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

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Message-ID: <b4b11c59-975d-26c7-043a-6acddff78dfd@nvidia.com>
Date:   Tue, 30 Mar 2021 20:56:38 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Alistair Popple <apopple@...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 3/30/21 3:56 PM, Alistair Popple wrote:
...
>> +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?
> 

Quite confused by this naming idea. Because: try_to_munlock() returns
void, so a boolean-style name such as "page_mlocked()" is already not a
good fit.

Even more important, though, is that try_to_munlock() is mlock-ing the
page, right? Is there some subtle point I'm missing? It really is doing
an mlock to the best of my knowledge here. Although the kerneldoc
comment for try_to_munlock() seems questionable too:

/**
  * 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.
  */

...because I don't see where, in *this* routine, it clears PG_mlocked!

Obviously we agree that a routine should be named based on what it does,
rather than on who calls it. So I think that still leads to:

      try_to_munlock() --> try_to_mlock()
      try_to_munlock_one() --> try_to_mlock_one()

Sorry if I'm missing something really obvious.

> 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.
> 

This is actually rather bad advice! page_referenced() returns an
int-that-is-really-a-boolean, whereas try_to_munlock(), at least as it
stands now, returns void. Usually when I'm writing a TODO item, I'm in a
hurry, and I think that's what probably happened here, too. :)

>> 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
> 

No objections here. :)

thanks,
-- 
John Hubbard
NVIDIA