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

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Message-ID: <Zaqw9k4x7IUh6ys-@google.com>
Date: Fri, 19 Jan 2024 17:27:18 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: 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>, 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, Jan 19, 2024 at 06:49:00AM +0100, Greg Kroah-Hartman wrote:
> On Thu, Jan 18, 2024 at 07:33:48PM +0000, Carlos Llamas wrote:
> > 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.
> 
> 
> <formletter>
> 
> This is not the correct way to submit patches for inclusion in the
> stable kernel tree.  Please read:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly.
> 
> </formletter>

Oops, here is the complete info:

Commit ID: c6d05e0762ab276102246d24affd1e116a46aa0c
Subject:   "binder: fix unused alloc->free_async_space"
Reason:    Fixes an incorrect calculation of available space.
Versions:  v4.19+

Note this patch will also have trivial conflicts in v4.19 and v5.4
kernels as commit 261e7818f06e is missing there. Please let me know and
I can send the corresponding patches separately.

Thanks,
--
Carlos Llamas