linux-kernel - Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY

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Message-ID: <e8db013bf06d2170dc48a8252c7049c6d1ee277a.camel@xry111.site>
Date: Tue, 25 Jun 2024 22:58:35 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Huacai Chen <chenhuacai@...nel.org>, Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	io-uring@...r.kernel.org, axboe@...nel.dk, torvalds@...ux-foundation.org, 
	loongarch@...ts.linux.dev
Subject: Re: [PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

On Tue, 2024-06-25 at 22:09 +0800, Huacai Chen wrote:
> On Tue, Jun 25, 2024 at 7:01 PM Mateusz Guzik <mjguzik@...il.com>
> wrote:
> > 
> > The newly used helper also checks for 0-sized buffers.
> > 
> > This avoids path lookup code, lockref management, memory allocation
> > and
> > in case of NULL path userspace memory access (which can be quite
> > expensive with SMAP on x86_64).
> > 
> > statx with AT_EMPTY_PATH paired with "" or NULL argument as
> > appropriate
> > issued on Sapphire Rapids (ops/s):
> > stock:     4231237
> > 0-check:   5944063 (+40%)
> > NULL path: 6601619 (+11%/+56%)
> > 
> > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> Hi, Ruoyao,
> 
> I'm a bit confused. Ii this patch a replacement of your recent patch?

Yes, both Linus and Christian hates introducing a new AT_ flag for this.

This patch just makes statx(fd, NULL, AT_EMPTY_PATH, ...) behave like
statx(fd, "", AT_EMPTY_PATH, ...) instead.  NULL avoids the performance
issue and it's also audit-able by seccomp BPF.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University