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Date:   Thu, 21 Dec 2017 09:31:50 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Jeff Moyer <jmoyer@...hat.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 14/15] dax: associate mappings with inodes, and warn if
 dma collides with truncate

On Thu, Dec 21, 2017 at 4:14 AM, Jan Kara <jack@...e.cz> wrote:
> On Wed 20-12-17 14:41:14, Dan Williams wrote:
>> On Wed, Dec 20, 2017 at 6:38 AM, Jan Kara <jack@...e.cz> wrote:
>> > On Tue 19-12-17 17:11:38, Dan Williams wrote:
>> >> On Fri, Nov 10, 2017 at 1:08 AM, Christoph Hellwig <hch@....de> wrote:
>> >> >> +             struct {
>> >> >> +                     /*
>> >> >> +                      * ZONE_DEVICE pages are never on an lru or handled by
>> >> >> +                      * a slab allocator, this points to the hosting device
>> >> >> +                      * page map.
>> >> >> +                      */
>> >> >> +                     struct dev_pagemap *pgmap;
>> >> >> +                     /*
>> >> >> +                      * inode association for MEMORY_DEVICE_FS_DAX page-idle
>> >> >> +                      * callbacks. Note that we don't use ->mapping since
>> >> >> +                      * that has hard coded page-cache assumptions in
>> >> >> +                      * several paths.
>> >> >> +                      */
>> >> >
>> >> > What assumptions?  I'd much rather fix those up than having two fields
>> >> > that have the same functionality.
>> >>
>> >> [ Reviving this old thread where you asked why I introduce page->inode
>> >> instead of reusing page->mapping ]
>> >>
>> >> For example, xfs_vm_set_page_dirty() assumes that page->mapping being
>> >> non-NULL indicates a typical page cache page, this is a false
>> >> assumption for DAX. My guess at a fix for this is to add
>> >> pagecache_page() checks to locations like this, but I worry about how
>> >> to find them all. Where pagecache_page() is:
>> >>
>> >> bool pagecache_page(struct page *page)
>> >> {
>> >>         if (!page->mapping)
>> >>                 return false;
>> >>         if (!IS_DAX(page->mapping->host))
>> >>                 return false;
>> >>         return true;
>> >> }
>> >>
>> >> Otherwise we go off the rails:
>> >>
>> >>  WARNING: CPU: 27 PID: 1783 at fs/xfs/xfs_aops.c:1468
>> >> xfs_vm_set_page_dirty+0xf3/0x1b0 [xfs]
>> >
>> > But this just shows that mapping->a_ops are wrong for this mapping, doesn't
>> > it? ->set_page_dirty handler for DAX mapping should just properly handle
>> > DAX pages... (and only those)
>>
>> Ah, yes. Now that I change ->mapping to be non-NULL for DAX pages I
>> enable all the address_space_operations to start firing. However,
>> instead of adding DAX specific address_space_operations it appears
>> ->mapping should never be set for DAX pages, because DAX pages are
>> disconnected from the page-writeback machinery.
>
> page->mapping is not only about page-writeback machinery. It is generally
> about page <-> inode relation and that still exists for DAX pages. We even
> reuse the mapping->page_tree to store DAX pages. Also requiring proper
> address_space_operations for DAX inodes is IMO not a bad thing as such.
>
> That being said I'm not 100% convinced we should really set page->mapping
> for DAX pages. After all they are not page cache pages but rather a
> physical storage for the data, don't ever get to LRU, etc. But if you need
> page->inode relation somewhere, that is a good indication to me that it
> might be just easier to set page->mapping and provide aops that do the
> right thing (i.e. usually not much) for them.
>
> BTW: the ->set_page_dirty() in particular actually *does* need to do
> something for DAX pages - corresponding radix tree entries should be
> marked dirty so that caches can get flushed when needed.

For this specific concern, the get_user_pages() path will have
triggered mkwrite, so the dax dirty tracking in the radix will have
already happened by the time we call ->set_page_dirty(). So, it's not
yet clear to me that we need that particular op.

>> In other words never
>> setting ->mapping bypasses all the possible broken assumptions and
>> code paths that take page-cache specific actions before calling an
>> address_space_operation.
>
> If there are any assumptions left after aops are set properly, then we can
> reconsider this but for now setting ->mapping and proper aops looks cleaner
> to me...

I'll try an address_space_operation with a nop ->set_page_dirty() and
see if anything else falls out.

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