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

> 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.


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