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

[Harry Yoo <harry@kernel.org>]

On Fri, Nov 06, 2026 at 02:09:34AM -0800, Breno Leitao wrote:
> 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,

Thanks for reviewing, Breno!

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

Good point, I think Fixes: f7d625adeb7b (6.6.y) would be fairly
reasonable as this is the first commit that reuses the buffer,
it is quite huge unaccounted memory waste on kernels with 64kB pages,
and the change is quite minimal (applies cleanly up to 6.6.y).

I'll respin it targeting net tree with Fixes: tag after waiting for
a few days.

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

Thanks!

-- 
Cheers,
Harry / Hyeonggon