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: <5a1796f3-e49c-80e8-2dd6-9a6e82939271@redhat.com>
Date:   Fri, 27 Jan 2023 00:41:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        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 26.01.23 23:15, Al Viro wrote:
> On Mon, Jan 23, 2023 at 02:24:13PM +0100, David Hildenbrand wrote:
>> On 23.01.23 14:19, David Howells wrote:
>>> David Hildenbrand <david@...hat.com> wrote:
>>>
>>>> Switching from FOLL_GET to FOLL_PIN was in the works by John H. Not sure what
>>>> the status is. Interestingly, Documentation/core-api/pin_user_pages.rst
>>>> already documents that "CASE 1: Direct IO (DIO)" uses FOLL_PIN ... which does,
>>>> unfortunately, no reflect reality yet.
>>>
>>> Yeah - I just came across that.
>>>
>>> Should iov_iter.c then switch entirely to using pin_user_pages(), rather than
>>> get_user_pages()?  In which case my patches only need keep track of
>>> pinned/not-pinned and never "got".
>>
>> 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 :)
> 
> Take a good look at iter_to_pipe().  It does *not* need to pin anything
> (we have an ITER_SOURCE there); with this approach it will.  And it
> will stuff those pinned references into a pipe, where they can sit
> indefinitely.
> 
> IOW, I don't believe it's a usable approach.
> 

Not sure what makes you believe that FOLL_GET is any better for this 
long-term pinning, I'd like to learn about that.

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. As also raised, that will fix the last remaining 
vmsplice()+hugetlb COW issue as tested by the cow.c vm selftest and make 
sure to migrate that memory off of MIGRATE_MOVABLE/CMA memory where we 
cannot tolerate to have long-term unmovable memory sitting around.

-- 
Thanks,

David / dhildenb