netdev - Re: [PATCH net] net/mlx5e: Do not recycle pages from emergency reserve

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Date:   Thu, 19 Jan 2017 11:25:49 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [PATCH net] net/mlx5e: Do not recycle pages from emergency
 reserve

On Thu, 2017-01-19 at 21:14 +0200, Saeed Mahameed wrote:
> On Thu, Jan 19, 2017 at 9:03 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > A driver using dev_alloc_page() must not reuse a page allocated from
> > emergency memory reserve.
> >
> > Otherwise all packets using this page will be immediately dropped,
> > unless for very specific sockets having SOCK_MEMALLOC bit set.
> >
> > This issue might be hard to debug, because only a fraction of received
> > packets would be dropped.
> 
> Hi Eric,
> 
> When you say reuse, you mean point to the same page from several SKBs ?
> 
> Because in our page cache implementation we don't reuse pages that
> already passed to the stack,
> we just keep them in the page cache until the ref count drop back to
> one, so we recycle them (i,e they will be re-used only when no one
> else is using them).

Look at other page_is_pfmemalloc() calls in other network drivers.

The point is :

We do not want to reuse a page that was initially allocated from
emergency reserve.

Chances are high that the packet will be simply dropped.

If your cache is not refilled with such problematic pages,
future dev_page_alloc() will likely give you sane pages, after stress
has ended.

By definition, emergency reserves are small, we do not want to keep
pages from the reserve in another cache, depleting the reserve for too
long.

dev_alloc_page() in mlx5 was introduced in commit
bc77b240b3c57236cdcc08d64ca390655d3a16ff
("net/mlx5e: Add fragmented memory support for RX multi packet WQE")

Maybe my Fixes: tag was slightly wrong.

> 
> on mlx5e rx re-post to the HW buffer we will call
> mlx5e_page_alloc_mapped->mlx5e_rx_cache_get, and  mlx5e_rx_cache_get
> will only return a page that is already freed by all stack users,
> otherwise it will allocated a brand new one.
> 
> a page form mlx5e_page_cache can have one of 2 states
> 1. passed to the stack (not free)
> 2. free (already used and freed by the stack)
> 
> a non free page will never get reused for another SKB.


Of course.

> 
> this is true for both mlx5 rx modes (striding and non-striding)
> 
> but in the striding mode regardless of page cache, small packets can
> always fit in one page and this page can get pointed to by several
> SKBs - should we be concerned about this case ?
> 
> Anyway, if you meant by this patch to avoid keeping such special pages
> in the mlx5 page cache,
> then it looks very good to me.
> 
> Thanks,
> Saeed.
> 
> 
> >
> > Fixes: 4415a0319f92 ("net/mlx5e: Implement RX mapped page cache for page recycle")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Tariq Toukan <tariqt@...lanox.com>
> > Cc: Saeed Mahameed <saeedm@...lanox.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlx5/core/en_rx.c |    3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > index 0e2fb3ed1790900ccdc0bbbef8bc5c33267df092..ce46409bde2736ea4d1246ca97855098f052f271 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > @@ -193,6 +193,9 @@ static inline bool mlx5e_rx_cache_put(struct mlx5e_rq *rq,
> >                 return false;
> >         }
> >
> > +       if (unlikely(page_is_pfmemalloc(dma_info->page)))
> > +               return false;
> > +
> >         cache->page_cache[cache->tail] = *dma_info;
> >         cache->tail = tail_next;
> >         return true;
> >
> >