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

[Carlos Llamas <cmllamas@google.com>]

On Mon, Jan 22, 2024 at 07:05:29AM -0800, Greg Kroah-Hartman wrote:
> On Mon, Jan 22, 2024 at 07:04:20AM -0800, Greg Kroah-Hartman wrote:
> > 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@google.com>
> > > > > > ---
> > > > > 
> > > > > Sorry, I forgot to Cc: stable@vger.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.
> > 
> > It doesn't even apply to 6.7.y either, so we need backports for all
> > affected trees, thanks.
> 
> Now I got it to apply, but we need backports for 5.4.y and 4.19.y,
> thanks.
> 
> greg k-h

Backports sent.

linux-4.19.y:
https://lore.kernel.org/all/20240122174250.2123854-2-cmllamas@google.com/

linux-5.4.y:
https://lore.kernel.org/all/20240122175751.2214176-2-cmllamas@google.com/

Thanks,
Carlos Llamas