linux-kernel - Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages

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Message-ID: <20200825015428.GU1236603@ZenIV.linux.org.uk>
Date:   Tue, 25 Aug 2020 02:54:28 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()

On Fri, Aug 21, 2020 at 09:20:54PM -0700, John Hubbard wrote:

> Direct IO behavior:
> 
>     ITER_IOVEC:
>         pin_user_pages_fast();
>         break;
> 
>     ITER_KVEC:    // already elevated page refcount, leave alone
>     ITER_BVEC:    // already elevated page refcount, leave alone
>     ITER_PIPE:    // just, no :)

Why?  What's wrong with splice to O_DIRECT file?