Re: [PATCH v3 12/13] dax: handle truncate of dma-busy pages

[Dan Williams]

On Fri, Oct 20, 2017 at 9:32 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> On Fri, Oct 20, 2017 at 08:42:00AM -0700, Dan Williams wrote:
>> I agree, but it needs quite a bit more thought and restructuring of
>> the truncate path. I also wonder how we reclaim those stranded
>> filesystem blocks, but a first approximation is wait for the
>> administrator to delete them or auto-delete them at the next mount.
>> XFS seems well prepared to reflink-swap these DMA blocks around, but
>> I'm not sure about EXT4.
>
> reflink still is an optional and experimental feature in XFS.  That
> being said we should not need to swap block pointers around on disk.
> We just need to prevent the block allocator from reusing the blocks
> for new allocations, and we have code for that, both for transactions
> that haven't been committed to disk yet, and for deleted blocks
> undergoing discard operations.
>
> But as mentioned in my second mail from this morning I'm not even
> sure we need that.  For short-term elevated page counts like normal
> get_user_pages users I think we can just wait for the page count
> to reach zero, while for abuses of get_user_pages for long term
> pinning memory (not sure if anyone but rdma is doing that) we'll need
> something like FL_LAYOUT leases to release the mapping.

I'll take a look at hooking this up through a page-idle callback. Can
I get some breadcrumbs to grep for from XFS folks on how to set/clear
the busy state of extents?