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

[Greg Kroah-Hartman]

On Fri, Jan 19, 2024 at 05:27:18PM +0000, Carlos Llamas wrote:
> 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@xxxxxxxxxx>
> > > > ---
> > > 
> > > Sorry, I forgot to Cc: stable@xxxxxxxxxxxxxxx.
> > 
> > 
> > <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.

It doesn't even apply to 6.7.y either, so we need backports for all
affected trees, thanks.

greg k-h