linux-kernel - Re: [PATCH v2 0/7] convert most filesystems to pin_user_pages

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Message-ID: <50d1d649-cb41-3031-c459-bbd38295c619@nvidia.com>
Date:   Tue, 6 Sep 2022 00:10:54 -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 0/7] convert most filesystems to pin_user_pages_fast()

On 9/5/22 23:36, Christoph Hellwig wrote:
> On Tue, Aug 30, 2022 at 09:18:36PM -0700, John Hubbard wrote:
>> The conversion is temporarily guarded by
>> CONFIG_BLK_USE_PIN_USER_PAGES_FOR_DIO. In the future (not part of this
>> series), when we are certain that all filesystems have converted their
>> Direct IO paths to FOLL_PIN, then we can do the final step, which is to
>> get rid of CONFIG_BLK_USE_PIN_USER_PAGES_FOR_DIO and search-and-replace
>> the dio_w_*() functions with their final names (see bvec.h changes).
> 
> What is the the point of these wrappers?  We should be able to
> convert one caller at a time in an entirely safe way.

I would be delighted if that were somehow possible. Every time I think
it's possible, it has fallen apart. The fact that bio_release_pages()
will need to switch over from put_page() to unpin_user_page(), combined
with the fact that there are a lot of callers that submit bios, has
led me to the current approach.

What did you have in mind?

thanks,

-- 
John Hubbard
NVIDIA