On 12/16/2015 07:50 PM, Jörn Engel wrote:
> On Wed, Dec 16, 2015 at 07:44:16PM +0100, Michael Kerrisk (man7.org) wrote:
>> On 28 October 2015 at 02:05, Jörn Engel <joern@xxxxxxxxxxxxxxx> wrote:
>>> Hello Michael!
>>>
>>> Just came across this. When reading the manpage MLOCK(2), I assume that
>>> mlockall(MCL_FUTURE) does _not_ imply MAP_POPULATE. But when reading
>>> the code, I see that it does.
>>>
>>> This little detail can be rather crucial for RT-people, so it might be
>>> worth spelling it out explicitly in the manpage.
>>
>> But, this detail doesn't seem so surprising to me. MCL_FUTURE == new
>> pages that are mapped will be locked. Of course they must be populated
>> into memory when the mapping is created. Or: to put it another way,
>> maybe it would help if you explain why you find the behavior
>> surprising. That might give me a clue about what should be fixed in
>> the man page.
>>
>> But, please take this thread to mtk.manpages@xxxxxxxxx, and cc
>> linux-man@xxxxxxxxxxxxxxx.
>
> Done. :)
>
> I guess it is a judgement call now. One fool (me) made the wrong
> assumption and had to a) get into an argument with a coworker and b)
> check the code to realize the mistake. If fools like me are common, it
> might be worth making this point explicit. If I am the oddball, it
> would be wasting file size and reading time for everyone else.
So, I just realized that a recent change to the API, plus its
associated documentation in the mlock(2) pages actually probably
lessens the chance of this mistake. The next release of man-pages
will include documentation of MCL_ONFAULT:
MCL_CURRENT Lock all pages which are currently mapped into the
address space of the process.
MCL_FUTURE Lock all pages which will become mapped into the
address space of the process in the future. These
could be, for instance, new pages required by a
growing heap and stack as well as new memory-mapped
files or shared memory regions.
MCL_ONFAULT (since Linux 4.4)
Used together with MCL_CURRENT, MCL_FUTURE, or
both. Mark all current (with MCL_CURRENT) or
future (with MCL_FUTURE) mappings to lock pages
when they are faulted in. When used with MCL_CUR‐
RENT, all present pages are locked, but mlockall()
will not fault in non-present pages. When used
with MCL_FUTURE, all future mappings will be marked
to lock pages when they are faulted in, but they
will not be populated by the lock when the mapping
is created. MCL_ONFAULT must be used with either
MCL_CURRENT or MCL_FUTURE or both.
Do you think that text helps?
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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