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

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Date:   Fri, 25 Feb 2022 14:20:25 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jens Axboe <axboe@...nel.dk>,
        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>,
        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 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