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

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Message-ID: <CAJfpegsDkpdCQiPmfKfX_b4-bkkj5N5vRhseifEH6woJ7r0S6A@mail.gmail.com>
Date:   Mon, 28 Feb 2022 16:59:33 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     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>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 6/6] fuse: convert direct IO paths to use FOLL_PIN

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,
Miklos