Message-ID: <20230502222038.57a47a85@meshulam.tesarici.cz>
Date:   Tue, 2 May 2023 22:20:38 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 03/40] fs: Convert alloc_inode_sb() to a macro

On Tue, 2 May 2023 15:57:51 -0400
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> On Tue, May 02, 2023 at 02:35:30PM +0200, Petr Tesařík wrote:
> > On Mon,  1 May 2023 09:54:13 -0700
> > Suren Baghdasaryan <surenb@...gle.com> wrote:
> >   
> > > From: Kent Overstreet <kent.overstreet@...ux.dev>
> > > 
> > > We're introducing alloc tagging, which tracks memory allocations by
> > > callsite. Converting alloc_inode_sb() to a macro means allocations will
> > > be tracked by its caller, which is a bit more useful.
> > > 
> > > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > > ---
> > >  include/linux/fs.h | 6 +-----
> > >  1 file changed, 1 insertion(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index 21a981680856..4905ce14db0b 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -2699,11 +2699,7 @@ int setattr_should_drop_sgid(struct mnt_idmap *idmap,
> > >   * This must be used for allocating filesystems specific inodes to set
> > >   * up the inode reclaim context correctly.
> > >   */
> > > -static inline void *
> > > -alloc_inode_sb(struct super_block *sb, struct kmem_cache *cache, gfp_t gfp)
> > > -{
> > > -	return kmem_cache_alloc_lru(cache, &sb->s_inode_lru, gfp);
> > > -}
> > > +#define alloc_inode_sb(_sb, _cache, _gfp) kmem_cache_alloc_lru(_cache, &_sb->s_inode_lru, _gfp)  
> > 
> > Honestly, I don't like this change. In general, pre-processor macros
> > are ugly and error-prone.  
> 
> It's a one line macro, it's fine.

It's not the same. A macro effectively adds a keyword, because it gets
expanded regardless of context; for example, you can't declare a local
variable called alloc_inode_sb, and the compiler errors may be quite
confusing at first. See also the discussion about patch 19/40 in this
series.

> > Besides, it works for you only because __kmem_cache_alloc_lru() is
> > declared __always_inline (unless CONFIG_SLUB_TINY is defined, but then
> > you probably don't want the tracking either). In any case, it's going
> > to be difficult for people to understand why and how this works.  
> 
> I think you must be confused. kmem_cache_alloc_lru() is a macro, and we
> need that macro to be expanded at the alloc_inode_sb() callsite. It's
> got nothing to do with whether or not __kmem_cache_alloc_lru() is inline
> or not.

Oh no, I am not confused. Look at the definition of
kmem_cache_alloc_lru():

void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
			   gfp_t gfpflags)
{
	return __kmem_cache_alloc_lru(s, lru, gfpflags);
}

See? No _RET_IP_ here. That's because it's here:

static __fastpath_inline
void *__kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
			     gfp_t gfpflags)
{
	void *ret = slab_alloc(s, lru, gfpflags, _RET_IP_, s->object_size);

	trace_kmem_cache_alloc(_RET_IP_, ret, s, gfpflags, NUMA_NO_NODE);

	return ret;
}

Now, if __kmem_cache_alloc_lru() is not inlined, then this _RET_IP_
will be somewhere inside kmem_cache_alloc_lru(), which is not very
useful.

But what is __fastpath_inline? Well, it depends:

#ifndef CONFIG_SLUB_TINY
#define __fastpath_inline __always_inline
#else
#define __fastpath_inline
#endif

In short, if CONFIG_SLUB_TINY is defined, it's up to the C compiler
whether __kmem_cache_alloc_lru() is inlined or not.

> > If the actual caller of alloc_inode_sb() is needed, I'd rather add it
> > as a parameter and pass down _RET_IP_ explicitly here.  
> 
> That approach was considered, but adding an ip parameter to every memory
> allocation function would've been far more churn.

See my reply to patch 19/40. Rename the original function, but add an
__always_inline function with the original signature, and let it take
care of _RET_IP_.

Petr T

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