Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 17 Jan 2024 at 14:56, Shakeel Butt <shakeelb@google.com> wrote:
> >
> > So I don't see how we can make it really cheap (say, less than 5% overhead)
> > without caching pre-accounted objects.
>
> Maybe this is what we want. Now we are down to just SLUB, maybe such
> caching of pre-accounted objects can be in SLUB layer and we can
> decide to keep this caching per-kmem-cache opt-in or always on.

So it turns out that we have another case of SLAB_ACCOUNT being quite
a big expense, and it's actually the normal - but failed - open() or
execve() case.

See the thread at

    https://lore.kernel.org/all/CAHk-=whw936qzDLBQdUz-He5WK_0fRSWwKAjtbVsMGfX70Nf_Q@mail.gmail.com/

and to see the effect in profiles, you can use this EXTREMELY stupid
test program:

    #include <fcntl.h>

    int main(int argc, char **argv)
    {
        for (int i = 0; i < 10000000; i++)
                open("nonexistent", O_RDONLY);
    }

where the point of course is that the "nonexistent" pathname doesn't
actually exist (so don't create a file called that for the test).

What happens is that open() allocates a 'struct file *' early from the
filp kmem_cache, which has SLAB_ACCOUNT set. So we'll do accounting
for it, failt the pathname open, and free it again, which uncharges
the accounting.

Now, in this case, I actually have a suggestion: could we please just
make SLAB_ACCOUNT be something that we do *after* the allocation, kind
of the same way the zeroing works?

IOW, I'd love to get rid of slab_pre_alloc_hook() entirely, and make
slab_post_alloc_hook() do all the "charge the memcg if required".

Obviously that means that now a failure to charge the memcg would have
to then de-allocate things, but that's an uncommon path and would be
marked unlikely and not be in the hot path at all.

Now, the reason I would prefer that is that the *second* step would be to

 (a) expose a "kmem_cache_charge()" function that takes a
*non*-accounted slab allocation, and turns it into an accounted one
(and obviously this is why you want to do everything in the post-alloc
hook: just try to share this code)

 (b) remote the SLAB_ACCOUNT from the filp_cachep, making all file
allocations start out unaccounted.

 (c) when we have *actually* looked up the pathname and open the file
successfully, at *that* point we'd do a

        error = kmem_cache_charge(filp_cachep, file);

    in do_dentry_open() to turn the unaccounted file pointer into an
accounted one (and if that fails, we do the cleanup and free it, of
course, exactly like we do when file_get_write_access() fails)

which means that now the failure case doesn't unnecessarily charge the
allocation that never ends up being finalized.

NOTE! I think this would clean up mm/slub.c too, simply because it
would get rid of that memcg_slab_pre_alloc_hook() entirely, and get
rid of the need to carry the "struct obj_cgroup **objcgp" pointer
along until the post-alloc hook: everything would be done post-alloc.

The actual kmem_cache_free() code already deals with "this slab hasn't
been accounted" because it obviously has to deal with allocations that
were done without __GFP_ACCOUNT anyway. So there's no change needed on
the freeing path, it already has to handle this all gracefully.

I may be missing something, but it would seem to have very little
downside, and fix a case that actually is visible right now.

              Linus