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

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Date:   Thu, 21 May 2020 12:57:01 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Bart Van Assche <bvanassche@....org>,
        LKML <linux-kernel@...r.kernel.org>
CC:     Kai Mäkisara <Kai.Makisara@...umbus.fi>,
        "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 2020-05-21 12:47, 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":
>>      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.
> 
> 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?
> 

No, I am not sure. I thought this was a DMA case (I'm not a SCSI Tape user,
so it *seemed* reasonable that a DMA engine was involved), but if it's really
direct IO, then we need to just drop this patch entirely. Because: I need to
convert the block/biovec code, including iov_iter_get_pages_alloc() and
friends, in order to handle direct IO. I'm working on that but it's not
ready yet.

(I was trying to get the smaller, non-direct-IO cases converted first.)

Thanks for spotting the discrepancy, and apologies for the confusion on this
end.

Also, I doubt if it's worth it, but do you want a patch to change SetPageDirty()
to set_page_dirty_lock(), meanwhile? It seems like if that's never come up, then
it's mostly a theoretical bug.

thanks,
-- 
John Hubbard
NVIDIA