RE: Users locking memory using futexes

[Perez-Gonzalez, Inaky]

> > Good thing is - I just found out after reading twice - that 
> FUTEX_FD does
> > not lock the page in memory, so that is one case less to 
> worry about. 
> 
> Oh yes it does - the page isn't unpinned until wakeup or close.
> See where it says in futex_fd():
> 
> 	page = NULL;
> out:
> 	if (page)
> 		unpin_page(page);

Bang, bang, bang ... assshoooole [hearing whispers in my ears]. Great point:
Inaky 0, Jamie 1 - this will teach me to read _three_ times on Monday
evenings. I am supposed to know all that code by heart ... oh well.

> Rusty's got a good point about pipe() though.

He does; grumble, grumble ... let's see ... with pipe you have an implicit
limit that controls you, the number of open files, that you also hit with
futex_fd() (in ... get_unused_fd()) - so that is covered. OTOH, with just
futex_wait(), if you are up to use one page per futex you lock on, you are
also limited by RLIMIT_NPROC for every process you lock on [asides from
wasting a lot of memory], so looks like there is another roadblock there to
control it.

Hum ... still I want to try Ingo's approach on the ptes; that is the part I
was missing [knowing that struct page * is not invariant as the pte number
... even being as obvious as it is].

Inaky Perez-Gonzalez -- Not speaking for Intel - opinions are my own [or my
fault]