linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Message-ID: <20160811221634.GU2356@ZenIV.linux.org.uk>
Date:	Thu, 11 Aug 2016 23:16:35 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@....de>,
	"Huang, Ying" <ying.huang@...el.com>,
	Dave Chinner <david@...morbit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, LKP <lkp@...org>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

On Thu, Aug 11, 2016 at 01:35:00PM -0700, Linus Torvalds wrote:

> The thing is, iov_iter_copy_from_user_atomic() doesn't itself enforce
> non-blocking user accesses, it depends on the caller blocking page
> faults.

Huh?  The very first thing it does is
        char *kaddr = kmap_atomic(page), *p = kaddr + offset;

If _that_ does not disable pagefaults, we are very deep in shit.  AFAICS,
all instances of that sucker do disable those, including the non-highmem
default (it's
static inline void *kmap_atomic(struct page *page)
{
        preempt_disable();
        pagefault_disable();
        return page_address(page);
}
)