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

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Message-ID: <20220906102106.q23ovgyjyrsnbhkp@quack3>
Date:   Tue, 6 Sep 2022 12:21:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     John Hubbard <jhubbard@...dia.com>,
        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 Tue 06-09-22 00:48:49, Christoph Hellwig wrote:
> On Tue, Sep 06, 2022 at 12:44:28AM -0700, John Hubbard wrote:
> > 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?
> 
> Do not change anyhing for FOLL_GET callers, as they are on the way out
> anyway.
> 
> For FOLL_PIN callers, never pin bvec and kvec pages:  For file systems
> not acquiring a reference is obviously safe, and the other callers will
> need an audit, but I can't think of why it woul  ever be unsafe.

Are you sure about "For file systems not acquiring a reference is obviously
safe"? I can see places e.g. in orangefs, afs, etc. which create bvec iters
from pagecache pages. And then we have iter_file_splice_write() which
creates bvec from pipe pages (which can also be pagecache pages if
vmsplice() is used). So perhaps there are no lifetime issues even without
acquiring a reference (but looking at the code I would not say it is
obvious) but I definitely don't see how it would be safe to not get a pin
to signal to filesystem backing the pagecache page that there is DMA
happening to/from the page.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR