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@google.com --- 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