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

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Message-ID: <20201102143259.GA1024551@kroah.com>
Date:   Mon, 2 Nov 2020 15:32:59 +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 Mon, Nov 02, 2020 at 02:08:36PM +0000, Matthew Wilcox wrote:
> On Mon, Nov 02, 2020 at 02:33:43PM +0100, Greg Kroah-Hartman wrote:
> > > 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...
> 
> See shmem_enabled_show() in mm/shmem.c (output at
> /sys/kernel/mm/transparent_hugepage/shmem_enabled on your machine).
> 
> I don't claim it's a good interface, but it exists.

Ok, that's a common pattern for sysfs files, not that bad.

What's wrong with using sysfs_emit_at()?  We want sysfs_emit() to "know"
that the buffer is PAGE_SIZE big, if you try to allow offsets in it,
that defeats the purpose of the check.

Or am I missing something else here?

thanks,

greg k-h