linux-kernel - Re: [PATCH 3/6] block, fs: assert that key paths use iovecs, and nothing else

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Message-ID: <36223c44-d23f-18be-186e-08bcfce496d2@nvidia.com>
Date:   Sun, 27 Feb 2022 19:29:39 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Al Viro <viro@...iv.linux.org.uk>, 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>,
        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: [PATCH 3/6] block, fs: assert that key paths use iovecs, and
 nothing else

On 2/27/22 14:15, Al Viro wrote:
> On Sun, Feb 27, 2022 at 01:34:31AM -0800, jhubbard.send.patches@...il.com wrote:
>> From: John Hubbard <jhubbard@...dia.com>
>>
>> Upcoming changes to Direct IO will change it from acquiring pages via
>> get_user_pages_fast(), to calling pin_user_pages_fast() instead.
>>
>> Place a few assertions at key points, that the pages are IOVEC (user
>> pages), to enforce the assumptions that there are no kernel or pipe or
>> other odd variations being passed.
> 
> Umm...  And what should happen when O_DIRECT file gets passed to splice()?

OK, right, this is not going to work as-is.

Any of the remaining iov_iter_get_pages*() callers that do direct IO
(vmsplice, ceph, ...) will end up calling bio_release_pages(), which is
now unconditionally calling unpin_user_pages(). So this is just
completely broken.

And so, once again, I'm back to, "must convert the whole pile of
iov_iter_get_page*() callers at once".

Back to the drawing board.

thanks,
-- 
John Hubbard
NVIDIA