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

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Date:   Fri, 25 Feb 2022 17:40:15 +0100
From:   Jan Kara <jack@...e.cz>
To:     Chaitanya Kulkarni <chaitanyak@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, 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>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-mm@...ck.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 Fri 25-02-22 16:14:14, Chaitanya Kulkarni wrote:
> On 2/25/22 04:05, Jan Kara wrote:
> > On Fri 25-02-22 00:50:18, 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().
> >>
> >> 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?
> 
> if I understand your comment correctly file systems which are listed in
> the list see [1] (all the credit goes to John to have a complete list)
> that are not using iomap but use XXX_XXX_direct_IO() should be fine,
> since in the callchain going from :-
> 
> XXX_XXX_direct_io()
>   __blkdev_direct_io()
>    do_direct_io()
> 
>    ...
> 
>      submit_page_selection()
>       get/put_page() <---
> 
> will take care of itself ?

Yes, John's changes to fs/direct-io.c should take care of these
filesystems using __blkdev_direct_io().

								Honza

> [1]
> 
> jfs_direct_IO()
> nilfs_direct_IO()
> ntfs_dirct_IO()
> reiserfs_direct_IO()
> udf_direct_IO()
> ocfs2_dirct_IO()
> affs_direct_IO()
> exfat_direct_IO()
> ext2_direct_IO()
> fat_direct_IO()
> hfs_direct_IO()
> hfs_plus_direct_IO()
> 
> -ck
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR