Re: [PATCH v2 1/1] Drivers: hv: vmbus: Calculate ring buffer size for more efficient use of memory

[Wei Liu]

On Wed, Feb 28, 2024 at 04:45:33PM -0800, mhkelley58@xxxxxxxxx wrote:
> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> 
> The VMBUS_RING_SIZE macro adds space for a ring buffer header to the
> requested ring buffer size.  The header size is always 1 page, and so
> its size varies based on the PAGE_SIZE for which the kernel is built.
> If the requested ring buffer size is a large power-of-2 size and the header
> size is small, the resulting size is inefficient in its use of memory.
> For example, a 512 Kbyte ring buffer with a 4 Kbyte page size results in
> a 516 Kbyte allocation, which is rounded to up 1 Mbyte by the memory
> allocator, and wastes 508 Kbytes of memory.
> 
> In such situations, the exact size of the ring buffer isn't that important,
> and it's OK to allocate the 4 Kbyte header at the beginning of the 512
> Kbytes, leaving the ring buffer itself with just 508 Kbytes. The memory
> allocation can be 512 Kbytes instead of 1 Mbyte and nothing is wasted.
> 
> Update VMBUS_RING_SIZE to implement this approach for "large" ring buffer
> sizes.  "Large" is somewhat arbitrarily defined as 8 times the size of
> the ring buffer header (which is of size PAGE_SIZE).  For example, for
> 4 Kbyte PAGE_SIZE, ring buffers of 32 Kbytes and larger use the first
> 4 Kbytes as the ring buffer header.  For 64 Kbyte PAGE_SIZE, ring buffers
> of 512 Kbytes and larger use the first 64 Kbytes as the ring buffer
> header.  In both cases, smaller sizes add space for the header so
> the ring size isn't reduced too much by using part of the space for
> the header.  For example, with a 64 Kbyte page size, we don't want
> a 128 Kbyte ring buffer to be reduced to 64 Kbytes by allocating half
> of the space for the header.  In such a case, the memory allocation
> is less efficient, but it's the best that can be done.
> 
> While the new algorithm slightly changes the amount of space allocated
> for ring buffers by drivers that use VMBUS_RING_SIZE, the devices aren't
> known to be sensitive to small changes in ring buffer size, so there
> shouldn't be any effect.
> 
> Fixes: c1135c7fd0e9 ("Drivers: hv: vmbus: Introduce types of GPADL")
> Fixes: 6941f67ad37d ("hv_netvsc: Calculate correct ring size when PAGE_SIZE is not 4 Kbytes")
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218502
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Reviewed-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Tested-by: Souradeep Chakrabarti <schakrabarti@xxxxxxxxxxxxxxxxxxx>

Applied to hyperv-fixes, thanks!