linux-kernel - Re: [PATCH RFC v3 12/13] mm: add SLAB_TYPESAFE_BY_RCU to files

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Message-ID: <CAJuCfpFrP-UpMih2j=Nxx=QSQm2k3QtJScLKniM9aXjbo5jCDw@mail.gmail.com>
Date: Tue, 13 Aug 2024 07:49:39 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org, 
	peterz@...radead.org, oleg@...hat.com, rostedt@...dmis.org, 
	mhiramat@...nel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	jolsa@...nel.org, paulmck@...nel.org, willy@...radead.org, 
	akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC v3 12/13] mm: add SLAB_TYPESAFE_BY_RCU to files_cache

On Mon, Aug 12, 2024 at 11:07 PM Mateusz Guzik <mjguzik@...il.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@...radead.org>
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.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.