linux-kernel - Re: [PATCH 03/40] fs: Convert alloc_inode

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Date:   Tue, 2 May 2023 15:57:51 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Petr Tesařík <petr@...arici.cz>
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, 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.

> 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.

> 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.