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

Suren Baghdasaryan
2024-08-13T14:49:53Z

On Mon, Aug 12, 2024 at 11:07 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> 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);

Oh, I completely missed the SLAB_TYPESAFE_BY_RCU for this cache, and
here I was telling Andrii that it's RCU unsafe to access
vma->vm_file... Mea culpa.

>
> 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

I believe you are correct.