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

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Date:   Wed, 31 Aug 2022 11:02:45 -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 8/31/22 02:43, Jan Kara wrote:
>> 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.
> 
> So after looking into that a bit more, I think a clean approach would be to
> provide iov_iter_pin_pages2() and iov_iter_pages_alloc2(), under the hood
> in __iov_iter_get_pages_alloc() make sure we use pin_user_page() instead of
> get_page() in all the cases (using this in pipe_get_pages() and
> iter_xarray_get_pages() is easy) and then make all bio handling use the
> pinning variants for iters. I think at least iov_iter_is_pipe() case needs
> to be handled as well because as I wrote above, pipe pages can enter direct
> IO code e.g. for splice(2).
> 

OK, yes.

> Also I think that all iov_iter_get_pages2() (or the _alloc2 variant) users
> actually do want the "pin page" semantics in the end (they are accessing
> page contents) so eventually we should convert them all to
> iov_iter_pin_pages2() and remove iov_iter_get_pages2() altogether. But this
> will take some more conversion work with networking etc. so I'd start with
> converting bios only.
> 
> 								Honza


I'll give this approach a spin, thanks very much for the guidance!


thanks,

-- 
John Hubbard
NVIDIA