Re: [PATCH RFC v3 12/13] mm: add SLAB_TYPESAFE_BY_RCU to files_cache

Mateusz Guzik
2024-08-13T06:07:40Z

On Mon, Aug 12, 2024 at 09:29:16PM -0700, Andrii Nakryiko wrote:
> Add RCU protection for file struct's backing memory by adding
> SLAB_TYPESAFE_BY_RCU flag to files_cachep. This will allow to locklessly
> access struct file's fields under RCU lock protection without having to
> take much more expensive and contended locks.
>
> This is going to be used for lockless uprobe look up in the next patch.
>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
> kernel/fork.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 76ebafb956a6..91ecc32a491c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -3157,8 +3157,8 @@ void __init proc_caches_init(void)
> NULL);
> files_cachep = kmem_cache_create("files_cache",
> sizeof(struct files_struct), 0,
> - SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
> - NULL);
> + SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_TYPESAFE_BY_RCU|
> + SLAB_ACCOUNT, NULL);
> fs_cachep = kmem_cache_create("fs_cache",
> sizeof(struct fs_struct), 0,
> SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,

Did you mean to add it to the cache backing 'struct file' allocations?

That cache is created in fs/file_table.c and already has the flag:
filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
SLAB_TYPESAFE_BY_RCU | SLAB_HWCACHE_ALIGN |
SLAB_PANIC | SLAB_ACCOUNT, NULL);

The cache you are modifying in this patch contains the fd array et al
and is of no consequence to "uprobes: add speculative lockless VMA to
inode resolution".

iow this patch needs to be dropped