Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path

[Jason Gunthorpe]

On Wed, Mar 17, 2021 at 09:08:28PM -0700, Dan Williams wrote:
> Now that device-dax and filesystem-dax are guaranteed to unmap all user
> mappings of devmap / DAX pages before tearing down the 'struct page'
> array, get_user_pages_fast() can rely on its traditional synchronization
> method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> device shutdown. Specifically the unmap guarantee ensures that gup-fast
> either succeeds in taking a page reference (lock-less), or it detects a
> need to fall back to the slow path where the device presence can be
> revalidated with locks held.
> 
> Reported-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  mm/gup.c |   38 ++++++++++++++++----------------------
>  1 file changed, 16 insertions(+), 22 deletions(-)

I'm happy to see this, and it is really the right thing that PTEs are
properly removed before anything happens to the pages under them.

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason