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Message-ID: <a23c8d6f-6311-f74f-b0fe-26682e16ee98@nvidia.com>
Date:   Fri, 25 Feb 2022 14:20:25 -0800
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Subject: Re: [RFC PATCH 0/7] block, fs: convert Direct IO to FOLL_PIN
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From:   John Hubbard <jhubbard@nvidia.com>
To:     Jan Kara <jack@suse.cz>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <dchinner@redhat.com>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
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On 2/25/22 11:36, John Hubbard wrote:
> On 2/25/22 04:05, Jan Kara wrote:
> ...
>> With having modified fs/direct-io.c and fs/iomap/direct-io.c which
>> filesystems do you know are missing conversion? Or is it that you just want
>> to make sure with audit everything is fine? The only fs I could find
>> unconverted by your changes is ceph. Am I missing something?
>>
>>                                 Honza
> 
> There are a few more filesystems that call iov_iter_get_pages() or
> iov_iter_get_pages_alloc(), plus networking things as well, plus some
> others that are hard to categorize, such as vhost. So we have:
> 
> * ceph
> * rds
> * cifs
> * p9
> * net: __zerocopy_sg_from_iter(), tls_setup_from_iter(),
> * crypto: af_alg_make_sg() (maybe N/A)
> * vmsplice() (as David Hildenbrand mentioned)
> * vhost: vhost_scsi_map_to_sgl()

...although...if each filesystem does *not* require custom attention,
then there is another, maybe better approach, which is: factor out an
iovec-only pair of allocators, and transition each subsystem when it's
ready. So:

     dio_iov_iter_get_pages()
     dio_iov_iter_get_pages_alloc()

That would allow doing this a bit at a time, and without the horrible
CONFIG parameter that is switched over all at once.

The bio_release_pages() still calls unpin_user_page(), though, so that
means that all Direct IO callers would still have to change over at
once.


thanks,
-- 
John Hubbard
NVIDIA