linux-kernel - Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin

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Date:   Fri, 23 Sep 2022 04:19:42 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

On Thu, Sep 22, 2022 at 01:29:35PM +0200, Jan Kara wrote:

> > This rule would mostly work, as long as we can relax it in some cases, to
> > allow pinning of both source and dest pages, instead of just destination
> > pages, in some cases. In particular, bio_release_pages() has lost all
> > context about whether it was a read or a write request, as far as I can
> > tell. And bio_release_pages() is the primary place to unpin pages for
> > direct IO.
> 
> Well, we already do have BIO_NO_PAGE_REF bio flag that gets checked in
> bio_release_pages(). I think we can easily spare another bio flag to tell
> whether we need to unpin or not. So as long as all the pages in the created
> bio need the same treatment, the situation should be simple.

Yes.  Incidentally, the same condition is already checked by the creators
of those bio - see the assorted should_dirty logics.

While we are at it - how much of the rationale around bio_check_pages_dirty()
doing dirtying is still applicable with pinning pages before we stick them
into bio?  We do dirty them before submitting bio, then on completion
bio_check_pages_dirty() checks if something has marked them clean while
we'd been doing IO; if all of them are still dirty we just drop the pages
(well, unpin and drop), otherwise we arrange for dirty + unpin + drop
done in process context (via schedule_work()).  Can they be marked clean by
anyone while they are pinned?  After all, pinning is done to prevent
writeback getting done on them while we are modifying the suckers...