Date: Wed, 26 Oct 2016 09:55:47 +0100
From: Lorenzo Stoakes <lstoakes@gmail.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Hugh Dickins <hughd@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/10] mm: adjust get_user_pages* functions to explicitly
 pass FOLL_* flags
Message-ID: <20161026085547.GA3737@lucifer>
References: <20161013002020.3062-1-lstoakes@gmail.com>
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On Tue, Oct 18, 2016 at 05:30:50PM +0200, Michal Hocko wrote:
>I am wondering whether we can go further. E.g. it is not really clear to
>me whether we need an explicit FOLL_REMOTE when we can in fact check
>mm != current->mm and imply that. Maybe there are some contexts which
>wouldn't work, I haven't checked.
>
>Then I am also wondering about FOLL_TOUCH behavior.
>__get_user_pages_unlocked has only few callers which used to be
>get_user_pages_unlocked before 1e9877902dc7e ("mm/gup: Introduce
>get_user_pages_remote()"). To me a dropped FOLL_TOUCH seems
>unintentional. Now that get_user_pages_unlocked has gup_flags argument I
>guess we might want to get rid of the __g-u-p-u version altogether, no?
>
>__get_user_pages is quite low level and imho shouldn't be exported. It's
>only user - kvm - should rather pull those two functions to gup instead
>and export them. There is nothing really KVM specific in them.

I believe I've attacked each of these, other than the use of explicit
FOLL_REMOTE which was explained by Dave.

> I also cannot say I would be entirely thrilled about get_user_pages_locked,
> we only have one user which can simply do lock g-u-p unlock AFAICS.

The principle difference here seems to be the availability of VM_FAULT_RETRY
behaviour (by passing a non-NULL locked argument), and indeed the comments in
gup.c recommends using get_user_pages_locked() if possible (though it seems not
to have been heeded too much :), so I'm not sure if this would be a fruitful
refactor, let me know!