linux-kernel - Re: [RFC PATCH 1/2] vmsplice unmap gifted pages for recipient

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Date:	Fri, 26 Jul 2013 09:47:26 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matt Helsley <matt.helsley@...il.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Michael Roth <mdroth@...ux.vnet.ibm.com>,
	Lei Li <lilei@...ux.vnet.ibm.com>,
	Leonardo Garcia <lagarcia@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 1/2] vmsplice unmap gifted pages for recipient

On 07/26/2013 08:16 AM, Robert Jennings wrote:
>>> > > +				if ((spd->flags & SPLICE_F_MOVE) &&
>>> > > +				    !buf->offset && (buf->len == PAGE_SIZE))
>>> > > +					/* Can move page aligned buf */
>>> > > +					zap_buf_page(useraddr);
>>> > >  				buf->flags |= PIPE_BUF_FLAG_GIFT;
>>> > > +			}
>> > 
>> > There isn't quite enough context here, but is it going to do this
>> > zap_buf_page() very often?  Seems a bit wasteful to do the up/down and
>> > find_vma() every trip through the loop.
> The call to zap_buf_page() is in a loop where each pipe buffer is being
> processed, but in that loop we have a pipe_wait() where we schedule().
> So as things are structured I don't have the ability to hold mmap_sem
> for multiple find_vma() calls.

You can hold a semaphore over a schedule(). :)

You could also theoretically hold mmap_sem and only drop it on actual
cases when you reschedule if you were afraid of holding mmap_sem for
long periods of time (even though it's a read).


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