[PATCH v12 4/9] hugetlb: disable region_add file_region coalescing

From: Mina Almasry
Date: Tue Feb 11 2020 - 16:32:24 EST

A follow up patch in this series adds hugetlb cgroup uncharge info the
file_region entries in resv->regions. The cgroup uncharge info may
differ for different regions, so they can no longer be coalesced at
region_add time. So, disable region coalescing in region_add in this
patch.

Behavior change:

Say a resv_map exists like this [0->1], [2->3], and [5->6].

Then a region_chg/add call comes in region_chg/add(f=0, t=5).

Old code would generate resv->regions: [0->5], [5->6].
New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
[5->6].

Special care needs to be taken to handle the resv->adds_in_progress
variable correctly. In the past, only 1 region would be added for every
region_chg and region_add call. But now, each call may add multiple
regions, so we can no longer increment adds_in_progress by 1 in region_chg,
or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
region_chg calls add_reservation_in_range() to count the number of regions
needed and allocates those, and that info is passed to region_add and
region_abort to decrement adds_in_progress correctly.

We've also modified the assumption that region_add after region_chg
never fails. region_chg now pre-allocates at least 1 region for
region_add. If region_add needs more regions than region_chg has
allocated for it, then it may fail.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>

---

Changes in v12:
- Removed Mike's Reviewed-by until coalescing is reviewed per his
request.
- Added VM_BUG_ON that was mistakingly in a follow up patch.

Changes in v9:
- Added clarifications in the comments and addressed minor issues from
code review.

Changes in v7:
- region_chg no longer allocates (t-f) / 2 file_region entries.

Changes in v6:
- Fix bug in number of region_caches allocated by region_chg

---
mm/hugetlb.c | 328 +++++++++++++++++++++++++++++++++++----------------
1 file changed, 224 insertions(+), 104 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac7bab0afafe7..57739e1aedeca 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -245,110 +245,180 @@ struct file_region {
long to;
};

+/* Helper that removes a struct file_region from the resv_map cache and returns
+ * it for use.
+ */
+static struct file_region *
+get_file_region_entry_from_cache(struct resv_map *resv, long from, long to)
+{
+ struct file_region *nrg = NULL;
+
+ VM_BUG_ON(resv->region_cache_count <= 0);
+
+ resv->region_cache_count--;
+ nrg = list_first_entry(&resv->region_cache, struct file_region, link);
+ VM_BUG_ON(!nrg);
+ list_del(&nrg->link);
+
+ nrg->from = from;
+ nrg->to = to;
+
+ return nrg;
+}
+
/* Must be called with resv->lock held. Calling this with count_only == true
* will count the number of pages to be added but will not modify the linked
- * list.
+ * list. If regions_needed != NULL and count_only == true, then regions_needed
+ * will indicate the number of file_regions needed in the cache to carry out to
+ * add the regions for this range.
*/
static long add_reservation_in_range(struct resv_map *resv, long f, long t,
- bool count_only)
+ long *regions_needed, bool count_only)
{
- long chg = 0;
+ long add = 0;
struct list_head *head = &resv->regions;
+ long last_accounted_offset = f;
struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;

- /* Locate the region we are before or in. */
- list_for_each_entry(rg, head, link)
- if (f <= rg->to)
- break;
-
- /* Round our left edge to the current segment if it encloses us. */
- if (f > rg->from)
- f = rg->from;
+ if (regions_needed)
+ *regions_needed = 0;

- chg = t - f;
+ /* In this loop, we essentially handle an entry for the range
+ * [last_accounted_offset, rg->from), at every iteration, with some
+ * bounds checking.
+ */
+ list_for_each_entry_safe(rg, trg, head, link) {
+ /* Skip irrelevant regions that start before our range. */
+ if (rg->from < f) {
+ /* If this region ends after the last accounted offset,
+ * then we need to update last_accounted_offset.
+ */
+ if (rg->to > last_accounted_offset)
+ last_accounted_offset = rg->to;
+ continue;
+ }

- /* Check for and consume any regions we now overlap with. */
- nrg = rg;
- list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
- if (&rg->link == head)
- break;
+ /* When we find a region that starts beyond our range, we've
+ * finished.
+ */
if (rg->from > t)
break;

- /* We overlap with this area, if it extends further than
- * us then we must extend ourselves. Account for its
- * existing reservation.
+ /* Add an entry for last_accounted_offset -> rg->from, and
+ * update last_accounted_offset.
*/
- if (rg->to > t) {
- chg += rg->to - t;
- t = rg->to;
+ if (rg->from > last_accounted_offset) {
+ add += rg->from - last_accounted_offset;
+ if (!count_only) {
+ nrg = get_file_region_entry_from_cache(
+ resv, last_accounted_offset, rg->from);
+ list_add(&nrg->link, rg->link.prev);
+ } else if (regions_needed)
+ *regions_needed += 1;
}
- chg -= rg->to - rg->from;

- if (!count_only && rg != nrg) {
- list_del(&rg->link);
- kfree(rg);
- }
+ last_accounted_offset = rg->to;
}

- if (!count_only) {
- nrg->from = f;
- nrg->to = t;
+ /* Handle the case where our range extends beyond
+ * last_accounted_offset.
+ */
+ if (last_accounted_offset < t) {
+ add += t - last_accounted_offset;
+ if (!count_only) {
+ nrg = get_file_region_entry_from_cache(
+ resv, last_accounted_offset, t);
+ list_add(&nrg->link, rg->link.prev);
+ } else if (regions_needed)
+ *regions_needed += 1;
}

- return chg;
+ VM_BUG_ON(add < 0);
+ return add;
}

/*
* Add the huge page range represented by [f, t) to the reserve
- * 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.
+ * map. Regions will be taken from the cache to fill in this range.
+ * Sufficient regions should exist in the cache due to the previous
+ * call to region_chg with the same range, but in some cases the cache will not
+ * have sufficient entries due to races with other code doing region_add or
+ * region_del. The extra needed entries will be allocated.
+ *
+ * regions_needed is the out value provided by a previous call to region_chg.
*
- * Return the number of new huge pages added to the map. This
- * number is greater than or equal to zero.
+ * Return the number of new huge pages added to the map. This number is greater
+ * than or equal to zero. If file_region entries needed to be allocated for
+ * this operation and we were not able to allocate, it ruturns -ENOMEM.
+ * region_add of regions of length 1 never allocate file_regions and cannot
+ * fail; region_chg will always allocate at least 1 entry and a region_add for
+ * 1 page will only require at most 1 entry.
*/
-static long region_add(struct resv_map *resv, long f, long t)
+static long region_add(struct resv_map *resv, long f, long t,
+ long in_regions_needed)
{
- struct list_head *head = &resv->regions;
- struct file_region *rg, *nrg;
- long add = 0;
+ long add = 0, actual_regions_needed = 0, i = 0;
+ struct file_region *trg = NULL, *rg = NULL;
+ struct list_head allocated_regions;
+
+ INIT_LIST_HEAD(&allocated_regions);

spin_lock(&resv->lock);
- /* Locate the region we are either in or before. */
- list_for_each_entry(rg, head, link)
- if (f <= rg->to)
- break;
+retry:
+
+ /* Count how many regions are actually needed to execute this add. */
+ add_reservation_in_range(resv, f, t, &actual_regions_needed, true);

/*
- * If no region exists which can be expanded to include the
- * specified range, pull a region descriptor from the cache
- * and use it for this range.
+ * Check for sufficient descriptors in the cache to accommodate
+ * this add operation. Note that actual_regions_needed may be greater
+ * than in_regions_needed. In this case, we need to make sure that we
+ * allocate extra entries, such that we have enough for all the
+ * existing adds_in_progress, plus the excess needed for this
+ * operation.
*/
- if (&rg->link == head || t < rg->from) {
- VM_BUG_ON(resv->region_cache_count <= 0);
+ if (resv->region_cache_count <
+ resv->adds_in_progress +
+ (actual_regions_needed - in_regions_needed)) {
+ /* region_add operation of range 1 should never need to
+ * allocate file_region entries.
+ */
+ VM_BUG_ON(t - f <= 1);

- resv->region_cache_count--;
- nrg = list_first_entry(&resv->region_cache, struct file_region,
- link);
- list_del(&nrg->link);
+ /* Must drop lock to allocate a new descriptor. */
+ spin_unlock(&resv->lock);
+ for (i = 0; i < (actual_regions_needed - in_regions_needed);
+ i++) {
+ trg = kmalloc(sizeof(*trg), GFP_KERNEL);
+ if (!trg)
+ goto out_of_memory;
+ list_add(&trg->link, &allocated_regions);
+ }
+ spin_lock(&resv->lock);

- nrg->from = f;
- nrg->to = t;
- list_add(&nrg->link, rg->link.prev);
+ list_for_each_entry_safe(rg, trg, &allocated_regions, link) {
+ list_del(&rg->link);
+ list_add(&rg->link, &resv->region_cache);
+ resv->region_cache_count++;
+ }

- add += t - f;
- goto out_locked;
+ goto retry;
}

- add = add_reservation_in_range(resv, f, t, false);
+ add = add_reservation_in_range(resv, f, t, NULL, false);
+
+ resv->adds_in_progress -= in_regions_needed;

-out_locked:
- resv->adds_in_progress--;
spin_unlock(&resv->lock);
VM_BUG_ON(add < 0);
return add;
+
+out_of_memory:
+ list_for_each_entry_safe(rg, trg, &allocated_regions, link) {
+ list_del(&rg->link);
+ kfree(rg);
+ }
+ return -ENOMEM;
}

/*
@@ -358,49 +428,79 @@ 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. 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.
+ * map. A number of new file_region structures is added to the cache as a
+ * placeholder, for the subsequent region_add call to use. At least 1
+ * file_region structure is added.
+ *
+ * out_regions_needed is the number of regions added to the
+ * resv->adds_in_progress. This value needs to be provided to a follow up call
+ * to region_add or region_abort for proper accounting.
*
* 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
* zero. -ENOMEM is returned if a new file_region structure or cache entry
* is needed and can not be allocated.
*/
-static long region_chg(struct resv_map *resv, long f, long t)
+static long region_chg(struct resv_map *resv, long f, long t,
+ long *out_regions_needed)
{
- long chg = 0;
+ struct file_region *trg = NULL, *rg = NULL;
+ long chg = 0, i = 0, to_allocate = 0;
+ struct list_head allocated_regions;
+
+ INIT_LIST_HEAD(&allocated_regions);

spin_lock(&resv->lock);
-retry_locked:
- resv->adds_in_progress++;
+
+ /* Count how many hugepages in this range are NOT respresented. */
+ chg = add_reservation_in_range(resv, f, t, out_regions_needed, true);
+
+ if (*out_regions_needed == 0)
+ *out_regions_needed = 1;
+
+ resv->adds_in_progress += *out_regions_needed;

/*
* Check for sufficient descriptors in the cache to accommodate
* the number of in progress add operations.
*/
- if (resv->adds_in_progress > resv->region_cache_count) {
- struct file_region *trg;
-
- VM_BUG_ON(resv->adds_in_progress - resv->region_cache_count > 1);
- /* Must drop lock to allocate a new descriptor. */
- resv->adds_in_progress--;
+ while (resv->region_cache_count < resv->adds_in_progress) {
+ to_allocate = resv->adds_in_progress - resv->region_cache_count;
+
+ /* Must drop lock to allocate a new descriptor. Note that even
+ * though we drop the lock here, we do not make another call to
+ * add_reservation_in_range after re-acquiring the lock.
+ * Essentially this branch makes sure that we have enough
+ * descriptors in the cache as suggested by the first call to
+ * add_reservation_in_range. If more regions turn out to be
+ * required, region_add will deal with it.
+ */
spin_unlock(&resv->lock);
-
- trg = kmalloc(sizeof(*trg), GFP_KERNEL);
- if (!trg)
- return -ENOMEM;
+ for (i = 0; i < to_allocate; i++) {
+ trg = kmalloc(sizeof(*trg), GFP_KERNEL);
+ if (!trg)
+ goto out_of_memory;
+ list_add(&trg->link, &allocated_regions);
+ }

spin_lock(&resv->lock);
- list_add(&trg->link, &resv->region_cache);
- resv->region_cache_count++;
- goto retry_locked;
- }

- chg = add_reservation_in_range(resv, f, t, true);
+ list_for_each_entry_safe(rg, trg, &allocated_regions, link) {
+ list_del(&rg->link);
+ list_add(&rg->link, &resv->region_cache);
+ resv->region_cache_count++;
+ }
+ }

spin_unlock(&resv->lock);
return chg;
+
+out_of_memory:
+ list_for_each_entry_safe(rg, trg, &allocated_regions, link) {
+ list_del(&rg->link);
+ kfree(rg);
+ }
+ return -ENOMEM;
}

/*
@@ -408,17 +508,20 @@ static long region_chg(struct resv_map *resv, long f, long t)
* of the resv_map keeps track of the operations in progress between
* calls to region_chg and region_add. Operations are sometimes
* aborted after the call to region_chg. In such cases, region_abort
- * is called to decrement the adds_in_progress counter.
+ * is called to decrement the adds_in_progress counter. regions_needed
+ * is the value returned by the region_chg call, it is used to decrement
+ * the adds_in_progress counter.
*
* NOTE: The range arguments [f, t) are not needed or used in this
* routine. They are kept to make reading the calling code easier as
* arguments will match the associated region_chg call.
*/
-static void region_abort(struct resv_map *resv, long f, long t)
+static void region_abort(struct resv_map *resv, long f, long t,
+ long regions_needed)
{
spin_lock(&resv->lock);
VM_BUG_ON(!resv->region_cache_count);
- resv->adds_in_progress--;
+ resv->adds_in_progress -= regions_needed;
spin_unlock(&resv->lock);
}

@@ -1898,6 +2001,7 @@ static long __vma_reservation_common(struct hstate *h,
struct resv_map *resv;
pgoff_t idx;
long ret;
+ long dummy_out_regions_needed;

resv = vma_resv_map(vma);
if (!resv)
@@ -1906,20 +2010,29 @@ static long __vma_reservation_common(struct hstate *h,
idx = vma_hugecache_offset(h, vma, addr);
switch (mode) {
case VMA_NEEDS_RESV:
- ret = region_chg(resv, idx, idx + 1);
+ ret = region_chg(resv, idx, idx + 1, &dummy_out_regions_needed);
+ /* We assume that vma_reservation_* routines always operate on
+ * 1 page, and that adding to resv map a 1 page entry can only
+ * ever require 1 region.
+ */
+ VM_BUG_ON(dummy_out_regions_needed != 1);
break;
case VMA_COMMIT_RESV:
- ret = region_add(resv, idx, idx + 1);
+ ret = region_add(resv, idx, idx + 1, 1);
+ /* region_add calls of range 1 should never fail. */
+ VM_BUG_ON(ret < 0);
break;
case VMA_END_RESV:
- region_abort(resv, idx, idx + 1);
+ region_abort(resv, idx, idx + 1, 1);
ret = 0;
break;
case VMA_ADD_RESV:
- if (vma->vm_flags & VM_MAYSHARE)
- ret = region_add(resv, idx, idx + 1);
- else {
- region_abort(resv, idx, idx + 1);
+ if (vma->vm_flags & VM_MAYSHARE) {
+ ret = region_add(resv, idx, idx + 1, 1);
+ /* region_add calls of range 1 should never fail. */
+ VM_BUG_ON(ret < 0);
+ } else {
+ region_abort(resv, idx, idx + 1, 1);
ret = region_del(resv, idx, idx + 1);
}
break;
@@ -4569,12 +4682,12 @@ int hugetlb_reserve_pages(struct inode *inode,
struct vm_area_struct *vma,
vm_flags_t vm_flags)
{
- long ret, chg;
+ long ret, chg, add = -1;
struct hstate *h = hstate_inode(inode);
struct hugepage_subpool *spool = subpool_inode(inode);
struct resv_map *resv_map;
struct hugetlb_cgroup *h_cg;
- long gbl_reserve;
+ long gbl_reserve, regions_needed = 0;

/* This should never happen */
if (from > to) {
@@ -4604,7 +4717,7 @@ int hugetlb_reserve_pages(struct inode *inode,
*/
resv_map = inode_resv_map(inode);

- chg = region_chg(resv_map, from, to);
+ chg = region_chg(resv_map, from, to, &regions_needed);

} else {
/* Private mapping. */
@@ -4670,9 +4783,14 @@ int hugetlb_reserve_pages(struct inode *inode,
* else has to be done for private mappings here
*/
if (!vma || vma->vm_flags & VM_MAYSHARE) {
- long add = region_add(resv_map, from, to);
-
- if (unlikely(chg > add)) {
+ add = region_add(resv_map, from, to, regions_needed);
+
+ if (unlikely(add < 0)) {
+ hugetlb_acct_memory(h, -gbl_reserve);
+ /* put back original number of pages, chg */
+ (void)hugepage_subpool_put_pages(spool, chg);
+ goto out_err;
+ } else if (unlikely(chg > add)) {
/*
* pages in this range were added to the reserve
* map between region_chg and region_add. This
@@ -4690,9 +4808,11 @@ int hugetlb_reserve_pages(struct inode *inode,
return 0;
out_err:
if (!vma || vma->vm_flags & VM_MAYSHARE)
- /* Don't call region_abort if region_chg failed */
- if (chg >= 0)
- region_abort(resv_map, from, to);
+ /* Only call region_abort if the region_chg succeeded but the
+ * region_add failed or didn't run.
+ */
+ if (chg >= 0 && add < 0)
+ region_abort(resv_map, from, to, regions_needed);
if (vma && is_vma_resv_set(vma, HPAGE_RESV_OWNER))
kref_put(&resv_map->refs, resv_map_release);
return ret;
--
2.25.0.225.g125e21ebc7-goog