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

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Date:   Wed, 9 Feb 2022 22:01:24 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yu Zhao <yuzhao@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 00/13] mm/munlock: rework of mlock+munlock page handling

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.
 
> 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.

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.
-- 
Michal Hocko
SUSE Labs