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Message-ID: <87sf2klez8.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 26 Jan 2024 15:40:27 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gourry.memverge@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-api@...r.kernel.org, corbet@....net, akpm@...ux-foundation.org,
gregory.price@...verge.com, honggyu.kim@...com, rakie.kim@...com,
hyeongtak.ji@...com, mhocko@...nel.org, vtavarespetr@...ron.com,
jgroves@...ron.com, ravis.opensrc@...ron.com, sthanneeru@...ron.com,
emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
hannes@...xchg.org, dan.j.williams@...el.com
Subject: Re: [PATCH v3 4/4] mm/mempolicy: change cur_il_weight to atomic and
carry the node with it
Gregory Price <gourry.memverge@...il.com> writes:
> In the prior patch, we carry only the current weight for a weighted
> interleave round with us across calls through the allocator path.
>
> node = next_node_in(current->il_prev, pol->nodemask)
> pol->cur_il_weight <--- this weight applies to the above node
>
> This separation of data can cause a race condition.
>
> If a cgroup-initiated task migration or mems_allowed change occurs
> from outside the context of the task, this can cause the weight to
> become stale, meaning we may end using that weight to allocate
> memory on the wrong node.
>
> Example:
> 1) task A sets (cur_il_weight = 8) and (current->il_prev) to
> node0. node1 is the next set bit in pol->nodemask
> 2) rebind event occurs, removing node1 from the nodemask.
> node2 is now the next set bit in pol->nodemask
> cur_il_weight is now stale.
> 3) allocation occurs, next_node_in(il_prev, nodes) returns
> node2. cur_il_weight is now applied to the wrong node.
>
> The upper level allocator logic must still enforce mems_allowed,
> so this isn't dangerous, but it is innaccurate.
>
> Just clearing the weight is insufficient, as it creates two more
> race conditions. The root of the issue is the separation of weight
> and node data between nodemask and cur_il_weight.
>
> To solve this, update cur_il_weight to be an atomic_t, and place the
> node that the weight applies to in the upper bits of the field:
>
> atomic_t cur_il_weight
> node bits 32:8
> weight bits 7:0
>
> Now retrieving or clearing the active interleave node and weight
> is a single atomic operation, and we are not dependent on the
> potentially changing state of (pol->nodemask) to determine what
> node the weight applies to.
>
> Two special observations:
> - if the weight is non-zero, cur_il_weight must *always* have a
> valid node number, e.g. it cannot be NUMA_NO_NODE (-1).
IIUC, we don't need that, "MAX_NUMNODES-1" is used instead.
> This is because we steal the top byte for the weight.
>
> - MAX_NUMNODES is presently limited to 1024 or less on every
> architecture. This would permanently limit MAX_NUMNODES to
> an absolute maximum of (1 << 24) to avoid overflows.
>
> Per some reading and discussion, it appears that max nodes is
> limited to 1024 so that zone type still fits in page flags, so
> this method seemed preferable compared to the alternatives of
> trying to make all or part of mempolicy RCU protected (which
> may not be possible, since it is often referenced during code
> chunks which call operations that may sleep).
>
> Signed-off-by: Gregory Price <gregory.price@...verge.com>
> ---
> include/linux/mempolicy.h | 2 +-
> mm/mempolicy.c | 93 +++++++++++++++++++++++++--------------
> 2 files changed, 61 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> index c644d7bbd396..8108fc6e96ca 100644
> --- a/include/linux/mempolicy.h
> +++ b/include/linux/mempolicy.h
> @@ -56,7 +56,7 @@ struct mempolicy {
> } w;
>
> /* Weighted interleave settings */
> - u8 cur_il_weight;
> + atomic_t cur_il_weight;
If we use this field for node and weight, why not change the field name?
For example, cur_wil_node_weight.
> };
>
> /*
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 5a517511658e..41b5fef0a6f5 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -321,7 +321,7 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
> policy->mode = mode;
> policy->flags = flags;
> policy->home_node = NUMA_NO_NODE;
> - policy->cur_il_weight = 0;
> + atomic_set(&policy->cur_il_weight, 0);
>
> return policy;
> }
> @@ -356,6 +356,7 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
> tmp = *nodes;
>
> pol->nodes = tmp;
> + atomic_set(&pol->cur_il_weight, 0);
> }
>
> static void mpol_rebind_preferred(struct mempolicy *pol,
> @@ -973,8 +974,10 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
> *policy = next_node_in(current->il_prev, pol->nodes);
> } else if (pol == current->mempolicy &&
> (pol->mode == MPOL_WEIGHTED_INTERLEAVE)) {
> - if (pol->cur_il_weight)
> - *policy = current->il_prev;
> + int cweight = atomic_read(&pol->cur_il_weight);
> +
> + if (cweight & 0xFF)
> + *policy = cweight >> 8;
Please define some helper functions or macros instead of operate on bits
directly.
> else
> *policy = next_node_in(current->il_prev,
> pol->nodes);
If we record current node in pol->cur_il_weight, why do we still need
curren->il_prev. Can we only use pol->cur_il_weight? And if so, we can
even make current->il_prev a union.
--
Best Regards,
Huang, Ying
[snip]
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