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

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Date:   Sun, 1 Nov 2020 22:06:04 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Joe Perches <joe@...ches.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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 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?

+++ 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);
        va_end(args);
 
        return len;