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

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Date:   Mon, 23 Jan 2023 15:20:41 +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 23.01.23 14:38, David Howells wrote:
> David Hildenbrand <david@...hat.com> wrote:
> 
>> That would be the ideal case: whenever intending to access page content, use
>> FOLL_PIN instead of FOLL_GET.
>>
>> The issue that John was trying to sort out was that there are plenty of
>> callsites that do a simple put_page() instead of calling
>> unpin_user_page(). IIRC, handling that correctly in existing code -- what was
>> pinned must be released via unpin_user_page() -- was the biggest workitem.
>>
>> Not sure how that relates to your work here (that's why I was asking): if you
>> could avoid FOLL_GET, that would be great :)
> 
> Well, it simplifies things a bit.
> 
> I can make the new iov_iter_extract_pages() just do "pin" or "don't pin" and
> do no ref-getting at all.  Things can be converted over to "unpin the pages or
> doing nothing" as they're converted over to using iov_iter_extract_pages()
> from iov_iter_get_pages*().
> 
> The block bio code then only needs a single bit of state: pinned or not
> pinned.

Unfortunately, I'll have to let BIO experts comment on that :) I only 
know the MM side of things here.

> 
> For cifs RDMA, do I need to make it pass in FOLL_LONGTERM?  And does that need
> a special cleanup?

Anything that holds pins "possibly forever" should that. vmsplice() is 
another example that should use it, once properly using FOLL_PIN. 
[FOLL_GET | FOLL_LONGTERM is not really used/defined with semantics]

> 
> sk_buff fragment handling could still be tricky.  I'm thinking that in that
> code I'll need to store FOLL_GET/PIN in the bottom two bits of the frag page
> pointer.  Sometimes it allocates a new page and attaches it (have ref);
> sometimes it does zerocopy to/from a page (have pin) and sometimes it may be
> pointing to a kernel buffer (don't pin or ref).
> 
> David
> 

-- 
Thanks,

David / dhildenb