linux-kernel - [PATCH v3] mm: hugetlb_vmemmap: provide stronger vmemmap allocation guarantees

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Date:   Mon,  8 May 2023 23:40:59 +0000
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     pasha.tatashin@...een.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, mhocko@...e.com, muchun.song@...ux.dev,
        rientjes@...gle.com, souravpanda@...gle.com
Subject: [PATCH v3] mm: hugetlb_vmemmap: provide stronger vmemmap allocation guarantees

HugeTLB pages have a struct page optimizations where struct pages for tail
pages are freed. However, when HugeTLB pages are destroyed, the memory for
struct pages (vmemmap) need to be allocated again.

Currently, __GFP_NORETRY flag is used to allocate the memory for vmemmap,
but given that this flag makes very little effort to actually reclaim
memory the returning of huge pages back to the system can be problem. Lets
use __GFP_RETRY_MAYFAIL instead. This flag is also performs graceful
reclaim without causing ooms, but at least it may perform a few retries,
and will fail only when there is genuinely little amount of unused memory
in the system.

Freeing a 1G page requires 16M of free memory. A machine might need to be
reconfigured from one task to another, and release a large number of 1G pages
back to the system if allocating 16M fails, the release won't work.

Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
Suggested-by: David Rientjes <rientjes@...gle.com>
---
 mm/hugetlb_vmemmap.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

Changelog:
v3
  - updated patch log to include details about when the failure can happen.
v2
  - removed gfp_mask argument from alloc_vmemmap_page_list as suggested by
    David Rientjes.
  - Fixed spelling in the patch title.

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 27f001e0f0a2..f42079b73f82 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -384,8 +384,9 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
 }
 
 static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
-				   gfp_t gfp_mask, struct list_head *list)
+				   struct list_head *list)
 {
+	gfp_t gfp_mask = GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_THISNODE;
 	unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
 	int nid = page_to_nid((struct page *)start);
 	struct page *page, *next;
@@ -413,12 +414,11 @@ static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
  * @reuse:	reuse address.
- * @gfp_mask:	GFP flag for allocating vmemmap pages.
  *
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
-			       unsigned long reuse, gfp_t gfp_mask)
+			       unsigned long reuse)
 {
 	LIST_HEAD(vmemmap_pages);
 	struct vmemmap_remap_walk walk = {
@@ -430,7 +430,7 @@ static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
 	/* See the comment in the vmemmap_remap_free(). */
 	BUG_ON(start - reuse != PAGE_SIZE);
 
-	if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
+	if (alloc_vmemmap_page_list(start, end, &vmemmap_pages))
 		return -ENOMEM;
 
 	mmap_read_lock(&init_mm);
@@ -476,8 +476,7 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 	 * When a HugeTLB page is freed to the buddy allocator, previously
 	 * discarded vmemmap pages must be allocated and remapping.
 	 */
-	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse,
-				  GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
+	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse);
 	if (!ret) {
 		ClearHPageVmemmapOptimized(head);
 		static_branch_dec(&hugetlb_optimize_vmemmap_key);
-- 
2.40.1.521.gf1e218fcd8-goog