[PATCH v3 01/13] mm: hugetlb: Track used_hpages when getting/putting pages from subpool
From: Ackerley Tng via B4 Relay
Date: Mon Jul 20 2026 - 22:56:23 EST
From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
hugepage_subpool_put_pages() currently has two distinct responsibilities
that conflict:
1. When size is specified for the mount, max_hpages != -1: Keep track of
total active pages (allocated + reserved) and decrement this count
(used_hpages) when a page is freed or allocation fails.
2. When min_size is specified for the mount, min_hpages != -1: Ensure we
don't drop below the guaranteed minimum, and restore a reservation
(rsv_hpages) if we do.
This causes trouble because when allocation fails (refer to
alloc_hugetlb_folio()) if gbl_chg = 1 (i.e. no subpool reservation was
taken):
+ To keep used_hpages consistent, HugeTLB needs to call
hugepage_subpool_put_pages() to restore undo used_hpages being
incremented
+ But can't call hugepage_subpool_put_pages() if no reservation was
consumed.
One option would be to conditionally do subpool tracking updates outside of
the hugepage_subpool_put_pages() function, but that would spread logic all
over.
Instead, always track used_hpages, regardless of whether a max_size was
requested for the mount, so that the subpool always knows how many pages
were allocated through it. Every page allocated through the subpool
increments used_hpages, regardless of whether a reservation was taken from
it.
Conceptually, now, every allocation involving a subpool uses a page from
the subpool, which must be returned to the subpool. Every page taken from
the subpool tries to use a subpool reservation. Restoring a page to the
subpool reservations only if the page was taken from subpool
reservations. (If used_hpages >= min_hpages, the page must have not have
been taken from the reservations.)
Always tracking used_hpages provides the subpool with information of both
used and reserved counts to make the correct decision for both max_size and
min_size correctly.
With used_hpages always tracked,
+ subpool_is_free() can be simplified, such that the subpool can be
declared free if there are no more pages in use.
+ open-coding in hugetlb_reserve_pages() can be removed.
Also update the documentation for used_hpages, since it no longer matters
whether the used pages count against the maximum.
Also update statfs reporting. Previously, if max_hpages is negative,
used_hpages is static at 0, so returning max_hpages - used_hpages returns
-1 and is always correct. Now, if the subpool doesn't have a maximum
requested size, indicate no limit for free pages (-1). If it does have a
maximum size, report the difference between the requested size and the
number of used pages. This difference is always positive, because if the
mount does have a maximum size, hugepage_subpool_get_pages() ensures that
the subpool usage never exceeds the maximum.
This fixes a bug in hugetlb_unreserve_pages(), where pages are returned to
the subpool regardless of whether it consumed a reservation. The
corresponding bug in the failure handling path of alloc_hugetlb_folio() was
fixed in a833a693a490e.
Fixes: 1c5ecae3a93fa ("hugetlbfs: add minimum size accounting to subpools")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
fs/hugetlbfs/inode.c | 8 ++++--
include/linux/hugetlb.h | 4 +--
mm/hugetlb.c | 71 +++++++++++++++++++++++--------------------------
3 files changed, 41 insertions(+), 42 deletions(-)
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 216e1a0dd0b23..26c0187340636 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1109,8 +1109,12 @@ static int hugetlbfs_statfs(struct dentry *dentry, struct kstatfs *buf)
spin_lock_irq(&sbinfo->spool->lock);
buf->f_blocks = sbinfo->spool->max_hpages;
- free_pages = sbinfo->spool->max_hpages
- - sbinfo->spool->used_hpages;
+ if (sbinfo->spool->max_hpages == -1) {
+ free_pages = -1;
+ } else {
+ free_pages = sbinfo->spool->max_hpages -
+ sbinfo->spool->used_hpages;
+ }
buf->f_bavail = buf->f_bfree = free_pages;
spin_unlock_irq(&sbinfo->spool->lock);
buf->f_files = sbinfo->max_inodes;
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 2abaf99321e90..34b9a3e1be0fa 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -38,8 +38,8 @@ struct hugepage_subpool {
spinlock_t lock;
long count;
long max_hpages; /* Maximum huge pages or -1 if no maximum. */
- long used_hpages; /* Used count against maximum, includes */
- /* both allocated and reserved pages. */
+ long used_hpages; /* Used page count, includes both */
+ /* allocated and reserved pages. */
struct hstate *hstate;
long min_hpages; /* Minimum huge pages or -1 if no minimum. */
long rsv_hpages; /* Pages reserved against global pool to */
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 571212b80835e..eef9610a0593c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -129,12 +129,8 @@ static inline bool subpool_is_free(struct hugepage_subpool *spool)
{
if (spool->count)
return false;
- if (spool->max_hpages != -1)
- return spool->used_hpages == 0;
- if (spool->min_hpages != -1)
- return spool->rsv_hpages == spool->min_hpages;
- return true;
+ return spool->used_hpages == 0;
}
static inline void unlock_or_release_subpool(struct hugepage_subpool *spool,
@@ -205,15 +201,14 @@ static long hugepage_subpool_get_pages(struct hugepage_subpool *spool,
spin_lock_irq(&spool->lock);
- if (spool->max_hpages != -1) { /* maximum size accounting */
- if ((spool->used_hpages + delta) <= spool->max_hpages)
- spool->used_hpages += delta;
- else {
- ret = -ENOMEM;
- goto unlock_ret;
- }
+ if (spool->max_hpages != -1 &&
+ spool->used_hpages + delta > spool->max_hpages) {
+ ret = -ENOMEM;
+ goto unlock_ret;
}
+ spool->used_hpages += delta;
+
/* minimum size accounting */
if (spool->min_hpages != -1 && spool->rsv_hpages) {
if (delta > spool->rsv_hpages) {
@@ -251,19 +246,24 @@ static long hugepage_subpool_put_pages(struct hugepage_subpool *spool,
spin_lock_irqsave(&spool->lock, flags);
- if (spool->max_hpages != -1) /* maximum size accounting */
- spool->used_hpages -= delta;
+ spool->used_hpages -= delta;
/* minimum size accounting */
if (spool->min_hpages != -1 && spool->used_hpages < spool->min_hpages) {
- if (spool->rsv_hpages + delta <= spool->min_hpages)
+ /*
+ * limit is the maximum number of reservations that
+ * can be restored to this subpool.
+ */
+ long limit = spool->min_hpages - spool->used_hpages;
+
+ if (spool->rsv_hpages + delta <= limit)
ret = 0;
else
- ret = spool->rsv_hpages + delta - spool->min_hpages;
+ ret = spool->rsv_hpages + delta - limit;
spool->rsv_hpages += delta;
- if (spool->rsv_hpages > spool->min_hpages)
- spool->rsv_hpages = spool->min_hpages;
+ if (spool->rsv_hpages > limit)
+ spool->rsv_hpages = limit;
}
/*
@@ -6542,7 +6542,7 @@ long hugetlb_reserve_pages(struct inode *inode,
struct vm_area_struct *vma,
vma_flags_t vma_flags)
{
- long chg = -1, add = -1, spool_resv, gbl_resv;
+ long chg = -1, add = -1, gbl_resv;
struct hstate *h = hstate_inode(inode);
struct hugepage_subpool *spool = subpool_inode(inode);
struct resv_map *resv_map;
@@ -6687,26 +6687,21 @@ long hugetlb_reserve_pages(struct inode *inode,
}
return chg;
-out_put_pages:
- spool_resv = chg - gbl_reserve;
- if (spool_resv) {
- /* put sub pool's reservation back, chg - gbl_reserve */
- gbl_resv = hugepage_subpool_put_pages(spool, spool_resv);
- /*
- * subpool's reserved pages can not be put back due to race,
- * return to hstate.
- */
- hugetlb_acct_memory(h, -gbl_resv);
- }
- /* Restore used_hpages for pages that failed global reservation */
- if (gbl_reserve && spool) {
- unsigned long flags;
+ out_put_pages:
+ /*
+ * Return all that was requested from the subpool, let subpool
+ * tell us the new number of reservations that need to be
+ * returned to the global pool.
+ */
+ gbl_resv = hugepage_subpool_put_pages(spool, chg);
+ /*
+ * There may be a difference between the number of
+ * reservations to consume and the number to restore now if
+ * there are multiple threads interacting with the subpool -
+ * restore the difference.
+ */
+ hugetlb_acct_memory(h, -(gbl_resv - gbl_reserve));
- spin_lock_irqsave(&spool->lock, flags);
- if (spool->max_hpages != -1)
- spool->used_hpages -= gbl_reserve;
- unlock_or_release_subpool(spool, flags);
- }
out_uncharge_cgroup:
hugetlb_cgroup_uncharge_cgroup_rsvd(hstate_index(h),
chg * pages_per_huge_page(h), h_cg);
--
2.55.0.229.g6434b31f56-goog