linux-kernel - Re: [PATCH 5/6] NFS: direct-io: convert to FOLL

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Date:   Mon, 29 Aug 2022 12:59:26 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

On 8/29/22 09:08, Jan Kara wrote:
>> However, the core block/bio conversion in patch 4 still does depend upon
>> a key assumption, which I got from a 2019 email discussion with
>> Christoph Hellwig and others here [1], which says:
>>
>>     "All pages released by bio_release_pages should come from
>>      get_get_user_pages...".
>>
>> I really hope that still holds true. Otherwise this whole thing is in
>> trouble.
>>
>> [1] https://lore.kernel.org/kvm/20190724053053.GA18330@infradead.org/
> 
> Well as far as I've checked that discussion, Christoph was aware of pipe
> pages etc. (i.e., bvecs) entering direct IO code. But he had some patches
> [2] which enabled GUP to work for bvecs as well (using the kernel mapping
> under the hood AFAICT from a quick glance at the series). I suppose we
> could also handle this in __iov_iter_get_pages_alloc() by grabbing pin
> reference instead of plain get_page() for the case of bvec iter. That way
> we should have only pinned pages in bio_release_pages() even for the bvec
> case.

OK, thanks, that looks viable. So, that approach assumes that the
remaining two cases in __iov_iter_get_pages_alloc() will never end up
being released via bio_release_pages():

    iov_iter_is_pipe(i)
    iov_iter_is_xarray(i)

I'm actually a little worried about ITER_XARRAY, which is a recent addition.
It seems to be used in ways that are similar to ITER_BVEC, and cephfs is
using it. It's probably OK for now, for this series, which doesn't yet
convert cephfs.


> 
> [2] http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec

Yes, I had looked through that again before sending this. The problem
for me was that that it didn't have to deal with releasing pages
differently (and therefore, differentiating between FOLL_PIN and
FOLL_GET pages). But it did enable GUP to handle bvecs, so with that
applied, one could then make the original claim about bio_release_pages()
and GUP, yes.


thanks,

-- 
John Hubbard
NVIDIA