Re: [PATCH] scsi: st: convert convert get_user_pages() --> pin_user_pages()

[Bart Van Assche]

On 2020-05-18 21:55, John Hubbard wrote:
> This code was using get_user_pages*(), in a "Case 2" scenario
> (DMA/RDMA), using the categorization from [1]. That means that it's
> time to convert the get_user_pages*() + put_page() calls to
> pin_user_pages*() + unpin_user_pages() calls.
> 
> There is some helpful background in [2]: basically, this is a small
> part of fixing a long-standing disconnect between pinning pages, and
> file systems' use of those pages.
> 
> Note that this effectively changes the code's behavior as well: it now
> ultimately calls set_page_dirty_lock(), instead of SetPageDirty().This
> is probably more accurate.
> 
> As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
> dealing with a file backed page where we have reference on the inode it
> hangs off." [3]
> 
> Also, this deletes one of the two FIXME comments (about refcounting),
> because there is nothing wrong with the refcounting at this point.
> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
>     https://lwn.net/Articles/807108/
> 
> [3] https://lore.kernel.org/r/20190723153640.GB720@xxxxxx

Kai, why is the st driver calling get_user_pages_fast() directly instead
of calling blk_rq_map_user()? blk_rq_map_user() is already used in
st_scsi_execute(). I think that the blk_rq_map_user() implementation is
also based on get_user_pages_fast(). See also iov_iter_get_pages_alloc()
in lib/iov_iter.c.

John, why are the get_user_pages_fast() calls in the st driver modified
but not the blk_rq_map_user() call? Are you sure that the modified code
is a "case 2" scenario and not a "case 1" scenario?

Thanks,

Bart.