netdev - Re: [net-next][PATCH v2 1/2] rds: handle unsupported rdma request to fs dax memory

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Date:   Fri, 10 May 2019 09:54:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Santosh Shilimkar <santosh.shilimkar@...cle.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [net-next][PATCH v2 1/2] rds: handle unsupported rdma request to
 fs dax memory

On Mon, Apr 29, 2019 at 04:37:19PM -0700, Santosh Shilimkar wrote:
> From: Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
> 
> RDS doesn't support RDMA on memory apertures that require On Demand
> Paging (ODP), such as FS DAX memory. User applications can try to use
> RDS to perform RDMA over such memories and since it doesn't report any
> failure, it can lead to unexpected issues like memory corruption when
> a couple of out of sync file system operations like ftruncate etc. are
> performed.

This comment doesn't make any sense..

> The patch adds a check so that such an attempt to RDMA to/from memory
> apertures requiring ODP will fail.
> 
> Reviewed-by: Håkon Bugge <haakon.bugge@...cle.com>
> Reviewed-tested-by: Zhu Yanjun <yanjun.zhu@...cle.com>
> Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...cle.com>
>  net/rds/rdma.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/rds/rdma.c b/net/rds/rdma.c
> index 182ab84..e0a6b72 100644
> +++ b/net/rds/rdma.c
> @@ -158,8 +158,9 @@ static int rds_pin_pages(unsigned long user_addr, unsigned int nr_pages,
>  {
>  	int ret;
>  
> -	ret = get_user_pages_fast(user_addr, nr_pages, write, pages);
> -
> +	/* get_user_pages return -EOPNOTSUPP for fs_dax memory */
> +	ret = get_user_pages_longterm(user_addr, nr_pages,
> +				      write, pages, NULL);

GUP is supposed to fully work on DAX filesystems.

You only need to switch to the long term version if the duration of
the GUP is under control of user space - ie it may last forever.

Short duration pins in the kernel do not need long term. 

At a minimum the commit message needs re-writing to properly explain
the motivation here.

Jason