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Message-ID: <77f3fc56-05d8-def0-e518-0906c729e7df@redhat.com>
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
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