Re: [RFC 9/9] prd: Add support for page struct mapping

[Toshi Kani <toshi.kani@hp.com>]

On Wed, 2014-08-13 at 15:26 +0300, Boaz Harrosh wrote:
> From: Yigal Korman <yigal@plexistor.com>
> 
> One of the current short comings of the NVDIMM/PMEM
> support is that this memory does not have a page-struct(s)
> associated with its memory and therefor cannot be passed
> to a block-device or network or DMAed in any way through
> another device in the system.
> 
> This simple patch fixes all this. After this patch an FS
> can do:
> 	bdev_direct_access(,&pfn,);
> 	page = pfn_to_page(pfn);
> And use that page for a lock_page(), set_page_dirty(), and/or
> anything else one might do with a page *.
> (Note that with brd one can already do this)
> 
> [pmem-pages-ref-count]
> pmem will serve it's pages with ref==0. Once an FS does
> an blkdev_get_XXX(,FMODE_EXCL,), that memory is own by the FS.
> The FS needs to manage its allocation, just as it already does
> for its disk blocks. The fs should set page->count = 2, before
> submission to any Kernel subsystem so when it returns it will
> never be released to the Kernel's page-allocators. (page_freeze)
> 
> All is actually needed for this is to allocate page-sections
> and map them into kernel virtual memory. Note that these sections
> are not associated with any zone, because that would add them to
> the page_allocators.

Can we just use memory hotplug and call add_memory(), instead of
directly calling sparse_add_one_section()?  Memory hotplug adds memory
as off-line state, and sets all pages reserved.  So, I do not think the
page allocators will mess with them (unless you put them online).  It
can also maps the pages with large page size.

Thanks,
-Toshi


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