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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.

Link: http://lkml.kernel.org/r/20190919200428.188797-2-almasrymina@google.com
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mina Almasry 2019-11-30 17:56:54 -08:00 committed by Linus Torvalds
parent 930668c344
commit 5c91195420
1 changed files with 11 additions and 52 deletions
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63
mm/hugetlb.c
View File

@ -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;
}