• Paul Menzel's avatar
    nfsd: Fix overflow causing non-working mounts on 1 TB machines · d4d4b66e
    Paul Menzel authored
    BugLink: https://bugs.launchpad.net/bugs/1840081
    
    [ Upstream commit 3b2d4dcf ]
    
    Since commit 10a68cdf (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 10a68cdf 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 10a68cdf `avail = 21845`), but
    `num = 4` remains the same.
    
    Fixes: c54f24e3 (nfsd: fix performance-limiting session calculation)
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarPaul Menzel <pmenzel@molgen.mpg.de>
    Signed-off-by: default avatarJ. Bruce Fields <bfields@redhat.com>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
    Signed-off-by: default avatarConnor Kuehl <connor.kuehl@canonical.com>
    Signed-off-by: default avatarKleber Sacilotto de Souza <kleber.souza@canonical.com>
    d4d4b66e
nfs4state.c 177 KB