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SubjectRe: [PATCH] scsi: st: convert convert get_user_pages() --> pin_user_pages()
On 2020-05-22 01:32, "Kai Mäkisara (Kolumbus)" wrote:
>> On 21. May 2020, at 22.47, Bart Van Assche <> 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":
>>> [3]
>> 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.

John Hubbard

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