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References: <20230411030248.53356-1-chengen.du@canonical.com>
In-Reply-To: <20230411030248.53356-1-chengen.du@canonical.com>
From:   Daire Byrne <daire@dneg.com>
Date:   Wed, 19 Apr 2023 08:17:32 +0100
Message-ID: <CAPt2mGNqqDeRMeCSVh6oX_=nS0UcGCfhBfVcuaVG9HpdwVSzVg@mail.gmail.com>
Subject: Re: [PATCH] NFS: Add mount option 'nofasc'
To:     Chengen Du <chengen.du@canonical.com>
Cc:     trond.myklebust@hammerspace.com, anna@kernel.org,
        linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org
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Just to say, I am personally for this or some other way to opt out of
the increased ACCESS calls on select servers (e.g. ones with high
latency or with lots of multi user logins).

I see it as similar to "actimeo" and "nocto" options in allowing users
to reduce "correctness" at their own risk if it helps with their
workloads and they deem the risk worth it.

I am currently reverting the original patches in our kernel for our
nfs re-exporting workloads.

Daire


On Tue, 11 Apr 2023 at 04:03, Chengen Du <chengen.du@canonical.com> wrote:
>
> This mount option is used to skip clearing the file access cache
> upon login. Some users or applications switch to other privileged
> users via commands such as 'su' to operate on NFS-mounted folders.
> In such cases, the privileged user's login time will be renewed,
> and NFS ACCESS operations will need to be re-sent, potentially
> leading to performance degradation.
> In some production environments where the access cache can be
> trusted due to stable group membership, this option could be
> particularly useful.
>
> Signed-off-by: Chengen Du <chengen.du@canonical.com>
> ---
>  fs/nfs/dir.c              | 21 ++++++++++++---------
>  fs/nfs/fs_context.c       |  8 ++++++++
>  fs/nfs/super.c            |  1 +
>  include/linux/nfs_fs_sb.h |  1 +
>  4 files changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 6fbcbb8d6587..9a6d86e2044e 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -2953,12 +2953,14 @@ static struct nfs_access_entry *nfs_access_search_rbtree(struct inode *inode, co
>         return NULL;
>  }
>
> -static u64 nfs_access_login_time(const struct task_struct *task,
> -                                const struct cred *cred)
> +static inline
> +bool nfs_check_access_stale(const struct task_struct *task,
> +                           const struct cred *cred,
> +                           const struct nfs_access_entry *cache)
>  {
>         const struct task_struct *parent;
>         const struct cred *pcred;
> -       u64 ret;
> +       u64 login_time;
>
>         rcu_read_lock();
>         for (;;) {
> @@ -2968,15 +2970,15 @@ static u64 nfs_access_login_time(const struct task_struct *task,
>                         break;
>                 task = parent;
>         }
> -       ret = task->start_time;
> +       login_time = task->start_time;
>         rcu_read_unlock();
> -       return ret;
> +
> +       return ((s64)(login_time - cache->timestamp) > 0);
>  }
>
>  static int nfs_access_get_cached_locked(struct inode *inode, const struct cred *cred, u32 *mask, bool may_block)
>  {
>         struct nfs_inode *nfsi = NFS_I(inode);
> -       u64 login_time = nfs_access_login_time(current, cred);
>         struct nfs_access_entry *cache;
>         bool retry = true;
>         int err;
> @@ -3005,7 +3007,8 @@ static int nfs_access_get_cached_locked(struct inode *inode, const struct cred *
>                 retry = false;
>         }
>         err = -ENOENT;
> -       if ((s64)(login_time - cache->timestamp) > 0)
> +       if (!(NFS_SERVER(inode)->flags & NFS_MOUNT_NOFASC) &&
> +           nfs_check_access_stale(current, cred, cache))
>                 goto out;
>         *mask = cache->mask;
>         list_move_tail(&cache->lru, &nfsi->access_cache_entry_lru);
> @@ -3025,7 +3028,6 @@ static int nfs_access_get_cached_rcu(struct inode *inode, const struct cred *cre
>          * but do it without locking.
>          */
>         struct nfs_inode *nfsi = NFS_I(inode);
> -       u64 login_time = nfs_access_login_time(current, cred);
>         struct nfs_access_entry *cache;
>         int err = -ECHILD;
>         struct list_head *lh;
> @@ -3040,7 +3042,8 @@ static int nfs_access_get_cached_rcu(struct inode *inode, const struct cred *cre
>                 cache = NULL;
>         if (cache == NULL)
>                 goto out;
> -       if ((s64)(login_time - cache->timestamp) > 0)
> +       if (!(NFS_SERVER(inode)->flags & NFS_MOUNT_NOFASC) &&
> +           nfs_check_access_stale(current, cred, cache))
>                 goto out;
>         if (nfs_check_cache_invalid(inode, NFS_INO_INVALID_ACCESS))
>                 goto out;
> diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
> index 9bcd53d5c7d4..5cd9b2882c79 100644
> --- a/fs/nfs/fs_context.c
> +++ b/fs/nfs/fs_context.c
> @@ -88,6 +88,7 @@ enum nfs_param {
>         Opt_vers,
>         Opt_wsize,
>         Opt_write,
> +       Opt_fasc,
>  };
>
>  enum {
> @@ -194,6 +195,7 @@ static const struct fs_parameter_spec nfs_fs_parameters[] = {
>         fsparam_string("vers",          Opt_vers),
>         fsparam_enum  ("write",         Opt_write, nfs_param_enums_write),
>         fsparam_u32   ("wsize",         Opt_wsize),
> +       fsparam_flag_no("fasc",         Opt_fasc),
>         {}
>  };
>
> @@ -861,6 +863,12 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
>         case Opt_sloppy:
>                 ctx->sloppy = true;
>                 break;
> +       case Opt_fasc:
> +               if (result.negated)
> +                       ctx->flags |= NFS_MOUNT_NOFASC;
> +               else
> +                       ctx->flags &= ~NFS_MOUNT_NOFASC;
> +               break;
>         }
>
>         return 0;
> diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> index 05ae23657527..059513d403f8 100644
> --- a/fs/nfs/super.c
> +++ b/fs/nfs/super.c
> @@ -444,6 +444,7 @@ static void nfs_show_mount_options(struct seq_file *m, struct nfs_server *nfss,
>                 { NFS_MOUNT_NORDIRPLUS, ",nordirplus", "" },
>                 { NFS_MOUNT_UNSHARED, ",nosharecache", "" },
>                 { NFS_MOUNT_NORESVPORT, ",noresvport", "" },
> +               { NFS_MOUNT_NOFASC, ",nofasc", "" },
>                 { 0, NULL, NULL }
>         };
>         const struct proc_nfs_info *nfs_infop;
> diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
> index ea2f7e6b1b0b..a39ae1bd2217 100644
> --- a/include/linux/nfs_fs_sb.h
> +++ b/include/linux/nfs_fs_sb.h
> @@ -153,6 +153,7 @@ struct nfs_server {
>  #define NFS_MOUNT_WRITE_EAGER          0x01000000
>  #define NFS_MOUNT_WRITE_WAIT           0x02000000
>  #define NFS_MOUNT_TRUNK_DISCOVERY      0x04000000
> +#define NFS_MOUNT_NOFASC               0x08000000
>
>         unsigned int            fattr_valid;    /* Valid attributes */
>         unsigned int            caps;           /* server capabilities */
> --
> 2.37.2
>