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

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Message-ID: <Zal9HFZcC3rFjogI@google.com>
Date: Thu, 18 Jan 2024 19:33:48 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Christian Brauner <brauner@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Sherry Yang <sherryy@...roid.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 05/28] binder: fix unused alloc->free_async_space

On Fri, Dec 01, 2023 at 05:21:34PM +0000, Carlos Llamas wrote:
> 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>
> ---

Sorry, I forgot to Cc: stable@...r.kernel.org.

--
Carlos Llamas