Re: [PATCH 00/13] mm/munlock: rework of mlock+munlock page handling

[Hugh Dickins]

On Wed, 9 Feb 2022, Michal Hocko wrote:
> On Wed 09-02-22 08:21:17, Hugh Dickins wrote:
> > On Wed, 9 Feb 2022, Michal Hocko wrote:
> [...]
> > > The only thing that is not entirely clear to me at the moment is why you
> > > have chosen to ignore already mapped LOCKONFAULT pages. They will
> > > eventually get sorted out during the reclaim/migration but this can
> > > backfire if too many pages have been pre-faulted before LOCKONFAULT
> > > call. Maybe not an interesting case in the first place but I am still
> > > wondering why you have chosen that way.
> > 
> > I'm puzzled: what makes you think I'm ignoring already mapped LOCKONFAULT
> > pages?  I'd consider that a bug.
> 
> I've had the following path in mind
> __mm_populate
>   populate_vma_page_range
>     if (vma->vm_flags & VM_LOCKONFAULT)
>     	return nr_page
> 
> which means that __get_user_pages is not called at all. This also means
> that follow_page_mask is skipped. The page table walk used to mark
> already mapped pages as mlocked so unless I miss some other path it
> would stay on its original LRU list and only get real mlock protection
> when encountered by the reclaim or migration.

That is so until 04/13: but then, whenever a page is faulted into a
VM_LOCKED area (except when skipping pte-of-compound) it goes through
mlock_page().  So it will then lock-on-fault.

Ah, but you're worrying about any previously-mapped pages when
VM_LOCKONFAULT is established: those ones get done by mlock_pte_range()
in 07/13, same as when VM_LOCKED is established - I checked again,
VM_LOCKONFAULT is not set without VM_LOCKED.

>  
> > It is the case, isn't it, that a VM_LOCKONFAULT area always has VM_LOCKED
> > set too?  If I've got that wrong, yes, I'll need to revisit conditions.
> 
> Yes, I did't really mean we would lose the mlock protection. We would
> just do the lazy mlock also on pages which are already mapped. This is
> certainly not a correctness issue - althoug stats might be off - but it
> could polute existing LRUs with mlocked pages rather easily.

Even with the lazy mlock in reclaim, I'd still accuse myself of a bug
if I've got this wrong.

> 
> As I've said. Either I am still missing something or this might even not
> be a big deal in real life. I was mostly curious about the choice to
> exclude the page table walk for LOCKONFAULT.

It surprised me too: but looking at how FOLL_POPULATE was handled in
faultin_page(), it appeared to me to become redundant, once mlocking
was counted at fault time (and in mlock_pte_range() - maybe that's
the page table walk you're looking for, not excluded but moved).

It is a big deal for me: please don't let me fool you, please do
continue to worry at it until you're convinced or you convince me.

Thanks,
Hugh