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Message-Id: <20190919200428.188797-2-almasrymina@google.com>
Date: Thu, 19 Sep 2019 13:04:27 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: mike.kravetz@...cle.com
Cc: almasrymina@...gle.com, rientjes@...gle.com, shakeelb@...gle.com,
gthelen@...gle.com, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 1/2] hugetlb: region_chg provides only cache entry
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 another series that disables region
coalescing, which would be further complicated if the race with
region_del exists.
Signed-off-by: Mina Almasry <almasrymina@...gle.com>
Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
---
mm/hugetlb.c | 63 +++++++++-------------------------------------------
1 file changed, 11 insertions(+), 52 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6d7296dd11b8..a14f6047fc7e 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;
}
--
2.23.0.351.gc4317032e6-goog
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