linux-kernel - Re: [RFC PATCH] vfs: wrap CONFIG_READ_ONLY_THP_FOR

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Date: Mon, 24 Jun 2024 10:17:12 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: linux-mm@...ck.org
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] vfs: wrap CONFIG_READ_ONLY_THP_FOR_FS-related code
 with an ifdef

On Mon, Jun 24, 2024 at 09:41:34AM +0200, Mateusz Guzik wrote:
> On kernels compiled without this option (which is currently the default
> state) filemap_nr_thps expands to 0.
> 
> do_dentry_open has a big chunk dependent on it, most of which gets
> optimized away, except for a branch and a full fence:
> 
> if (f->f_mode & FMODE_WRITE) {
> [snip]
>         smp_mb();
>         if (filemap_nr_thps(inode->i_mapping)) {
> [snip]
> 	}
> }
> 
> While the branch is pretty minor the fence really does not need to be
> there.
> 

[the To: recipient bounces, thus got dropped]

Now that I sent this I remembered that some of the atomic ops in Linux
provide a full fence regardless of an architecture.

get_write_access uses atomic_inc_unless_negative which qualifies?

  1551 static __always_inline bool
  1552 atomic_inc_unless_negative(atomic_t *v)
  1553 {
  1554 │       kcsan_mb();
  1555 │       instrument_atomic_read_write(v, sizeof(*v));
  1556 │       return raw_atomic_inc_unless_negative(v);
  1557 }

If so, the ifdefed-out smp_mb can be eliminated altogether if I got my
fences right.

On top of that mnt_get_write_access injects another smp_mb() anyway.

> This is a bare-minimum patch which takes care of it until someone(tm)
> cleans this up. Notably it does not conditionally compile other spots
> which issue the matching fence.
> 
> I did not bother benchmarking it, not issuing a spurious full fence in
> the fast path does not warrant justification from perf standpoint.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> ---
> 
> I am not particularly familiar with any of this, the smp_mb in the open
> for write path was sticking out like a sore thumb on code read so I
> figured there may be One Weird Trick to whack it.
> 
> If the stock code is correct as is, then the ifdef as above is fine.
> 
> The ifdefed chunk is big enough that it should probably be its own
> routine. I don't want to bikeshed so I did not go for it.
> 
> For a moment I considered adding filemap_nr_thps_mb which would expand
> to 0 or issue the fence + do the read, but then I figured a routine
> claiming to post a fence and only conditionally do it is misleading at
> best.
> 
> As per the commit message fences in collapse_file remain compiled in.
> It is unclear to me if the code following them is doing anything useful
> on kernels !CONFIG_READ_ONLY_THP_FOR_FS.
> 
> All that said, if there is cosmetic touch ups you want done here, I can
> do them.
> 
> However, a nice full patch would take care of all of the above and I
> have neither the information needed to do it nor the interest to get it,
> so should someone insinst on a full version I'm going to suggest they
> write it themselves. I repeat this is merely a damage control until
> someone sorts thigs out.
> 
>  fs/open.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 28f2fcbebb1b..654c300b3c33 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -980,6 +980,7 @@ static int do_dentry_open(struct file *f,
>  	if ((f->f_flags & O_DIRECT) && !(f->f_mode & FMODE_CAN_ODIRECT))
>  		return -EINVAL;
>  
> +#ifdef CONFIG_READ_ONLY_THP_FOR_FS
>  	/*
>  	 * XXX: Huge page cache doesn't support writing yet. Drop all page
>  	 * cache for this file before processing writes.
> @@ -1007,6 +1008,7 @@ static int do_dentry_open(struct file *f,
>  			filemap_invalidate_unlock(inode->i_mapping);
>  		}
>  	}
> +#endif
>  
>  	return 0;
>  
> -- 
> 2.43.0
>