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

Andrii Nakryiko
2024-08-13T18:16:00Z

On Tue, Aug 13, 2024 at 7:49 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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?

Yep, thanks for catching this!

> >
> > 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.
>

Well, my bad for not double-checking and going just by the name.
filp_cachep vs files_cachep is easy to mix up.

> >
> > 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.
>

I'm happy that we already have SLAB_TYPESAFE_BY_RCU on filp_cachep,
I'll just drop this patch.