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

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Message-ID: <494478b6-9a8c-5271-fc9f-fd758af850c0@acm.org>
Date:   Thu, 21 May 2020 12:47:08 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     John Hubbard <jhubbard@...dia.com>,
        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 2020-05-18 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?

Thanks,

Bart.