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

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Message-ID: <d2b87357-baf9-ef1f-6e6a-18aab8e6d2fd@nvidia.com>
Date:   Fri, 25 Feb 2022 11:36:30 -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 04:05, Jan Kara wrote:
...
>> After quite some time exploring and consulting with people as well, it
>> is clear that this cannot be done in just one patchset. That's because,
>> not only is this large and time-consuming (for example, Chaitanya
>> Kulkarni's first reaction, after looking into the details, was, "convert
>> the remaining filesystems to use iomap, *then* convert to FOLL_PIN..."),
>> but it is also spread across many filesystems.
> 
> 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()

In addition to that, I was also worried that maybe the
blockdev_direct_IO() or iomap filesystems might be breaking encapsulation
occasionally, by calling put_page() on the direct IO user page buffer.
Perhaps in error paths.

Are you pretty sure that that last concern is not valid? That would be a
welcome bit of news.



thanks,
-- 
John Hubbard
NVIDIA