Re: [PATCH 2/2] iommufd: copy to and from folios in batch_rw()

[Jason Gunthorpe <jgg@ziepe.ca>]

On Sat, Oct 31, 2026 at 04:33:19PM -0400, Tal Zussman wrote:
> batch_rw() copies one page at a time and, for writes, dirties each page
> with set_page_dirty_lock(). Walk the pfn batch a folio at a time
> instead, copying with memcpy_to_folio() and memcpy_from_folio() and
> marking each folio dirty once. Convert the single-page copy in
> iopt_pages_rw_page() to the same helper.
> 
> iopt_pages_rw_access() rejects dmabuf pages, so only pinned user memory
> and memfd pages can reach batch_rw(), and those are always folios.
> 
> memcpy_to_folio() also flushes the dcache after the write, which the
> open-coded memcpy() didn't.
> 
> This removes the last use of set_page_dirty_lock() in iommufd.
> 
> With ftrace timing iopt_pages_rw_slow() through the mock driver on a
> non-debug kernel, slow-path writes to THP-backed pages drop from ~1310 to
> ~930 ns/page (now the same cost as a read), and writes to 4K folios drop
> ~9%.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>  drivers/iommu/iommufd/pages.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)

This looks reasonable to me

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason