryhl@google.com Link: https://lore.kernel.org/r/20231201172212.1813387-6-cmllamas@google.com [cmllamas: fix trivial conflict with missing 261e7818f06e.] Signed-off-by: Carlos Llamas cmllamas@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/android/binder_alloc.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-)
--- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -402,8 +402,7 @@ static struct binder_buffer *binder_allo /* Pad 0-size buffers so they get assigned unique addresses */ size = max(size, sizeof(void *));
- if (is_async && - alloc->free_async_space < size + sizeof(struct binder_buffer)) { + if (is_async && alloc->free_async_space < size) { binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC, "%d: binder_alloc_buf size %zd failed, no async space left\n", alloc->pid, size); @@ -509,7 +508,7 @@ static struct binder_buffer *binder_allo buffer->async_transaction = is_async; buffer->extra_buffers_size = extra_buffers_size; if (is_async) { - alloc->free_async_space -= size + sizeof(struct binder_buffer); + alloc->free_async_space -= size; binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC_ASYNC, "%d: binder_alloc_buf size %zd async free %zd\n", alloc->pid, size, alloc->free_async_space); @@ -631,8 +630,7 @@ static void binder_free_buf_locked(struc BUG_ON(buffer->data > alloc->buffer + alloc->buffer_size);
if (buffer->async_transaction) { - alloc->free_async_space += buffer_size + sizeof(struct binder_buffer); - + alloc->free_async_space += buffer_size; binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC_ASYNC, "%d: binder_free_buf size %zd async free %zd\n", alloc->pid, size, alloc->free_async_space);