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Message-ID: <b2fa24c3-c92f-6834-e6da-b952ac25e5f3@oracle.com>
Date: Mon, 16 Sep 2019 15:17:35 -0700
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: shuah@...nel.org, rientjes@...gle.com, shakeelb@...gle.com,
gthelen@...gle.com, akpm@...ux-foundation.org,
khalid.aziz@...cle.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
cgroups@...r.kernel.org, aneesh.kumar@...ux.vnet.ibm.com,
mkoutny@...e.com
Subject: Re: [PATCH v4 4/9] hugetlb: region_chg provides only cache entry
On 9/10/19 4:31 PM, Mina Almasry wrote:
> Current behavior is that region_chg provides both a cache entry in
> resv->region_cache, AND a placeholder entry in resv->regions. region_add
> first tries to use the placeholder, and if it finds that the placeholder
> has been deleted by a racing region_del call, it uses the cache entry.
>
> This behavior is completely unnecessary and is removed in this patch for
> a couple of reasons:
>
> 1. region_add needs to either find a cached file_region entry in
> resv->region_cache, or find an entry in resv->regions to expand. It
> does not need both.
> 2. region_chg adding a placeholder entry in resv->regions opens up
> a possible race with region_del, where region_chg adds a placeholder
> region in resv->regions, and this region is deleted by a racing call
> to region_del during region_chg execution or before region_add is
> called. Removing the race makes the code easier to reason about and
> maintain.
>
> In addition, a follow up patch in this series disables region
> coalescing, which would be further complicated if the race with
> region_del exists.
>
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
Thanks. I like this modification as it does simplify the code and could
be added as a general cleanup independent of the other changes.
Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
--
Mike Kravetz
> ---
> mm/hugetlb.c | 63 +++++++++-------------------------------------------
> 1 file changed, 11 insertions(+), 52 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index fbd7c52e17348..bea51ae422f63 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -246,14 +246,10 @@ struct file_region {
>
> /*
> * Add the huge page range represented by [f, t) to the reserve
> - * map. In the normal case, existing regions will be expanded
> - * to accommodate the specified range. Sufficient regions should
> - * exist for expansion due to the previous call to region_chg
> - * with the same range. However, it is possible that region_del
> - * could have been called after region_chg and modifed the map
> - * in such a way that no region exists to be expanded. In this
> - * case, pull a region descriptor from the cache associated with
> - * the map and use that for the new range.
> + * map. Existing regions will be expanded to accommodate the specified
> + * range, or a region will be taken from the cache. Sufficient regions
> + * must exist in the cache due to the previous call to region_chg with
> + * the same range.
> *
> * Return the number of new huge pages added to the map. This
> * number is greater than or equal to zero.
> @@ -272,9 +268,8 @@ static long region_add(struct resv_map *resv, long f, long t)
>
> /*
> * If no region exists which can be expanded to include the
> - * specified range, the list must have been modified by an
> - * interleving call to region_del(). Pull a region descriptor
> - * from the cache and use it for this range.
> + * specified range, pull a region descriptor from the cache
> + * and use it for this range.
> */
> if (&rg->link == head || t < rg->from) {
> VM_BUG_ON(resv->region_cache_count <= 0);
> @@ -339,15 +334,9 @@ static long region_add(struct resv_map *resv, long f, long t)
> * call to region_add that will actually modify the reserve
> * map to add the specified range [f, t). region_chg does
> * not change the number of huge pages represented by the
> - * map. However, if the existing regions in the map can not
> - * be expanded to represent the new range, a new file_region
> - * structure is added to the map as a placeholder. This is
> - * so that the subsequent region_add call will have all the
> - * regions it needs and will not fail.
> - *
> - * Upon entry, region_chg will also examine the cache of region descriptors
> - * associated with the map. If there are not enough descriptors cached, one
> - * will be allocated for the in progress add operation.
> + * map. A new file_region structure is added to the cache
> + * as a placeholder, so that the subsequent region_add
> + * call will have all the regions it needs and will not fail.
> *
> * Returns the number of huge pages that need to be added to the existing
> * reservation map for the range [f, t). This number is greater or equal to
> @@ -357,10 +346,9 @@ static long region_add(struct resv_map *resv, long f, long t)
> static long region_chg(struct resv_map *resv, long f, long t)
> {
> struct list_head *head = &resv->regions;
> - struct file_region *rg, *nrg = NULL;
> + struct file_region *rg;
> long chg = 0;
>
> -retry:
> spin_lock(&resv->lock);
> retry_locked:
> resv->adds_in_progress++;
> @@ -378,10 +366,8 @@ static long region_chg(struct resv_map *resv, long f, long t)
> spin_unlock(&resv->lock);
>
> trg = kmalloc(sizeof(*trg), GFP_KERNEL);
> - if (!trg) {
> - kfree(nrg);
> + if (!trg)
> return -ENOMEM;
> - }
>
> spin_lock(&resv->lock);
> list_add(&trg->link, &resv->region_cache);
> @@ -394,28 +380,6 @@ static long region_chg(struct resv_map *resv, long f, long t)
> if (f <= rg->to)
> break;
>
> - /* If we are below the current region then a new region is required.
> - * Subtle, allocate a new region at the position but make it zero
> - * size such that we can guarantee to record the reservation. */
> - if (&rg->link == head || t < rg->from) {
> - if (!nrg) {
> - resv->adds_in_progress--;
> - spin_unlock(&resv->lock);
> - nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> - if (!nrg)
> - return -ENOMEM;
> -
> - nrg->from = f;
> - nrg->to = f;
> - INIT_LIST_HEAD(&nrg->link);
> - goto retry;
> - }
> -
> - list_add(&nrg->link, rg->link.prev);
> - chg = t - f;
> - goto out_nrg;
> - }
> -
> /* Round our left edge to the current segment if it encloses us. */
> if (f > rg->from)
> f = rg->from;
> @@ -439,11 +403,6 @@ static long region_chg(struct resv_map *resv, long f, long t)
> }
>
> out:
> - spin_unlock(&resv->lock);
> - /* We already know we raced and no longer need the new region */
> - kfree(nrg);
> - return chg;
> -out_nrg:
> spin_unlock(&resv->lock);
> return chg;
> }
> --
> 2.23.0.162.g0b9fbb3734-goog
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