[PATCH v3 1/4] mm: hugetlb: Track used_hpages when getting/putting pages from subpool
From: Ackerley Tng via B4 Relay
Date: Wed Sep 16 2026 - 19:40:13 EST
From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
HugeTLB subpools currently only track used pages (used_hpages) when a
maximum size limit (max_hpages) is configured.
This breaks minimum size (min_hpages) guarantees. The subpool guarantee
requires that the sum of used pages and remaining subpool reserves
satisfies the configured minimum:
used_hpages + rsv_hpages >= min_hpages
where used_hpages includes both allocated folios and active reservations.
Therefore, when pages or reservations are released, the maximum number
of reservations the subpool can absorb is:
limit = max(0, min_hpages - used_hpages)
When used_hpages is untracked and remains zero,
hugepage_subpool_put_pages() assumes limit == min_hpages. It falsely
restores reservations to the subpool even when existing allocations
already satisfy the guarantee.
This false restoration leads to:
1. Leaked global reservations: resv_huge_pages is not decremented,
reducing the huge pages available for use across the host.
2. Premature freeing: the subpool considers itself unreferenced on
unmount (rsv_hpages == min_hpages), freeing the subpool while folios
are still active in page cache and leading to a use-after-free.
Fix this by unconditionally tracking used_hpages in the subpool, and
capping the reservations the subpool can absorb to
(min_hpages - used_hpages).
With used_hpages always tracked:
- subpool_is_free() is simplified to check whether used_hpages is zero.
- hugetlbfs_statfs() is updated to explicitly check for max_hpages == -1;
previously it relied on (-1 - 0) evaluating to -1, which would now
underflow and report negative free block counts.
Trace of a false restoration:
1. Mount time:
+ spool->min_hpages = 1 (user requested min_size=2M)
+ spool->max_hpages = -1 (no maximum size specified)
+ spool->rsv_hpages = 1 (reserve min_hpages)
+ spool->used_hpages = 0 (untracked when max_hpages == -1)
+ h->resv_huge_pages = 1 (reserved by hugetlb_acct_memory(h, 1))
2. Shared mapping of 4MB (2 pages) created (mmap with MAP_SHARED):
+ In hugetlb_reserve_pages(), region_chg() finds chg = 2 (pages 0
and 1 need reservations)
+ hugepage_subpool_get_pages(spool, 2):
+ spool->rsv_hpages = 0 (consumed the 1 subpool reservation)
+ Returns 1 (since this subpool only had 1 reservation)
+ hugetlb_acct_memory(h, 1):
+ h->resv_huge_pages = 2 (incremented from 1 to 2 for the
global reservation)
+ region_add() records reservations for pages 0 and 1 in
resv_map
3. Process touches and populates Page 0:
+ hugetlb_no_page() calls alloc_hugetlb_folio()
+ Page 0 reuses the existing reservation (vma_needs_reservation()
returns 0 => map_chg = MAP_CHG_REUSE = 0)
+ hugepage_subpool_get_pages() is not called (map_chg == 0)
+ dequeue_hugetlb_folio_nodemask() consumes 1 reservation:
h->resv_huge_pages = 1 (decremented from 2 to 1)
4. Process closes the file and exits:
+ For MAP_SHARED mappings, reservations persist in inode resv_map
+ Page 1 reservation remains active
+ h->resv_huge_pages = 1 (retained for Page 1)
5. File is truncated to 2MB (truncate -s 2M):
+ Truncation invokes remove_inode_hugepages() for range [1, LONG_MAX)
+ Page 1 was never faulted into page cache => freed = 0
+ Calls hugetlb_unreserve_pages(inode, 1, LONG_MAX, freed = 0)
+ region_del() removes Page 1 from resv_map => chg = 1
6. Inside hugetlb_unreserve_pages(): hugepage_subpool_put_pages(1):
+ delta = chg - freed = 1 - 0 = 1
+ Because spool->max_hpages == -1, spool->used_hpages always = 0
+ spool->used_hpages < spool->min_hpages (0 < 1 => true)
<<== subpool assumes 0 pages are in use, ignoring allocated Page 0
+ spool->rsv_hpages + delta <= spool->min_hpages (0 + 1 <= 1 => true)
+ spool->rsv_hpages += 1 => spool->rsv_hpages = 1
<<== false reservation restored to subpool
+ Returns 0 (subpool absorbed the reservation)
7. Back in hugetlb_unreserve_pages(): hugetlb_acct_memory():
+ hugetlb_acct_memory(h, -0) does nothing
+ h->resv_huge_pages remains stuck at 1 even though both reservations
have ended (Page 0 allocated, Page 1 truncated)
8. Later during unmounting:
+ subpool_is_free() checks spool->rsv_hpages == spool->min_hpages
(1 == 1 => true)
+ The subpool is erroneously considered completely free
+ hugetlb_acct_memory(spool->hstate, -spool->min_hpages) decrements
h->resv_huge_pages by 1 (1 - 1 = 0), masking the leak on unmount
9. If the folio outlives the inode:
+ When Page 0 is freed, free_huge_folio() attempts to access the
freed subpool pointer, leading to a use-after-free and double-free.
Fixes: 09a95e29cb30 ("mm/hugetlb: optimize minimum size (min_size) accounting")
Fixes: 1c5ecae3a93fa ("hugetlbfs: add minimum size accounting to subpools")
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Documentation/mm/hugetlbfs_reserv.rst | 17 +----
.../translations/zh_CN/mm/hugetlbfs_reserv.rst | 11 +---
fs/hugetlbfs/inode.c | 8 ++-
include/linux/hugetlb.h | 4 +-
mm/hugetlb.c | 73 +++++++++++++---------
5 files changed, 55 insertions(+), 58 deletions(-)
diff --git a/Documentation/mm/hugetlbfs_reserv.rst b/Documentation/mm/hugetlbfs_reserv.rst
index a49115db18c76..d244583fdcbc3 100644
--- a/Documentation/mm/hugetlbfs_reserv.rst
+++ b/Documentation/mm/hugetlbfs_reserv.rst
@@ -314,21 +314,8 @@ huge pages. If they can not be reserved, the mount fails.
The routines hugepage_subpool_get/put_pages() are called when pages are
obtained from or released back to a subpool. They perform all subpool
accounting, and track any reservations associated with the subpool.
-hugepage_subpool_get/put_pages are passed the number of huge pages by which
-to adjust the subpool 'used page' count (down for get, up for put). Normally,
-they return the same value that was passed or an error if not enough pages
-exist in the subpool.
-
-However, if reserves are associated with the subpool a return value less
-than the passed value may be returned. This return value indicates the
-number of additional global pool adjustments which must be made. For example,
-suppose a subpool contains 3 reserved huge pages and someone asks for 5.
-The 3 reserved pages associated with the subpool can be used to satisfy part
-of the request. But, 2 pages must be obtained from the global pools. To
-relay this information to the caller, the value 2 is returned. The caller
-is then responsible for attempting to obtain the additional two pages from
-the global pools.
-
+hugepage_subpool_get/put_pages() use the number of huge pages passed to adjust
+the subpool 'used page' count.
COW and Reservations
====================
diff --git a/Documentation/translations/zh_CN/mm/hugetlbfs_reserv.rst b/Documentation/translations/zh_CN/mm/hugetlbfs_reserv.rst
index 20947f8bd0654..ae1f1f31477fc 100644
--- a/Documentation/translations/zh_CN/mm/hugetlbfs_reserv.rst
+++ b/Documentation/translations/zh_CN/mm/hugetlbfs_reserv.rst
@@ -246,15 +246,8 @@ hugepage_subpool的min_hpages字段中被跟踪。在挂载时，hugetlb_acct_me
被调用以预留指定数量的巨页。如果它们不能被预留，挂载就会失败。
当从子池中获取或释放页面时，会调用hugepage_subpool_get/put_pages()函数。
-hugepage_subpool_get/put_pages被传递给巨页数量，以此来调整子池的 “已用页面” 计数
-（get为下降，put为上升）。通常情况下，如果子池中没有足够的页面，它们会返回与传递的相同的值或
-一个错误。
-
-然而，如果预留与子池相关联，可能会返回一个小于传递值的返回值。这个返回值表示必须进行的额外全局
-池调整的数量。例如，假设一个子池包含3个预留的巨页，有人要求5个。与子池相关的3个预留页可以用来
-满足部分请求。但是，必须从全局池中获得2个页面。为了向调用者转达这一信息，将返回值2。然后，调用
-者要负责从全局池中获取另外两个页面。
-
+它们负责所有子池的统计核算，并跟踪与子池相关联的预留。
+hugepage_subpool_get/put_pages()函数使用传入的巨页数量来调整子池的“已用页面”计数。
COW和预留
==========
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 7611a8470ea26..5113f743f6fc7 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 16c4c4caa126c..4551ff3023640 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -39,8 +39,8 @@ 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 4f6f58bf3db6c..e72e22f887478 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -130,12 +130,8 @@ 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,
@@ -193,13 +189,18 @@ void hugepage_put_subpool(struct hugepage_subpool *spool)
unlock_or_release_subpool(spool, flags);
}
-/*
- * Subpool accounting for allocating and reserving pages.
- * Return -ENOMEM if there are not enough resources to satisfy the
- * request. Otherwise, return the number of pages by which the
- * global pools must be adjusted (upward). The returned value may
- * only be different than the passed value (delta) in the case where
- * a subpool minimum size must be maintained.
+/**
+ * hugepage_subpool_get_pages - Get pages from a subpool
+ * @spool: pointer to subpool structure (may be NULL)
+ * @delta: number of pages to allocate or reserve
+ *
+ * Check and update subpool page usage counts when allocating or
+ * reserving @delta hugepages.
+ *
+ * Context: Takes spool->lock using spin_lock_irq().
+ * Return: Non-negative number of reservations that cannot be
+ * satisfied by the subpool, or -ENOMEM if the subpool maximum
+ * limit would be exceeded.
*/
static long hugepage_subpool_get_pages(struct hugepage_subpool *spool,
long delta)
@@ -211,15 +212,14 @@ 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) {
@@ -240,11 +240,19 @@ static long hugepage_subpool_get_pages(struct hugepage_subpool *spool,
return ret;
}
-/*
- * Subpool accounting for freeing and unreserving pages.
- * Return the number of global page reservations that must be dropped.
- * The return value may only be different than the passed value (delta)
- * in the case where a subpool minimum size must be maintained.
+/**
+ * hugepage_subpool_put_pages - Release pages back to a subpool
+ * @spool: pointer to subpool structure (may be NULL)
+ * @delta: number of pages to free or unreserve
+ *
+ * Check and update subpool page usage counts when freeing or
+ * unreserving @delta hugepages.
+ *
+ * Context: Takes spool->lock using spin_lock_irqsave(). May release
+ * and free @spool if its usage count and references reach
+ * zero.
+ * Return: Non-negative number of reservations that the subpool cannot
+ * absorb.
*/
static long hugepage_subpool_put_pages(struct hugepage_subpool *spool,
long delta)
@@ -257,19 +265,24 @@ 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;
}
/*
--
2.55.0.1082.g2b9226bbc0-goog