[PATCH v3 1/4] mm: hugetlb: Track used_hpages when getting/putting pages from subpool

Patch content 5 changed files

From: Ackerley Tng <ackerleytng@google.com> HugeTLB subpools currently only track used pages (used_hpages) when amaximum size limit (max_hpages) is configured. This breaks minimum size (min_hpages) guarantees. The subpool guaranteerequires that the sum of used pages and remaining subpool reservessatisfies 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 numberof 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 falselyrestores reservations to the subpool even when existing allocationsalready 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, andcapping 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@google.com>Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>Cc: stable@vger.kernel.org--- 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.rstindex 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.rstindex 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.cindex 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.hindex 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.cindex 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