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

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Message-ID: <YxhaJktqtHw3QTSG@infradead.org>
Date:   Wed, 7 Sep 2022 01:45:26 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        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 Tue, Sep 06, 2022 at 12:21:06PM +0200, Jan Kara wrote:
> > For FOLL_PIN callers, never pin bvec and kvec pages:  For file systems
> > not acquiring a reference is obviously safe, and the other callers will
> > need an audit, but I can't think of why it woul  ever be unsafe.
> 
> Are you sure about "For file systems not acquiring a reference is obviously
> safe"? I can see places e.g. in orangefs, afs, etc. which create bvec iters
> from pagecache pages. And then we have iter_file_splice_write() which
> creates bvec from pipe pages (which can also be pagecache pages if
> vmsplice() is used). So perhaps there are no lifetime issues even without
> acquiring a reference (but looking at the code I would not say it is
> obvious) but I definitely don't see how it would be safe to not get a pin
> to signal to filesystem backing the pagecache page that there is DMA
> happening to/from the page.

I mean in the context of iov_iter_get_pages callers, that is direct
I/O.  Direct callers of iov_iter_bvec which then pass that iov to
->read_iter / ->write_iter will need to hold references (those are
the references that the callers of iov_iter_get_pages rely on!).