linux-kernel - Re: [PATCH] scsi: st: convert convert get_user_pages() --> pin_user

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Date:   Tue, 26 May 2020 11:15:30 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Kai Mäkisara (Kolumbus) 
        <kai.makisara@...umbus.fi>, Bart Van Assche <bvanassche@....org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "James E . J . Bottomley" <jejb@...ux.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        <linux-scsi@...r.kernel.org>
Subject: Re: [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 <bvanassche@....org> 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@lst.de
>>
>> 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