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

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Date:   Fri, 25 Feb 2022 16:14:14 +0000
From:   Chaitanya Kulkarni <chaitanyak@...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>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>,
        "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 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 ?

> 
> 								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