linux-kernel - Re: [PATCH] erofs: fix file-backed mounts over FUSE

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Message-ID: <20241114063422.GM3387508@ZenIV>
Date: Thu, 14 Nov 2024 06:34:22 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: linux-erofs@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
	syzbot+0b1279812c46e48bb0c1@...kaller.appspotmail.com
Subject: Re: [PATCH] erofs: fix file-backed mounts over FUSE

On Thu, Nov 14, 2024 at 02:23:27PM +0800, Gao Xiang wrote:

> > 3) AFAICS, (buf->kmap_type == EROFS_KMAP) == (buf->base != NULL).  What's
> > the point of having that as a separate field?
> 
> Once buf->kmap_type has EROFS_KMAP and EROFS_KMAP_ATOMIC, but it
> seems that it can be cleaned up now.
> 
> I will clean up later but it's a seperate story.
> 
> > 
> > 4) Why bother with union?  Just have buf->file serve as your buf->use_fp
> > and be done with that...
> 
> I'd like to leave `struct erofs_buf` as small as possible since
> it's on stack.

enum + bool eats at least as much as a pointer, and if it's on stack...
an extra word is really noise - it's not as if you had a plenty of
those in the current call chain at any given point.