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: <5f1764fa-0252-9de5-9522-294a0749103f@nvidia.com>
Date:   Mon, 23 Jan 2023 11:56:32 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     David Hildenbrand <david@...hat.com>,
        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>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list
 from an iterator

On 1/23/23 05:24, 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.
>>

Yes, that part of the documentation is...aspirational. :) But this
series is taking us there, so good. Let me go review v8 of the series in
actual detail to verify but it sounds very promising.

>> 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 :)
> 

The largest part of this problem has been: __iov_iter_get_pages_alloc()
calls get_user_pages_fast() (so, FOLL_GET), as part of the Direct IO
path. And that __iov_iter_get_pages_alloc() is also called by a wide
variety of things that are not Direct IO: networking, crytpo, RDS.

So splitting out a variant that only Direct IO uses is a great move and
should allow conversion of Direct IO to FOLL_PIN. Again, let me go do an
actual review to check on that.

thanks,
-- 
John Hubbard
NVIDIA