Subject: Re: [PATCH V4 4/6] mm: mlock: Introduce VM_LOCKONFAULT and add mlock flags to enable it

From: Eric B Munson <[email-masked]>
Date: Wed, 22 Jul 2015 14:43:43 -0400

On Wed, 22 Jul 2015, Vlastimil Babka wrote:

> On 07/21/2015 09:59 PM, Eric B Munson wrote:
> >The cost of faulting in all memory to be locked can be very high when
> >working with large mappings.  If only portions of the mapping will be
> >used this can incur a high penalty for locking.
> >
> >For the example of a large file, this is the usage pattern for a large
> >statical language model (probably applies to other statical or graphical
> >models as well).  For the security example, any application transacting
> >in data that cannot be swapped out (credit card data, medical records,
> >etc).
> >
> >This patch introduces the ability to request that pages are not
> >pre-faulted, but are placed on the unevictable LRU when they are finally
> >faulted in.  This can be done area at a time via the
> >mlock2(MLOCK_ONFAULT) or the mlockall(MCL_ONFAULT) system calls.  These
> >calls can be undone via munlock2(MLOCK_ONFAULT) or
> >munlockall2(MCL_ONFAULT).
> >
> >Applying the VM_LOCKONFAULT flag to a mapping with pages that are
> >already present required the addition of a function in gup.c to pin all
> >pages which are present in an address range.  It borrows heavily from
> >__mm_populate().
> >
> >To keep accounting checks out of the page fault path, users are billed
> >for the entire mapping lock as if MLOCK_LOCKED was used.
> 
> Hi,
> 
> I think you should include a complete description of which
> transitions for vma states and mlock2/munlock2 flags applied on them
> are valid and what they do. It will also help with the manpages.
> You explained some to Jon in the last thread, but I think there
> should be a canonical description in changelog (if not also
> Documentation, if mlock is covered there).
> 
> For example the scenario Jon asked, what happens after a
> mlock2(MLOCK_ONFAULT) followed by mlock2(MLOCK_LOCKED), and that the
> answer is "nothing". Your promised code comment for
> apply_vma_flags() doesn't suffice IMHO (and I'm not sure it's there,
> anyway?).

I missed adding that comment to the code, will be there in V5 along with
the description in the changelog.

> 
> But the more I think about the scenario and your new VM_LOCKONFAULT
> vma flag, it seems awkward to me. Why should munlocking at all care
> if the vma was mlocked with MLOCK_LOCKED or MLOCK_ONFAULT? In either
> case the result is that all pages currently populated are munlocked.
> So the flags for munlock2 should be unnecessary.

Say a user has a large area of interleaved MLOCK_LOCK and MLOCK_ONFAULT
mappings and they want to unlock only the ones with MLOCK_LOCK.  With
the current implementation, this is possible in a single system call
that spans the entire region.  With your suggestion, the user would have
to know what regions where locked with MLOCK_LOCK and call munlock() on
each of them.  IMO, the way munlock2() works better mirrors the way
munlock() currently works when called on a large area of interleaved
locked and unlocked areas.

> 
> I also think VM_LOCKONFAULT is unnecessary. VM_LOCKED should be
> enough - see how you had to handle the new flag in all places that
> had to handle the old flag? I think the information whether mlock
> was supposed to fault the whole vma is obsolete at the moment mlock
> returns. VM_LOCKED should be enough for both modes, and the flag to
> mlock2 could just control whether the pre-faulting is done.
> 
> So what should be IMHO enough:
> - munlock can stay without flags
> - mlock2 has only one new flag MLOCK_ONFAULT. If specified,
> pre-faulting is not done, just set VM_LOCKED and mlock pages already
> present.
> - same with mmap(MAP_LOCKONFAULT) (need to define what happens when
> both MAP_LOCKED and MAP_LOCKONFAULT are specified).
> 
> Now mlockall(MCL_FUTURE) muddles the situation in that it stores the
> information for future VMA's in current->mm->def_flags, and this
> def_flags would need to distinguish VM_LOCKED with population and
> without. But that could be still solvable without introducing a new
> vma flag everywhere.

With you right up until that last paragraph.  I have been staring at
this a while and I cannot come up a way to handle the
mlockall(MCL_ONFAULT) without introducing a new vm flag.  It doesn't
have to be VM_LOCKONFAULT, we could use the model that Michal Hocko
suggested with something like VM_FAULTPOPULATE.  However, we can't
really use this flag anywhere except the mlock code becuase we have to
be able to distinguish a caller that wants to use MLOCK_LOCK with
whatever control VM_FAULTPOPULATE might grant outside of mlock and a
caller that wants MLOCK_ONFAULT.  That was a long way of saying we need
an extra vma flag regardless.  However, if that flag only controls if
mlock pre-populates it would work and it would do away with most of the
places I had to touch to handle VM_LOCKONFAULT properly.

I picked VM_LOCKONFAULT because it is explicit about what it is for and
there is little risk of someone coming along in 5 years and saying "why
not overload this flag to do this other thing completely unrelated to
mlock?".  A flag for controling speculative population is more likely to
be overloaded outside of mlock().

If you have a sane way of handling mlockall(MCL_ONFAULT) without a new
VMA flag, I am happy to give it a try, but I haven't been able to come
up with one that doesn't have its own gremlins.