When inode_lru_isolate() drops the page cache of an unused inode, it has to unlock the list, so it returns LRU_RETRY and the walk restarts at the head of the list, where the inode still is. If invalidate_mapping_pages() could not empty the mapping, the restarted walk strips the same inode again, and so on until nr_to_walk runs out. Every walk of that list then ends on this inode, and nothing behind it is reclaimed for as long as its cache stays. Before commit 2a0629834cd8 ("vfs: Don't evict inode under the inode lru traversing context") the walker pinned the inode with a reference, and dropping it in iput_final() set I_REFERENCED, so the restarted walk rotated the inode to the tail. Set I_REFERENCED again when the mapping is still populated after the invalidation, in the same i_lock section that clears I_LRU_ISOLATING. inode_unpin_lru_isolating() now expects i_lock to be held, like inode_pin_lru_isolating(). On CONFIG_HIGHMEM kernels this can happen to any unused inode: a folio is left behind if it is locked, under writeback, spliced into a pipe, or still in another CPU's LRU-add batch after being read. Without HIGHMEM it happens to DAX inodes whose only entry is at index 0, which mapping_shrinkable() lets onto the LRU and invalidate_mapping_pages() never removes. Fixes: 2a0629834cd8 ("vfs: Don't evict inode under the inode lru traversing context") Cc: stable@vger.kernel.org Assisted-by: LLM Signed-off-by: Nikolaos Karaolidis --- Found while working on reclaim of inodes pinned by page cache in ZONE_MOVABLE. Tested in QEMU/KVM on master af32da41b032 (v7.3-rc6-232), with lockdep and KASAN enabled. A small init program makes one unused inode whose page can't be invalidated, lets it reach the head of the inode LRU, opens 2000 other files, does one "echo 2 > /proc/sys/vm/drop_caches" and counts the inodes left via umount; kprobes count the invalidate_mapping_pages() calls: without patch with patch i386 HIGHMEM, file read 2000 of 2000 left, none left, on another CPU 10300 strips, 1 inode 4 strips i386 HIGHMEM, page same same spliced into a pipe x86_64, ext4 -o dax, same same file mmap()ed controls (no leftover, none left none left shadow entry at index 0) 2a0629834cd8^ behaves like the patched kernel, 2a0629834cd8 like master. On 5.15.y x86_64, where every unused inode with page cache is on the LRU, the read-on-another-CPU case blocks the walk as well. With "echo 64M > /sys/fs/cgroup/memory.reclaim" instead of drop_caches, the i386 pipe case does 262450 strips and frees nothing without the patch, 40 strips and frees everything with it. In the DAX case that write takes ~400ms with the patch instead of ~4ms: invalidate_mapping_pages() counts the DAX entry it leaves behind as reclaimed, so unpatched, the 64M are "reclaimed" by re-stripping one inode in a tight loop while nothing is freed. The reproducer is a single C file; I can post it. An LLM was used to check the analysis against the code, write the reproducer and draft the changelog; I have reviewed all of it. fs/inode.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/fs/inode.c b/fs/inode.c index a9d37be390a1..1614a97ce953 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -593,12 +593,11 @@ static void inode_pin_lru_isolating(struct inode *inode) static void inode_unpin_lru_isolating(struct inode *inode) { - spin_lock(&inode->i_lock); + lockdep_assert_held(&inode->i_lock); WARN_ON(!(inode_state_read(inode) & I_LRU_ISOLATING)); inode_state_clear(inode, I_LRU_ISOLATING); /* Called with inode->i_lock which ensures memory ordering. */ inode_wake_up_bit(inode, __I_LRU_ISOLATING); - spin_unlock(&inode->i_lock); } static void inode_wait_for_lru_isolating(struct inode *inode) @@ -981,7 +980,13 @@ static enum lru_status inode_lru_isolate(struct list_head *item, else __count_vm_events(PGINODESTEAL, reap); mm_account_reclaimed_pages(reap); + + spin_lock(&inode->i_lock); + /* The walk restarts here: don't retry leftovers right away */ + if (!mapping_empty(&inode->i_data)) + inode_state_set(inode, I_REFERENCED); inode_unpin_lru_isolating(inode); + spin_unlock(&inode->i_lock); return LRU_RETRY; } base-commit: af32da41b0327b9c6a37856ba82b6760d6c8d10e -- 2.55.0