Date: Fri, 8 May 2015 10:20:44 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Ingo Molnar <mingo@kernel.org>
Cc: Jerome Glisse <j.glisse@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Boaz Harrosh <boaz@plexistor.com>, Jan Kara <jack@suse.cz>,
        Mike Snitzer <snitzer@redhat.com>, Neil Brown <neilb@suse.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, Chris Mason <clm@fb.com>,
        Paul Mackerras <paulus@samba.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Christoph Hellwig <hch@lst.de>, Alasdair Kergon <agk@redhat.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Mel Gorman <mgorman@suse.de>, Matthew Wilcox <willy@linux.intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Jens Axboe <axboe@kernel.dk>, "Theodore Ts'o" <tytso@mit.edu>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Julia Lawall <Julia.Lawall@lip6.fr>, Tejun Heo <tj@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t
Message-ID: <20150508092044.GH889@ZenIV.linux.org.uk>
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On Fri, May 08, 2015 at 07:37:59AM +0200, Ingo Molnar wrote:

>     same as iov_iter_get_pages(), except that pages array is allocated
>     (kmalloc if possible, vmalloc if that fails) and left for caller to
>     free.  Lustre and NFS ->direct_IO() switched to it.
>     
>     Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> 
> So if code does iov_iter_get_pages_alloc() on a user address that has 
> a real struct page behind it - and some other code does a regular 
> get_user_pages() on it, we'll have two sets of struct page 
> descriptors, the 'real' one, and a fake allocated one, right?

Huh?  iov_iter_get_pages() is given an array of pointers to struct page,
which it fills with what it finds.  iov_iter_get_pages_alloc() *allocates*
such an array, fills that with what it finds and gives the allocated array
to caller.

We are not allocating any struct page instances in either of those.
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