Re: [PATCH] nfsd: fix incorrect high limit in clamp() on over-allocation

Chuck Lever
2024-12-09T14:47:37Z

On 12/9/24 7:25 AM, Vincent Mailhol via B4 Relay wrote:
> From: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
>
> If over allocation occurs in nfsd4_get_drc_mem(), total_avail is set
> to zero. Consequently,
>
> clamp_t(unsigned long, avail, slotsize, total_avail/scale_factor);
>
> gives:
>
> clamp_t(unsigned long, avail, slotsize, 0);
>
> resulting in a clamp() call where the high limit is smaller than the
> low limit, which is undefined: the result could be either slotsize or
> zero depending on the order of evaluation.
>
> Luckily, the two instructions just below the clamp() recover the
> undefined behaviour:
>
> num = min_t(int, num, avail / slotsize);
> num = max_t(int, num, 1);
>
> If avail = slotsize, the min_t() sets it back to 1. If avail = 0, the
> max_t() sets it back to 1.
>
> So this undefined behaviour has no visible effect.
>
> Anyway, remove the undefined behaviour in clamp() by only calling it
> and only doing the calculation of num if memory is still available.
> Otherwise, if over-allocation occurred, directly set num to 1 as
> intended by the author.
>
> While at it, apply below checkpatch fix:
>
> WARNING: min() should probably be min_t(unsigned long, NFSD_MAX_MEM_PER_SESSION, total_avail)
> #100: FILE: fs/nfsd/nfs4state.c:1954:
> + avail = min((unsigned long)NFSD_MAX_MEM_PER_SESSION, total_avail);
>
> Fixes: 7f49fd5d7acd ("nfsd: handle drc over-allocation gracefully.")
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> ---
> Found by applying below patch from David:
>
> https://lore.kernel.org/all/34d53778977747f19cce2abb287bb3e6@AcuMS.aculab.com/
>
> Doing so yield this report:
>
> In function ‘nfsd4_get_drc_mem’,
> inlined from ‘check_forechannel_attrs’ at fs/nfsd/nfs4state.c:3791:16,
> inlined from ‘nfsd4_create_session’ at fs/nfsd/nfs4state.c:3864:11:
> ././include/linux/compiler_types.h:542:38: error: call to ‘__compiletime_assert_3707’ declared with attribute error: clamp() low limit (unsigned long)(slotsize) greater than high limit (unsigned long)(total_avail/scale_factor)
> 542 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^
> ././include/linux/compiler_types.h:523:4: note: in definition of macro ‘__compiletime_assert’
> 523 | prefix ## suffix(); \
> | ^~~~~~
> ././include/linux/compiler_types.h:542:2: note: in expansion of macro ‘_compiletime_assert’
> 542 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> | ^~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> | ^~~~~~~~~~~~~~~~~~
> ./include/linux/minmax.h:114:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
> 114 | BUILD_BUG_ON_MSG(statically_true(ulo > uhi), \
> | ^~~~~~~~~~~~~~~~
> ./include/linux/minmax.h:121:2: note: in expansion of macro ‘__clamp_once’
> 121 | __clamp_once(val, lo, hi, __UNIQUE_ID(v_), __UNIQUE_ID(l_), __UNIQUE_ID(h_))
> | ^~~~~~~~~~~~
> ./include/linux/minmax.h:275:36: note: in expansion of macro ‘__careful_clamp’
> 275 | #define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))
> | ^~~~~~~~~~~~~~~
> fs/nfsd/nfs4state.c:1972:10: note: in expansion of macro ‘clamp_t’
> 1972 | avail = clamp_t(unsigned long, avail, slotsize,
> | ^~~~~~~
>
> Because David's patch is targetting Andrew's mm tree, I would suggest
> that my patch also goes to that tree.
> ---
> fs/nfsd/nfs4state.c | 46 +++++++++++++++++++++++++---------------------
> 1 file changed, 25 insertions(+), 21 deletions(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 741b9449f727defc794347f1b116c955d715e691..eb91460c434e30f6df70f66d937f8c0f334b8e1b 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1944,35 +1944,39 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca, struct nfsd_net *nn
> {
> u32 slotsize = slot_bytes(ca);
> u32 num = ca->maxreqs;
> - unsigned long avail, total_avail;
> - unsigned int scale_factor;
>
> spin_lock(&nfsd_drc_lock);
> - if (nfsd_drc_max_mem > nfsd_drc_mem_used)
> + if (nfsd_drc_max_mem > nfsd_drc_mem_used) {
> + unsigned long avail, total_avail;
> + unsigned int scale_factor;
> +
> total_avail = nfsd_drc_max_mem - nfsd_drc_mem_used;
> - else
> + avail = min_t(unsigned long,
> + NFSD_MAX_MEM_PER_SESSION, total_avail);
> + /*
> + * Never use more than a fraction of the remaining memory,
> + * unless it's the only way to give this client a slot.
> + * The chosen fraction is either 1/8 or 1/number of threads,
> + * whichever is smaller. This ensures there are adequate
> + * slots to support multiple clients per thread.
> + * Give the client one slot even if that would require
> + * over-allocation--it is better than failure.
> + */
> + scale_factor = max_t(unsigned int,
> + 8, nn->nfsd_serv->sv_nrthreads);
> +
> + avail = clamp_t(unsigned long, avail, slotsize,
> + total_avail/scale_factor);
> + num = min_t(int, num, avail / slotsize);
> + num = max_t(int, num, 1);
> + } else {
> /* We have handed out more space than we chose in
> * set_max_drc() to allow. That isn't really a
> * problem as long as that doesn't make us think we
> * have lots more due to integer overflow.
> */
> - total_avail = 0;
> - avail = min((unsigned long)NFSD_MAX_MEM_PER_SESSION, total_avail);
> - /*
> - * Never use more than a fraction of the remaining memory,
> - * unless it's the only way to give this client a slot.
> - * The chosen fraction is either 1/8 or 1/number of threads,
> - * whichever is smaller. This ensures there are adequate
> - * slots to support multiple clients per thread.
> - * Give the client one slot even if that would require
> - * over-allocation--it is better than failure.
> - */
> - scale_factor = max_t(unsigned int, 8, nn->nfsd_serv->sv_nrthreads);
> -
> - avail = clamp_t(unsigned long, avail, slotsize,
> - total_avail/scale_factor);
> - num = min_t(int, num, avail / slotsize);
> - num = max_t(int, num, 1);
> + num = 1;
> + }
> nfsd_drc_mem_used += num * slotsize;
> spin_unlock(&nfsd_drc_lock);
>
>
> ---
> base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4
> change-id: 20241209-nfs4state_fix-bc6f1c1fc1d1
>
> Best regards,

Hi Vincent -

We're replacing this code wholesale in 6.14. See:

https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=nfsd-testing&id=8233f78fbd970cbfcb9f78c719ac5a3aac4ea053

--
Chuck Lever