Re: [PATCH v4] mm/swap: Fix release_pages() when releasing devmap pages

[Jason Gunthorpe]

On Wed, Jun 05, 2019 at 02:49:22PM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> release_pages() is an optimized version of a loop around put_page().
> Unfortunately for devmap pages the logic is not entirely correct in
> release_pages().  This is because device pages can be more than type
> MEMORY_DEVICE_PUBLIC.  There are in fact 4 types, private, public, FS
> DAX, and PCI P2PDMA.  Some of these have specific needs to "put" the
> page while others do not.
> 
> This logic to handle any special needs is contained in
> put_devmap_managed_page().  Therefore all devmap pages should be
> processed by this function where we can contain the correct logic for a
> page put.
> 
> Handle all device type pages within release_pages() by calling
> put_devmap_managed_page() on all devmap pages.  If
> put_devmap_managed_page() returns true the page has been put and we
> continue with the next page.  A false return of
> put_devmap_managed_page() means the page did not require special
> processing and should fall to "normal" processing.
> 
> This was found via code inspection while determining if release_pages()
> and the new put_user_pages() could be interchangeable.[1]
> 
> [1] https://lore.kernel.org/lkml/20190523172852.GA27175@xxxxxxxxxxxxxxxxxxxxxxxxx/
> 
> Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> Changes from V3:
> 	Update comment to the one provided by John
> 
> Changes from V2:
> 	Update changelog for more clarity as requested by Michal
> 	Update comment WRT "failing" of put_devmap_managed_page()
> 
> Changes from V1:
> 	Add comment clarifying that put_devmap_managed_page() can still
> 	fail.
> 	Add Reviewed-by tags.
> 
>  mm/swap.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)

Andrew,

As per the discussion on the hmm thread I took this patch into the
hmm.git as the conflict that was created with CH's rework was tricky -
the resolution is simple, but keeping Ira's hunk instead of the delete
is, IMHO, subtle.

Regards, 
Jason