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From: Christoph Hellwig <hch@lst.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>,
	Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	Michael Ellerman <mpe@ellerman.id.au>,
	the arch/x86 maintainers <x86@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()
Date: Thu, 3 Sep 2020 09:11:44 +0200	[thread overview]
Message-ID: <20200903071144.GA19247@lst.de> (raw)
In-Reply-To: <CAHk-=wiDCcxuHgENo3UtdFi2QW9B7yXvNpG5CtF=A6bc6PTTgA@mail.gmail.com>

On Wed, Sep 02, 2020 at 11:02:22AM -0700, Linus Torvalds wrote:
> I don't see why this change would make any difference.

Me neither, but while looking at a different project I did spot places
that actually do an access_ok with len 0, that's why I wanted him to
try.

That being said: Christophe are these number stables?  Do you get
similar numbers with multiple runs?

> And btw, why do the 32-bit and 64-bit checks even differ? It's not
> like the extra (single) instruction should even matter. I think the
> main reason is that the simpler 64-bit case could stay as a macro
> (because it only uses "addr" and "size" once), but honestly, that
> "simplification" doesn't help when you then need to have that #ifdef
> for the 32-bit case and an inline function anyway.

I'll have to leave that to the powerpc folks.  The intent was to not
change the behavior (and I even fucked that up for the the size == 0
case).

> However, I suspect a bigger reason for the actual performance
> degradation would be the patch that makes things use "write_iter()"
> for writing, even when a simpler "write()" exists.

Except that we do not actually have such a patch.  For normal user
writes we only use ->write_iter if ->write is not present.  But what
shows up in the profile is that /dev/zero only has a read_iter op and
not a normal read.  I've added a patch below that implements a normal
read which might help a tad with this workload, but should not be part
of a regression.

Also Christophe:  can you bisect which patch starts this?  Is it really
this last patch in the series?

---
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index abd4ffdc8cdebc..1dc99ab158457a 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -726,6 +726,27 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
 	return written;
 }
 
+static ssize_t read_zero(struct file *file, char __user *buf,
+			 size_t count, loff_t *ppos)
+{
+	size_t cleared = 0;
+
+	while (count) {
+		size_t chunk = min_t(size_t, count, PAGE_SIZE);
+
+		if (clear_user(buf + cleared, chunk))
+			return cleared ? cleared : -EFAULT;
+		cleared += chunk;
+		count -= chunk;
+
+		if (signal_pending(current))
+			return cleared ? cleared : -ERESTARTSYS;
+		cond_resched();
+	}
+
+	return cleared;
+}
+
 static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 {
 #ifndef CONFIG_MMU
@@ -921,6 +942,7 @@ static const struct file_operations zero_fops = {
 	.llseek		= zero_lseek,
 	.write		= write_zero,
 	.read_iter	= read_iter_zero,
+	.read		= read_zero,
 	.write_iter	= write_iter_zero,
 	.mmap		= mmap_zero,
 	.get_unmapped_area = get_unmapped_area_zero,

  reply	other threads:[~2020-09-03  7:11 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-27 15:00 remove the last set_fs() in common code, and remove it for x86 and powerpc v2 Christoph Hellwig
2020-08-27 15:00 ` [PATCH 01/10] fs: don't allow kernel reads and writes without iter ops Christoph Hellwig
2020-08-27 15:58   ` David Laight
2020-08-29  9:23     ` 'Christoph Hellwig'
     [not found]   ` <20200901064849.GI4299@shao2-debian>
2020-09-01  7:08     ` [fs] ef30fb3c60: kernel write not supported for file /sys/kernel/softlockup_panic Christoph Hellwig
2020-08-27 15:00 ` [PATCH 02/10] fs: don't allow splice read/write without explicit ops Christoph Hellwig
2020-08-27 15:00 ` [PATCH 03/10] uaccess: add infrastructure for kernel builds with set_fs() Christoph Hellwig
2020-08-27 15:00 ` [PATCH 04/10] test_bitmap: skip user bitmap tests for !CONFIG_SET_FS Christoph Hellwig
2020-08-27 15:00 ` [PATCH 05/10] lkdtm: disable set_fs-based " Christoph Hellwig
2020-08-27 18:06   ` Linus Torvalds
2020-08-29  9:24     ` Christoph Hellwig
2020-09-01 18:52       ` Kees Cook
2020-09-01 18:57       ` Kees Cook
2020-09-02  8:09         ` Christoph Hellwig
2020-08-27 15:00 ` [PATCH 06/10] x86: move PAGE_OFFSET, TASK_SIZE & friends to page_{32,64}_types.h Christoph Hellwig
2020-08-27 15:00 ` [PATCH 07/10] x86: make TASK_SIZE_MAX usable from assembly code Christoph Hellwig
2020-08-27 15:00 ` [PATCH 08/10] x86: remove address space overrides using set_fs() Christoph Hellwig
2020-08-27 18:15   ` Linus Torvalds
2020-08-29  9:25     ` Christoph Hellwig
2020-08-27 15:00 ` [PATCH 09/10] powerpc: use non-set_fs based maccess routines Christoph Hellwig
2020-08-27 15:00 ` [PATCH 10/10] powerpc: remove address space overrides using set_fs() Christoph Hellwig
2020-09-02  6:15   ` Christophe Leroy
2020-09-02 12:36     ` Christoph Hellwig
2020-09-02 13:13       ` David Laight
2020-09-02 13:24         ` Christophe Leroy
2020-09-02 13:51           ` David Laight
2020-09-02 14:12             ` Christophe Leroy
2020-09-02 15:02               ` David Laight
2020-09-02 15:17       ` Christophe Leroy
2020-09-02 18:02         ` Linus Torvalds
2020-09-03  7:11           ` Christoph Hellwig [this message]
2020-09-03  7:27             ` Christophe Leroy
2020-09-03  8:55             ` Christophe Leroy
2020-09-03  7:20           ` Christophe Leroy
2020-08-27 15:31 ` remove the last set_fs() in common code, and remove it for x86 and powerpc v2 Christoph Hellwig
2020-09-01 17:13 ` Christophe Leroy
2020-09-01 17:25   ` Al Viro
2020-09-01 17:42     ` Matthew Wilcox
2020-09-01 18:39     ` Christophe Leroy
2020-09-01 19:01     ` Christophe Leroy
2020-09-02  8:10     ` Christoph Hellwig
2020-10-27  9:29 ` [PATCH 02/10] fs: don't allow splice read/write without explicit ops David Howells
2020-10-27  9:51 ` David Howells
2020-10-27  9:54   ` Christoph Hellwig
2020-10-27 10:38   ` David Howells

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