Re: [PATCH v2 05/28] binder: fix unused alloc->free_async_space

[Alice Ryhl]

> 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@xxxxxxxxxx>

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>