linux-kernel - Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

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Message-ID: <Y89zZofd63LCZYcG@infradead.org>
Date:   Mon, 23 Jan 2023 21:57:58 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-mm@...ck.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list
 from an iterator

On Mon, Jan 23, 2023 at 03:07:48PM -0800, John Hubbard wrote:
> On 1/23/23 08:11, Jan Kara wrote:
> > > For cifs RDMA, do I need to make it pass in FOLL_LONGTERM?  And does that need
> > > a special cleanup?
> > 
> > FOLL_LONGTERM doesn't need a special cleanup AFAIK. It should be used
> > whenever there isn't reasonably bound time after which the page is
> > unpinned. So in case CIFS sets up RDMA and then it is up to userspace how
> > long the RDMA is going to be running it should be using FOLL_LONGTERM. The
> 
> Yes, we have been pretty consistently deciding that RDMA generally
> implies FOLL_LONGTERM. (And furthermore, FOLL_LONGTERM implies
> FOLL_PIN--that one is actually enforced by the gup/pup APIs.)

That's weird.  For storage or file systems, pages are pinnen just as
long when using RDMA as when using local DMA, in fact if you do RDMA
to really fast remote media vs slow local media (e.g. SSD vs disk) you
might pin it shorter when using RDMA.

I think FOLL_LONGTERM makes sense for non-ODP user space memory
registrations for RDMA, which will last basically forever.  It does
not really make much sense at all for in-kernel memory registration for
RDMA that are very short term.