linux-kernel - Re: [PATCH] NFS: check for allocation failure from mempool

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Date:   Mon, 2 Mar 2020 10:30:43 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NFS: check for allocation failure from mempool_alloc

On Wed, Feb 26, 2020 at 11:43:20PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> It is possible for mempool_alloc to return null when using
> the GFP_KERNEL flag, so return NULL and avoid a null pointer
> dereference on the following memset of the null pointer.
> 
> Addresses-Coverity: ("Dereference null return")
> Fixes: 2b17d725f9be ("NFS: Clean up writeback code")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  fs/nfs/write.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index c478b772cc49..7ca036660dd1 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -106,6 +106,9 @@ static struct nfs_pgio_header *nfs_writehdr_alloc(void)
>  {
>  	struct nfs_pgio_header *p = mempool_alloc(nfs_wdata_mempool, GFP_KERNEL);
>  
> +	if (!p)

The fixes tag was wrong.  When I searched for the correct fixes tag,
it turned out this was intentional.  See commit 237f8306c302
("NFS: don't expect errors from mempool_alloc().") and commit 518662e0fcb9
("NFS: fix usage of mempools.").

    When passed GFP flags that allow sleeping (such as
    GFP_NOIO), mempool_alloc() will never return NULL, it will
    wait until memory is available.

regards,
dan carpenter