linux-kernel - Re: [PATCH v2 05/28] binder: fix unused alloc->free_async

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Date:   Mon,  4 Dec 2023 11:56:52 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     cmllamas@...gle.com
Cc:     arve@...roid.com, brauner@...nel.org, gregkh@...uxfoundation.org,
        joel@...lfernandes.org, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org, maco@...roid.com,
        sherryy@...roid.com, surenb@...gle.com, tkjos@...roid.com,
        Alice Ryhl <aliceryhl@...gle.com>
Subject: Re: [PATCH v2 05/28] binder: fix unused alloc->free_async_space

> 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@...gle.com>

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>