linux-kernel - Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove 'page-less' support

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Date:   Fri, 20 Oct 2017 11:31:48 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Michal Hocko <mhocko@...e.com>,
        Jan Kara <jack@...e.cz>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dave Chinner <david@...morbit.com>,
        "J. Bruce Fields" <bfields@...ldses.org>, linux-mm@...ck.org,
        Paul Mackerras <paulus@...ba.org>,
        Sean Hefty <sean.hefty@...el.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-rdma@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        Jeff Moyer <jmoyer@...hat.com>, hch@....de,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Doug Ledford <dledford@...hat.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-nvdimm@...ts.01.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-fsdevel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove
        'page-less' support

On Fri, Oct 20, 2017 at 09:47:50AM +0200, Christoph Hellwig wrote:
> I'd like to brainstorm how we can do something better.
> 
> How about:
> 
> If we hit a page with an elevated refcount in truncate / hole puch
> etc for a DAX file system we do not free the blocks in the file system,
> but add it to the extent busy list.  We mark the page as delayed
> free (e.g. page flag?) so that when it finally hits refcount zero we
> call back into the file system to remove it from the busy list.

Brainstorming some more:

Given that on a DAX file there shouldn't be any long-term page
references after we unmap it from the page table and don't allow
get_user_pages calls why not wait for the references for all
DAX pages to go away first?  E.g. if we find a DAX page in
truncate_inode_pages_range that has an elevated refcount we set
a new flag to prevent new references from showing up, and then
simply wait for it to go away.  Instead of a busy way we can
do this through a few hashed waitqueued in dev_pagemap.  And in
fact put_zone_device_page already gets called when putting the
last page so we can handle the wakeup from there.

In fact if we can't find a page flag for the stop new callers
things we could probably come up with a way to do that through
dev_pagemap somehow, but I'm not sure how efficient that would
be.