[PATCH 1/2] ceph: use GFP_NOFS for cap flush allocation in writeback path
From: Xiubo Li via B4 Relay
Date: Thu Jul 23 2026 - 01:52:15 EST
From: Xiubo Li <xiubo.li@xxxxxxxxx>
ceph_alloc_cap_flush() is called from ceph_writepages_start() inside
the writeback layer, where other allocations in the same path
(ceph_osdc_alloc_request, ceph_osdc_alloc_messages) already use
GFP_NOFS. A GFP_KERNEL allocation here can trigger direct reclaim
that recursively enters the filesystem writeback path:
ceph_writepages_start() // inode A writeback
ceph_alloc_cap_flush()
kmem_cache_alloc(..., GFP_KERNEL)
[direct reclaim]
try_to_free_pages()
shrink_slab()
super_cache_scan()
prune_icache_sb()
inode_lru_isolate()
iput() -> evict(inode_B)
[inode_B has dirty pages]
filemap_flush()
ceph_writepages_start() // re-enters writeback
ceph_alloc_cap_flush()
-> RECURSION / STACK OVERFLOW
All 11 callers of ceph_alloc_cap_flush() are in write or writeback
contexts: writepages (x2), write_iter, fallocate, copy_file_range,
setxattr, setattr, and page_mkwrite.
Signed-off-by: Xiubo Li <xiubo.li@xxxxxxxxx>
---
fs/ceph/caps.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index f8d898ad091e..cbaebeac06f7 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -1875,7 +1875,7 @@ struct ceph_cap_flush *ceph_alloc_cap_flush(void)
{
struct ceph_cap_flush *cf;
- cf = kmem_cache_alloc(ceph_cap_flush_cachep, GFP_KERNEL);
+ cf = kmem_cache_alloc(ceph_cap_flush_cachep, GFP_NOFS);
if (!cf)
return NULL;
--
2.53.0