linux-kernel - Re: [RFC PATCH 0/7] block, fs: convert Direct IO to FOLL

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Date:   Fri, 25 Feb 2022 14:12:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <kch@...dia.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/7] block, fs: convert Direct IO to FOLL_PIN

On 25.02.22 09:50, John Hubbard wrote:
> Hi,
> 
> Summary:
> 
> This puts some prerequisites in place, including a CONFIG parameter,
> making it possible to start converting and testing the Direct IO part of
> each filesystem, from get_user_pages_fast(), to pin_user_pages_fast().
> 
> It will take "a few" kernel releases to get the whole thing done.
> 
> Details:
> 
> As part of fixing the "get_user_pages() + file-backed memory" problem
> [1], and to support various COW-related fixes as well [2], we need to
> convert the Direct IO code from get_user_pages_fast(), to
> pin_user_pages_fast(). Because pin_user_pages*() calls require a
> corresponding call to unpin_user_page(), the conversion is more
> elaborate than just substitution.
> 
> Further complicating the conversion, the block/bio layers get their
> Direct IO pages via iov_iter_get_pages() and iov_iter_get_pages_alloc(),
> each of which has a large number of callers. All of those callers need
> to be audited and changed so that they call unpin_user_page(), rather
> than put_page().

vmsplice is another candidate that uses iov_iter_get_pages() and should
be converted to FOLL_PIN. For that particular user, we have to also pass
FOLL_LONGTERM -- vmsplice as it stands can block memory hotunplug / CMA
/ ... for all eternity.

-- 
Thanks,

David / dhildenb