Re: [PATCH] mm: avoid dirtying shared mappings on mlock

[Suleiman Souhlal <ssouhlal@freebsd.org>]

On Oct 12, 2007, at 7:58 AM, Peter Zijlstra wrote:

> On Fri, 2007-10-12 at 07:53 -0700, Arjan van de Ven wrote:
>> On Fri, 12 Oct 2007 12:50:22 +0200
>>>>> The pages will still be read-only due to dirty tracking, so the
>>>>> first write will still do page_mkwrite().
>>>>
>>>> Which can SIGBUS, no?
>>>
>>> Sure, but that is no different than any other mmap'ed write. I'm not
>>> seeing how an mlocked region is special here.
>>>
>>> I agree it would be nice if mmap'ed writes would have better error
>>> reporting than SIGBUS, but such is life.
>>
>> well... there's another consideration
>> people use mlock() in cases where they don't want to go to the
>> filesystem for paging and stuff as well (think the various iscsi
>> daemons and other things that get in trouble).. those kind of uses
>> really use mlock to avoid
>> 1) IO to the filesystem
>> 2) Needing memory allocations for pagefault like things
>> at least for the more "hidden" cases...
>>
>> prefaulting everything ready pretty much gives them that... letting
>> things fault on demand... nicely breaks that.
>
> Non of that is changed. So I'm a little puzzled as to which side you
> argue.

I think this might change the behavior in case you mlock sparse files.
I guess currently the holes disappear when you mlock them, but with  
the patch the blocks wouldn't get allocated until they get written to.

-- Suleiman

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