linux-kernel - Re: [PATCH 1/2] dax: prevent invalidation of mapped DAX entries

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Date:   Mon, 1 May 2017 10:54:51 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Kuznetsov <kuznet@...tuozzo.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        linux-cifs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nfs@...r.kernel.org,
        linux-nvdimm@...ts.01.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        Ron Minnich <rminnich@...dia.gov>,
        samba-technical@...ts.samba.org, Steve French <sfrench@...ba.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        v9fs-developer@...ts.sourceforge.net
Subject: Re: [PATCH 1/2] dax: prevent invalidation of mapped DAX entries

On Tue, Apr 25, 2017 at 12:10:41PM +0200, Jan Kara wrote:
> On Thu 20-04-17 21:44:36, Ross Zwisler wrote:
> > dax_invalidate_mapping_entry() currently removes DAX exceptional entries
> > only if they are clean and unlocked.  This is done via:
> > 
> > invalidate_mapping_pages()
> >   invalidate_exceptional_entry()
> >     dax_invalidate_mapping_entry()
> > 
> > However, for page cache pages removed in invalidate_mapping_pages() there
> > is an additional criteria which is that the page must not be mapped.  This
> > is noted in the comments above invalidate_mapping_pages() and is checked in
> > invalidate_inode_page().
> > 
> > For DAX entries this means that we can can end up in a situation where a
> > DAX exceptional entry, either a huge zero page or a regular DAX entry,
> > could end up mapped but without an associated radix tree entry. This is
> > inconsistent with the rest of the DAX code and with what happens in the
> > page cache case.
> > 
> > We aren't able to unmap the DAX exceptional entry because according to its
> > comments invalidate_mapping_pages() isn't allowed to block, and
> > unmap_mapping_range() takes a write lock on the mapping->i_mmap_rwsem.
> > 
> > Since we essentially never have unmapped DAX entries to evict from the
> > radix tree, just remove dax_invalidate_mapping_entry().
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > Fixes: c6dcf52c23d2 ("mm: Invalidate DAX radix tree entries only if appropriate")
> > Reported-by: Jan Kara <jack@...e.cz>
> > Cc: <stable@...r.kernel.org>    [4.10+]
> 
> Just as a side note - we wouldn't really have to unmap the mapping range
> covered by the DAX exceptional entry. It would be enough to find out
> whether such range is mapped and bail out in that case. But that would
> still be pretty expensive for DAX - we'd have to do rmap walk similar as in
> dax_mapping_entry_mkclean() and IMHO it is not worth it. So I agree with
> what you did. You can add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>

Yep, that makes sense.  Thanks for the review.