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

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Message-ID: <bce0605a-336f-99ba-5b65-a8e5a7e49e00@nvidia.com>
Date:   Tue, 30 Mar 2021 15:43:19 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alistair Popple <apopple@...dia.com>
CC:     <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 3:24 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.
> 

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.

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

thanks,
-- 
John Hubbard
NVIDIA