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Date: Wed, 26 Oct 2016 08:59:52 +0100
From: Lorenzo Stoakes <lstoakes@gmail.com>
To: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, Jan Kara <jack@suse.cz>,
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        Andrew Morton <akpm@linux-foundation.org>,
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        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: remove unnecessary __get_user_pages_unlocked() calls
Message-ID: <20161026075952.GA30977@lucifer>
References: <20161025233609.5601-1-lstoakes@gmail.com>
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On Wed, Oct 26, 2016 at 12:36:09AM +0100, Lorenzo Stoakes wrote:
> In hva_to_pfn_slow() we are able to replace __get_user_pages_unlocked() with
> get_user_pages_unlocked() since we can now pass gup_flags.
>
> In async_pf_execute() we need to pass different tsk, mm arguments so
> get_user_pages_remote() is the sane replacement here (having added manual
> acquisition and release of mmap_sem.)
>
> Since we pass a NULL pages parameter the subsequent call to
> __get_user_pages_locked() will have previously bailed any attempt at
> VM_FAULT_RETRY, so we do not change this behaviour by using
> get_user_pages_remote() which does not invoke VM_FAULT_RETRY logic at all.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>

Note that the use of get_user_pages_remote() in async_pf_execute() reintroduces
the use of the FOLL_TOUCH flag - I don't think this is a problem as this flag
was dropped by 1e987790 ("mm/gup: Introduce get_user_pages_remote()") which
states 'Without protection keys, this patch should not change any behavior', so
I don't think this was intentional.