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Date: Thu, 11 Aug 2016 23:16:35 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>, "Huang, Ying" <ying.huang@intel.com>,
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Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression
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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);
}
)