linux-kernel - Re: [PATCH 00/12] block/bio, fs: convert put_page() to put_user

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Date:   Mon, 5 Aug 2019 15:54:35 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>, <john.hubbard@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "David S . Miller" <davem@...emloft.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Latchesar Ionkov <lucho@...kov.net>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        <ceph-devel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-cifs@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-nfs@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, <netdev@...r.kernel.org>,
        <samba-technical@...ts.samba.org>,
        <v9fs-developer@...ts.sourceforge.net>,
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 00/12] block/bio, fs: convert put_page() to
 put_user_page*()

On 7/23/19 11:17 PM, Christoph Hellwig wrote:
> On Tue, Jul 23, 2019 at 09:25:06PM -0700, john.hubbard@...il.com wrote:
>> * Store, in the iov_iter, a "came from gup (get_user_pages)" parameter.
>>   Then, use the new iov_iter_get_pages_use_gup() to retrieve it when
>>   it is time to release the pages. That allows choosing between put_page()
>>   and put_user_page*().
>>
>> * Pass in one more piece of information to bio_release_pages: a "from_gup"
>>   parameter. Similar use as above.
>>
>> * Change the block layer, and several file systems, to use
>>   put_user_page*().
> 
> I think we can do this in a simple and better way.  We have 5 ITER_*
> types.  Of those ITER_DISCARD as the name suggests never uses pages, so
> we can skip handling it.  ITER_PIPE is rejected іn the direct I/O path,
> which leaves us with three.
> 

Hi Christoph,

Are you working on anything like this? Or on the put_user_bvec() idea?
Please let me know, otherwise I'll go in and implement something here.


thanks,
-- 
John Hubbard
NVIDIA

> Out of those ITER_BVEC needs a user page reference, so we want to call
> put_user_page* on it.  ITER_BVEC always already has page reference,
> which means in the block direct I/O path path we alread don't take
> a page reference.  We should extent that handling to all other calls
> of iov_iter_get_pages / iov_iter_get_pages_alloc.  I think we should
> just reject ITER_KVEC for direct I/O as well as we have no users and
> it is rather pointless.  Alternatively if we see a use for it the
> callers should always have a life page reference anyway (or might
> be on kmalloc memory), so we really should not take a reference either.
> 
> In other words:  the only time we should ever have to put a page in
> this patch is when they are user pages.  We'll need to clean up
> various bits of code for that, but that can be done gradually before
> even getting to the actual put_user_pages conversion.
>