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SubjectRe: [PATCH] nfsd: Fix overflow causing non-working mounts on 1 TB machines
Good catch!  And thanks for the detailed explanation.  Applying for 5.2
and stable.

On Wed, Jul 03, 2019 at 02:54:43PM +0200, Paul Menzel wrote:
> Date: Wed, 3 Jul 2019 13:28:15 +0200
>
> Since commit 10a68cdf10 (nfsd: fix performance-limiting session
> calculation) (Linux 5.1-rc1 and 4.19.31), shares from NFS servers with
> 1 TB of memory cannot be mounted anymore. The mount just hangs on the
> client.
>
> The gist of commit 10a68cdf10 is the change below.
>
> -avail = clamp_t(int, avail, slotsize, avail/3);
> +avail = clamp_t(int, avail, slotsize, total_avail/3);
>
> Here are the macros.
>
> #define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
> #define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
>
> `total_avail` is 8,434,659,328 on the 1 TB machine. `clamp_t()` casts
> the values to `int`, which for 32-bit integers can only hold values
> −2,147,483,648 (−2^31) through 2,147,483,647 (2^31 − 1).
>
> `avail` (in the function signature) is just 65536, so that no overflow
> was happening. Before the commit the assignment would result in 21845,
> and `num = 4`.
>
> When using `total_avail`, it is causing the assignment to be
> 18446744072226137429 (printed as %lu), and `num` is then 4164608182.
>
> My next guess is, that `nfsd_drc_mem_used` is then exceeded, and the
> server thinks there is no memory available any more for this client.
>
> Updating the arguments of `clamp_t()` and `min_t()` to `unsigned long`
> fixes the issue.
>
> Now, `avail = 65536` (before commit 10a68cdf10 `avail = 21845`), but
> `num = 4` remains the same.
>
> Fixes: 10a68cdf10 (nfsd: fix performance-limiting session calculation)
> Cc: stable@vger.kernel.org
> Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
> ---
>
> 1. No, idea if `min_t()` arguments also need updating.
> 2. Instead of `unsigned long`, should `size_t` be used?
>
> fs/nfsd/nfs4state.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 618e66078ee5..1a0cdeb3b875 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1563,7 +1563,7 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
> * Never use more than a third of the remaining memory,
> * unless it's the only way to give this client a slot:
> */
> - avail = clamp_t(int, avail, slotsize, total_avail/3);
> + avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
> num = min_t(int, num, avail / slotsize);
> nfsd_drc_mem_used += num * slotsize;
> spin_unlock(&nfsd_drc_lock);
> --
> 2.22.0
>


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