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

[John Hubbard]

On 2020-05-22 01:32, "Kai MÃkisara (Kolumbus)" wrote:
> > On 21. May 2020, at 22.47, Bart Van Assche <bvanassche@xxxxxxx> wrote:
> >
> > 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.
> The reason is that the blk_ functions were not available when that part
> of the code was done. Nobody has converted that to use the more
> modern functions because the old method still works.


Hi Kai, Bart,

Say, thinking about this some more, it seems like: as only sgl_unmap_user_pages()
is allowed to release the pages that were pinned in sgl_map_user_pages(), then
this is a clear case of Direct IO (not DMA/RDMA, as I first thought) that needs
to be handled via pin_user_pages_fast().

I'll send a v2 with the commit log updated to refer to Direct IO, because this
does still apply, after all.

thanks,
--
John Hubbard
NVIDIA