Re: [PATCH net-next] net: ena: reuse the whole RX page on 64kB page kernels

[Breno Leitao <leitao@debian.org>]

On Fri, Nov 06, 2026 at 03:17:11AM +0000, Harry Yoo (Meta) wrote:
> Since commit ef5b0771d247 ("net: ena: fix driver when PAGE_SIZE ==
> 64kB"), the RX buffer given to the device is limited to ENA_PAGE_SIZE
> (16kB) because the buffer length field in the RX descriptor is 16 bits.
> However, the driver still allocates a full page for each RX buffer, so
> on a 64kB page kernel only the first 16kB of the page is handed to the
> device and the remaining 48kB is never used.
> 
> The dynamic RX buffer recycling added by commit f7d625adeb7b ("net: ena:
> Add dynamic recycling mechanism for rx buffers") does not fully utilize
> the page because ena_try_rx_buf_page_reuse() only reuses what is left
> in the current buffer and ignores unused memory in the page.
> 
> This shows up when RX data stays in socket receive queues, as each
> queued skb keeps the whole 64kB page alive while at most 16kB of it
> holds data. On an EC2 t4g.large instance (64kB pages, 8GB memory) with
> 519MB left unread across 64 TCP sockets, RX pages pinned 2869MB of
> memory while the socket layer accounted only 687MB.
> 
> Fix this by mapping the whole page and letting
> ena_try_rx_buf_page_reuse() size the next buffer from the room left in
> the page instead of from what is left of ena_buf->len. ena_buf->len is
> then only the length written to the RX descriptor and stays capped at
> ENA_PAGE_SIZE. With this fixed, a comparable workload pins only 497MB.
> 
> When PAGE_SIZE equals ENA_PAGE_SIZE, the room left in the page equals
> ena_buf->len, so nothing changes.
> 
> Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>

Nice finding, this seems like a bug, no? Which means 'net' tree and
a Fix: tag.

Reviewed-by: Breno Leitao <leitao@debian.org>