linux-kernel - Re: [PATCH 4/5] mm: shmem: Convert shmem_enabled_show to use sysfs_emit

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Message-ID: <20201102133343.GA1011963@kroah.com>
Date:   Mon, 2 Nov 2020 14:33:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Joe Perches <joe@...ches.com>, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] mm: shmem: Convert shmem_enabled_show to use
 sysfs_emit_at

On Sun, Nov 01, 2020 at 10:06:04PM +0000, Matthew Wilcox wrote:
> On Sun, Nov 01, 2020 at 01:43:13PM -0800, Joe Perches wrote:
> > > Why did you change this?
> > 
> > Are you asking about the function argument alignment or the commit message?
> 
> The indentation.  Don't change the fucking indentation, Joe.
> 
> > > Look, this isn't performance sensitive code.  Just do something simple.
> > > 
> > > 		if (shmem_huge == values[i])
> > > 			buf += sysfs_emit(buf, "[%s]",
> > > 					shmem_format_huge(values[i]));
> > > 		else
> > > 			buf += sysfs_emit(buf, "%s",
> > > 					shmem_format_huge(values[i]));
> > > 		if (i == ARRAY_SIZE(values) - 1)
> > > 			buf += sysfs_emit(buf, "\n");
> > > 		else
> > > 			buf += sysfs_emit(buf, " ");
> > > 
> > > Shame there's no sysfs_emitc, but there you go.
> > 
> > I think what's there is simple.
> 
> Again, you're wrong.
> 
> > And your suggested code doesn't work.
> > sysfs_emit is used for single emits.
> > sysfs_emit_at is used for multiple emits.
> 
> Oh, ugh, sysfs_emit() should be able to work on a buffer that isn't
> page aligned.  Greg, how about this?

How can sysfs_emit() be called on a non-page-aligned buffer?  It's being
used on the buffer that was passed to the sysfs call.

And if you are writing multiple values to a single sysfs file output,
well, not good...

> 
> +++ b/fs/sysfs/file.c
> @@ -722,13 +722,13 @@ int sysfs_emit(char *buf, const char *fmt, ...)
>  {
>         va_list args;
>         int len;
> +       int start = offset_in_page(buf);
>  
> -       if (WARN(!buf || offset_in_page(buf),
> -                "invalid sysfs_emit: buf:%p\n", buf))
> +       if (WARN(!buf, "invalid sysfs_emit: buf:%p\n", buf))
>                 return 0;
>  
>         va_start(args, fmt);
> -       len = vscnprintf(buf, PAGE_SIZE, fmt, args);
> +       len = vscnprintf(buf, PAGE_SIZE - start, fmt, args);

That's a bit too 'tricky' for my taste, why is it somehow needed?

thanks,

greg k-h