Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

From: Christoph Hellwig <hch@infradead.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
	John Hubbard <jhubbard@nvidia.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>, Miklos Szeredi <miklos@szeredi.hu>,
	"Darrick J . Wong" <djwong@kernel.org>,
	Trond Myklebust <trond.myklebust@hammerspace.com>,
	Anna Schumaker <anna@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Logan Gunthorpe <logang@deltatee.com>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines
Date: Mon, 26 Sep 2022 08:53:43 -0700	[thread overview]
Message-ID: <YzHLB4lGa2vktN7W@infradead.org> (raw)
In-Reply-To: <Yy3bNjaiUoGv/djG@ZenIV>

On Fri, Sep 23, 2022 at 05:13:42PM +0100, Al Viro wrote:
> You are mixing two issues here - holding references to pages while using
> iov_iter instance is obvious; holding them until async IO is complete, even
> though struct iov_iter might be long gone by that point is a different
> story.

But someone needs to hold a refernce until the I/O is completed, because
the I/O obviously needs the pages.  Yes, we could say the callers holds
them and can drop the references right after I/O submission, while
the method needs to grab another reference.  But that is more
complicated and is more costly than just holding the damn reference.

> And originating iov_iter instance really can be long-gone by the time
> of IO completion - requirement to keep it around would be very hard to
> satisfy.  I've no objections to requiring the pages in ITER_BVEC to be
> preserved at least until the IO completion by means independent of
> whatever ->read_iter/->write_iter does to them, but
> 	* that needs to be spelled out very clearly and
> 	* we need to verify that it is, indeed, the case for all existing
> iov_iter_bvec callers, preferably with comments next to non-obvious ones
> (something that is followed only by the sync IO is obvious)

Agreed.

> That goes not just for bio - if we make get_pages *NOT* grab references
> on ITER_BVEC (and I'm all for it), we need to make sure that those
> pages won't be retained after the original protection runs out.

Yes.