linux-kernel - Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to

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Date:   Wed, 31 Mar 2021 09:56:20 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
CC:     Jason Gunthorpe <jgg@...dia.com>, <linux-mm@...ck.org>,
        <nouveau@...ts.freedesktop.org>, <bskeggs@...hat.com>,
        <akpm@...ux-foundation.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kvm-ppc@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <rcampbell@...dia.com>,
        <jglisse@...hat.com>, <hch@...radead.org>, <daniel@...ll.ch>,
        <willy@...radead.org>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap

On Wednesday, 31 March 2021 9:43:19 AM AEDT John Hubbard wrote:
> On 3/30/21 3:24 PM, Jason Gunthorpe wrote:
> ...
> >> As far as I can tell this has always been called try_to_munlock() even 
though
> >> it appears to do the opposite.
> > 
> > Maybe we should change it then?
> > 
> >>> /**
> >>>   * try_to_munlock - try to munlock a page
> >>>   * @page: the page to be munlocked
> >>>   *
> >>>   * Called from munlock code.  Checks all of the VMAs mapping the page
> >>>   * to make sure nobody else has this page mlocked. The page will be
> >>>   * returned with PG_mlocked cleared if no other vmas have it mlocked.
> >>>   */
> >>
> >> In other words it sets PG_mlocked if one or more vmas has it mlocked. So
> >> try_to_mlock() might be a better name, except that seems to have the 
potential
> >> for confusion as well because it's only called from the munlock code path 
and
> >> never for mlock.
> > 
> > That explanation makes more sense.. This function looks like it is
> > 'set PG_mlocked of the page if any vm->flags has VM_LOCKED'
> > 
> > Maybe call it check_vm_locked or something then and reword the above
> > comment?
> > 
> > (and why is it OK to read vm->flags for this without any locking?)
> > 
> >>> Something needs attention here..
> >>
> >> I think the code is correct, but perhaps the naming could be better. 
Would be
> >> interested hearing any thoughts on renaming try_to_munlock() to 
try_to_mlock()
> >> as the current name appears based on the context it is called from 
(munlock)
> >> rather than what it does (mlock).
> > 
> > The point of this patch is to make it clearer, after all, so I'd
> > change something and maybe slightly clarify the comment.
> > 

Yep, agree with that.
 
> I'd add that, after looking around the calling code, this is a really 
unhappy
> pre-existing situation. Anyone reading this has to remember at which point 
in the
> call stack the naming transitions from "do the opposite of what the name 
says",
> to "do what the name says".
>
> +1 for renaming "munlock*" items to "mlock*", where applicable. good grief.

At least the situation was weird enough to prompt further investigation :) 

Renaming to mlock* doesn't feel like the right solution to me either though. I 
am not sure if you saw me responding to myself earlier but I am thinking 
renaming try_to_munlock() -> page_mlocked() and try_to_munlock_one() -> 
page_mlock_one() might be better. Thoughts?

This is actually inspired from a suggestion in Documentation/vm/unevictable-
lru.rst which warns about this problem:

try_to_munlock() Reverse Map Scan
---------------------------------

.. warning::
   [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
   page_referenced() reverse map walker.

> Although, it seems reasonable to tack such renaming patches onto the tail 
end
> of this series. But whatever works.

Unless anyone objects strongly I will roll the rename into this patch as there 
is only one caller of try_to_munlock.

 - Alistair

> thanks,
>