From: Christoph Hellwig <hch@lst.de>
To: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Christoph Hellwig <hch@lst.de>,
Johannes Berg <johannes@sipsolutions.net>,
richard@nod.at, linux-um@lists.infradead.org
Subject: Re: [PATCH, RFC] um: remove set_fs
Date: Mon, 23 Aug 2021 14:17:41 +0200 [thread overview]
Message-ID: <20210823121741.GA12605@lst.de> (raw)
In-Reply-To: <169368f3-1651-8b5c-d979-6ec9e060f6ca@cambridgegreys.com>
On Mon, Aug 23, 2021 at 09:28:37AM +0100, Anton Ivanov wrote:
> In first read - you do not want to do that.
>
> buffer_op is slow (even with all the tweaks we have done to that). It is one of the reasons UML userspace is slower than it should be.
>
> The primary reason for that is that it never copies more than a page at a time and pages in/out a page at a time.
>
> We retain reasonable kernel speed because we bypass it for kernel - "if (uaccess_kernel())".
>
> Unless I am missing something, this change will use the slow path currently used for userspace for the kernel.
uaccess_kernel() is not the fast path, it is the exception path when
someone uses the uaccess helper on kernel pointer after doing a
set_fs(KERNEL_DS). And this path is entirel gone once CONFIG_SET_FS
is not set and replaced with calls to __get_kernel_nofault /
__put_kernel_nofault.
In other words - very little generated code actually changes with
this patch, except for the context switch path, which gets simplified.
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next prev parent reply other threads:[~2021-08-23 12:17 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-09 13:38 [PATCH, RFC] um: remove set_fs Christoph Hellwig
2021-07-09 14:47 ` Johannes Berg
2021-07-09 16:57 ` Christoph Hellwig
2021-08-23 7:04 ` Christoph Hellwig
2021-08-23 7:14 ` Anton Ivanov
2021-08-23 8:28 ` Anton Ivanov
2021-08-23 12:17 ` Christoph Hellwig [this message]
2021-08-23 12:29 ` Anton Ivanov
2021-08-26 19:15 ` Richard Weinberger
2021-08-26 21:12 ` Anton Ivanov
2021-08-26 22:04 ` Richard Weinberger
2021-08-27 5:40 ` hch
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