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Date:   Thu, 25 May 2023 02:51:59 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: Extending page pinning into fs/direct-io.c
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On Wed, May 24, 2023 at 09:47:10AM +0100, David Howells wrote:
> True - but I was thinking of just treating the zero_page specially and never
> hold a pin or a ref on it.  It can be checked by address, e.g.:
> 
>     static inline void bio_release_page(struct bio *bio, struct page *page)
>     {
> 	    if (page == ZERO_PAGE(0))
> 		    return;
> 	    if (bio_flagged(bio, BIO_PAGE_PINNED))
> 		    unpin_user_page(page);
> 	    else if (bio_flagged(bio, BIO_PAGE_REFFED))
> 		    put_page(page);
>     }

That does sound good as well to me.

> I was looking at this:
> 
>     static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
>     {
>     ...
> 	    if (dio->is_async && dio_op == REQ_OP_READ && dio->should_dirty)
> 		    bio_set_pages_dirty(bio);
>     ...
>     }
> 
> but looking again, the lock is taken briefly and the dirty bit is set - which
> is reasonable.  However, should we be doing it before starting the I/O?

It is done before starting the I/O - the submit_bio is just below this.