linux-kernel - Re: [PATCH] proc: fixup PDE allocation bloat

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Date:   Fri, 17 Aug 2018 16:28:36 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     dhowells@...hat.com, Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] proc: fixup PDE allocation bloat

On Thu, Jul 19, 2018 at 05:06:55PM -0700, Shakeel Butt wrote:
> On Sun, Jun 17, 2018 at 2:57 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
> >
> > commit 24074a35c5c975c94cd9691ae962855333aac47f
> > ("proc: Make inline name size calculation automatic")
> > started to put PDE allocations into kmalloc-256 which is unnecessary as
> > ~40 character names are very rare.
> >
> > Put allocation back into kmalloc-192 cache for 64-bit non-debug builds.
> >
> > Put BUILD_BUG_ON to know when PDE size is gotten out of control.
> >
> > Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> > ---
> >
> >  fs/proc/inode.c    |    6 ++++--
> >  fs/proc/internal.h |   17 +++++++----------
> >  2 files changed, 11 insertions(+), 12 deletions(-)
> >
> > --- a/fs/proc/inode.c
> > +++ b/fs/proc/inode.c
> > @@ -105,8 +105,10 @@ void __init proc_init_kmemcache(void)
> >                 kmem_cache_create("pde_opener", sizeof(struct pde_opener), 0,
> >                                   SLAB_ACCOUNT|SLAB_PANIC, NULL);
> >         proc_dir_entry_cache = kmem_cache_create_usercopy(
> > -               "proc_dir_entry", SIZEOF_PDE_SLOT, 0, SLAB_PANIC,
> > -               OFFSETOF_PDE_NAME, SIZEOF_PDE_INLINE_NAME, NULL);
> > +               "proc_dir_entry", SIZEOF_PDE, 0, SLAB_PANIC,
> 
> Hi Alexey, can you comment if proc_dir_entry_cache should or shouldn't
> have SLAB_ACCOUNT flag?

It should not (but see below):

SLAB_ACCOUNT is for allocations which can be done by userspace directly:
open(2) directly allocates "struct file".

But /proc entries aren't like that: say, /proc/cpuinfo is created by kernel
and userspace can't do anything about it.

Some subsystems create /proc entries based on userspace actions and
those aren't related to hardware (example: xt_hashlimit.c) but those are
few so kernel doesn't bother accounting those.

Or in other words: user can't mkdir(1) and touch(1) and ln(1) inside /proc
at will and therefore PDEs aren't accounted.