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: <3396eec4-a469-5b95-06db-c40e5616465b@redhat.com>
Date:   Fri, 27 Jan 2023 01:20:05 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        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>,
        John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list
 from an iterator

On 27.01.23 01:05, David Howells wrote:
> David Hildenbrand <david@...hat.com> wrote:
> 
>> As raised already somewhere in the whole discussion by me, the right way to
>> take such a long-term ping as vmsplice() does is to use
>> FOLL_PIN|FOLL_LONGTERM.
> 
> So the pipe infrastructure would have to be able to pin pages instead of
> carrying refs on them?  What about pages just allocated and added to the pipe
> ring in normal pipe use?
Ordinary kernel allocations (alloc_page() ...) always have to be freed 
via put_page() and friends. Such allocations are unmovable as default 
and don't require any special care.

Pages mapped into user space are movable as default and might be placed 
on ZONE_MOVABLE/CMA memory (well, and might be swapped out). 
FOLL_LONGTERM makes sure to migrate these pages off of such problematic 
physical memory regions, such that we can safely pin them until eternity.

-- 
Thanks,

David / dhildenb