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Message-ID: <91cf0982fcf9470d94c3d5c149cfd2bd@AcuMS.aculab.com>
Date: Mon, 9 Dec 2024 14:24:13 +0000
From: David Laight <David.Laight@...LAB.COM>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>, NeilBrown <neilb@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>, "J. Bruce Fields"
<bfields@...ldses.org>
CC: Chuck Lever <chuck.lever@...cle.com>, Jeff Layton <jlayton@...nel.org>,
Olga Kornievskaia <okorniev@...hat.com>, Dai Ngo <Dai.Ngo@...cle.com>, "Tom
Talpey" <tom@...pey.com>, "linux-nfs@...r.kernel.org"
<linux-nfs@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] nfsd: fix incorrect high limit in clamp() on
over-allocation
From: Vincent Mailhol
> Sent: 09 December 2024 12:26
>
> 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.
NAK:
The code is still wrong
> 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);
That was never a bug and checkpatch should never report it!
Casting one argument to min() has always been safer than using min_t().
Indeed it should really have been the preferred solition.
Consider what happens with min_t() if 'total_avail' happens to be 64bit
(with long being 32bit) - suddenly significant bit get masked off.
With the 'new improved' min() just delete the cast - it won't complain.
>
> Fixes: 7f49fd5d7acd ("nfsd: handle drc over-allocation gracefully.")
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
...
> 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;
You've only checked > the result can still be 1.
> - 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);
Shouldn't need to be max_t(), max() looks to be fine.
But can we please have the constants on the right?
> + 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;
> + }
I'd leave the logic alone and use explicit min() and max) instead of clamp().
(and hopefully checkpatch won't suggest clamp() again).
The clamp() is trying to increase 'avail' to 'slotsize' - that would
ensure the later max() does nothing.
So replace the clamp() with a max(), giving:
avail = max(avail, total_avail / max(nn->nfsd_serv->sv_nrthreads, 8));
num = min(ca->maxregs, avail / slotsize) ?: 1;
Unless I missed another assignment to 'num' that is probably equvalent.
David
> nfsd_drc_mem_used += num * slotsize;
> spin_unlock(&nfsd_drc_lock);
>
>
> ---
> base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4
> change-id: 20241209-nfs4state_fix-bc6f1c1fc1d1
>
> Best regards,
> --
> Vincent Mailhol <mailhol.vincent@...adoo.fr>
>
-
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