From: Bart Van Assche <bvanassche@acm.org>
To: John Hubbard <jhubbard@nvidia.com>, LKML <linux-kernel@vger.kernel.org>
Cc: "Kai Mäkisara" <Kai.Makisara@kolumbus.fi>,
"James E . J . Bottomley" <jejb@linux.ibm.com>,
"Martin K . Petersen" <martin.petersen@oracle.com>,
linux-scsi@vger.kernel.org
Subject: Re: [PATCH] scsi: st: convert convert get_user_pages() --> pin_user_pages()
Date: Thu, 21 May 2020 12:47:08 -0700 [thread overview]
Message-ID: <494478b6-9a8c-5271-fc9f-fd758af850c0@acm.org> (raw)
In-Reply-To: <20200519045525.2446851-1-jhubbard@nvidia.com>
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.
next prev parent reply other threads:[~2020-05-21 19:47 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-19 4:55 [PATCH] scsi: st: convert convert get_user_pages() --> pin_user_pages() John Hubbard
2020-05-19 20:12 ` John Hubbard
2020-05-20 1:57 ` Martin K. Petersen
2020-05-21 19:47 ` Bart Van Assche [this message]
2020-05-21 19:57 ` John Hubbard
2020-05-21 20:58 ` Bart Van Assche
2020-05-22 8:32 ` "Kai Mäkisara (Kolumbus)"
2020-05-26 18:15 ` John Hubbard
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=494478b6-9a8c-5271-fc9f-fd758af850c0@acm.org \
--to=bvanassche@acm.org \
--cc=Kai.Makisara@kolumbus.fi \
--cc=jejb@linux.ibm.com \
--cc=jhubbard@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-scsi@vger.kernel.org \
--cc=martin.petersen@oracle.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).