Re: [PATCH] fs: rotate inodes whose page cache could not be dropped

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2026/10/16 18:25, Nikolaos Karaolidis 写道:
> 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 <nick@karaolidis.com>
> ---
> 
> 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(-)
> 
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
> 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
>