linux-kernel - Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin

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Message-ID: <103fe662-3dc8-35cb-1a68-dda8af95c518@nvidia.com>
Date:   Tue, 6 Sep 2022 00:44:28 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>, Jan Kara <jack@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        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 v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

On 9/5/22 23:47, Christoph Hellwig wrote:
> I'd it one step back.  For BVECS we never need a get or pin.  The
> block layer already does this, an the other callers should as well.
> For KVEC the same is true.  For PIPE and xarray as you pointed out
> we can probably just do the pin, it is not like these are performance
> paths.
> 
> So, I'd suggest to:
> 
>  - factor out the user backed and bvec cases from
>    __iov_iter_get_pages_alloc into helper just to keep
>    __iov_iter_get_pages_alloc readable.

OK, that part is clear.

>  - for the pin case don't use the existing bvec helper at all, but
>    copy the logic for the block layer for not pinning.

I'm almost, but not quite sure I get the idea above. Overall, what
happens to bvec pages? Leave the get_page() pin in place for FOLL_GET
(or USE_FOLL_GET), I suppose, but do...what, for FOLL_PIN callers?

thanks,

-- 
John Hubbard
NVIDIA