linux-kernel - Re: [PATCH 6/6] fuse: convert direct IO paths to use FOLL

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Message-ID: <f0b158dc-5b01-67aa-1f49-331bf1ff2bfd@nvidia.com>
Date:   Mon, 28 Feb 2022 13:16:34 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Miklos Szeredi <miklos@...redi.hu>, jhubbard.send.patches@...il.com
Cc:     Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] fuse: convert direct IO paths to use FOLL_PIN

On 2/28/22 07:59, Miklos Szeredi wrote:
> On Sun, 27 Feb 2022 at 10:34, <jhubbard.send.patches@...il.com> wrote:
>>
>> From: John Hubbard <jhubbard@...dia.com>
>>
>> Convert the fuse filesystem to support the new iov_iter_get_pages()
>> behavior. That routine now invokes pin_user_pages_fast(), which means
>> that such pages must be released via unpin_user_page(), rather than via
>> put_page().
>>
>> This commit also removes any possibility of kernel pages being handled,
>> in the fuse_get_user_pages() call. Although this may seem like a steep
>> price to pay, Christoph Hellwig actually recommended it a few years ago
>> for nearly the same situation [1].
> 
> This might work for O_DIRECT, but fuse has this mode of operation
> which turns normal "buffered" I/O into direct I/O.  And that in turn
> will break execve of such files.
> 
> So AFAICS we need to keep kvec handing in some way.
> 

Thanks for bringing that up! Do you have any hints for me, to jump start
a deeper look? And especially, sample programs that exercise this?


thanks,
-- 
John Hubbard
NVIDIA