linux-kernel - Re: [RFC PATCH 1/7] mm/gup: introduce pin_user

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Date:   Tue, 1 Mar 2022 10:30:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chaitanya Kulkarni <kch@...dia.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/7] mm/gup: introduce pin_user_page()


>>>> That might be problematic and possibly the wrong approach, depending on
>> *what* we're actually pinning and what we're intending to do with that.
>>
>> My assumption would have been that this interface is to duplicate a pin
> 
> I see that I need to put more documentation here, so people don't have
> to assume things... :)
> 

Yes, please :)

>> on a page, which would be perfectly fine, because the page actually saw
>> a FOLL_PIN previously.
>>
>> We're taking a pin on a page that we haven't obtained via FOLL_PIN if I
>> understand correctly. Which raises the questions, how do we end up with
>> the pages here, and what are we doing to do with them (use them like we
>> obtained them via FOLL_PIN?)?
>>
>>
>> If it's converting FOLL_GET -> FOLL_PIN manually, then we're bypassing
>> FOLL_PIN special handling in GUP code:
>>
>> page = get_user_pages(FOLL_GET)
>> pin_user_page(page)
>> put_page(page)
> 
> No, that's not where this is going at all. The idea, which  I now see
> needs better documentation, is to handle file-backed pages. Only.
> 
> We're not converting from one type to another, nor are we doubling up.
> We're just keeping the pin type consistent so that the vast block-
> processing machinery can take pages in and handle them, then release
> them at the end with bio_release_pages(), which will call
> unpin_user_pages().
> 

Ah, okay, that makes sense. Glad to hear that we're intending to use
this with !anon pages only.

>>
>>
>> For anonymous pages, we'll bail out for example once we have
>>
>> https://lkml.kernel.org/r/20220224122614.94921-14-david@redhat.com
>>
>> Because the conditions for pinned anonymous pages might no longer hold.
>>
>> If we won't call pin_user_page() on anonymous pages, it would be fine.
> 
> We won't, and in fact, I should add WARN_ON_ONCE(PageAnon(page)) to
> this function.

Exactly what I would have suggested,

> 
>> But then, I still wonder how we come up the "struct page" here.
>>
> 
>  From the file system. For example, the NFS-direct and fuse conversions
> in the last patches show how that works.
> 
> Thanks for this feedback, this is very helpful.

Thanks for clarifying, John!

-- 
Thanks,

David / dhildenb