linux-pci.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, "Christoph Hellwig" <hch@lst.de>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Don Dutile" <ddutile@redhat.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Minturn Dave B" <dave.b.minturn@intel.com>,
	"Jason Ekstrand" <jason@jlekstrand.net>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Xiong Jianxin" <jianxin.xiong@intel.com>,
	"Bjorn Helgaas" <helgaas@kernel.org>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Martin Oliveira" <martin.oliveira@eideticom.com>,
	"Chaitanya Kulkarni" <ckulkarnilinux@gmail.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Stephen Bates" <sbates@raithlin.com>
Subject: Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages
Date: Fri, 23 Sep 2022 20:21:35 -0300	[thread overview]
Message-ID: <Yy4/f+s1jOCm7dFo@ziepe.ca> (raw)
In-Reply-To: <aa5d51dd-0b40-29c0-69af-e83043541d3e@deltatee.com>

On Fri, Sep 23, 2022 at 05:14:11PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2022-09-23 17:07, Jason Gunthorpe wrote:
> > On Fri, Sep 23, 2022 at 05:01:26PM -0600, Logan Gunthorpe wrote:
> >>
> >>
> >>
> >> On 2022-09-23 16:58, Jason Gunthorpe wrote:
> >>> On Fri, Sep 23, 2022 at 02:11:03PM -0600, Logan Gunthorpe wrote:
> >>>>
> >>>>
> >>>> On 2022-09-23 13:53, Jason Gunthorpe wrote:
> >>>>> On Fri, Sep 23, 2022 at 01:08:31PM -0600, Logan Gunthorpe wrote:
> >>>>> I'm encouraging Dan to work on better infrastructure in pgmap core
> >>>>> because every pgmap implementation has this issue currently.
> >>>>>
> >>>>> For that reason it is probably not so relavent to this series.
> >>>>>
> >>>>> Perhaps just clarify in the commit message that the FOLL_LONGTERM
> >>>>> restriction is to copy DAX until the pgmap page refcounts are fixed.
> >>>>
> >>>> Ok, I'll add that note.
> >>>>
> >>>> Per the fix for the try_grab_page(), to me it doesn't fit well in 
> >>>> try_grab_page() without doing a bunch of cleanup to change the
> >>>> error handling, and the same would have to be added to try_grab_folio().
> >>>> So I think it's better to leave it where it was, but move it below the 
> >>>> respective grab calls. Does the incremental patch below look correct?
> >>>
> >>> Oh? I was thinking of just a very simple thing:
> >>
> >> Really would like it to return -EREMOTEIO instead of -ENOMEM as that's the
> >> error used for bad P2PDMA page everywhere.
> > 
> > I'd rather not see GUP made more fragile just for that..
> 
> Not sure how that's more fragile... You're way seems more dangerous given
> the large number of call sites we are adding it to when it might not
> apply.

No, that is the point, it *always* applies. A devmap struct page of
the wrong type should never exit gup, from any path, no matter what.

We have two central functions that validate a page is OK to return,
that *everyone* must call.

If you don't put it there then we will probably miss copying it into a
call site eventually.

> > try_grab_page() calls folio_ref_inc(), that is only legal if it knows
> > the page is already a valid pointer under the PTLs, so it is safe to
> > check the pgmap as well.
> 
> My point is it doesn't get a reference or a pin unless FOLL_PIN or FOLL_GET is
> set and the documentation states that neither might be set, in which case 
> folio_ref_inc() will not be called...

That isn't how GUP is structured, all the calls to try_grab_page() are
in places where PIN/GET might be set and are safe for that usage.

If we know PIN/GET is not set then we don't even need to call the
function because it is a NOP.

Jason

  reply	other threads:[~2022-09-23 23:21 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-22 16:39 [PATCH v10 0/8] Userspace P2PDMA with O_DIRECT NVMe devices Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages Logan Gunthorpe
2022-09-23 18:13   ` Jason Gunthorpe
2022-09-23 19:08     ` Logan Gunthorpe
2022-09-23 19:53       ` Jason Gunthorpe
2022-09-23 20:11         ` Logan Gunthorpe
2022-09-23 22:58           ` Jason Gunthorpe
2022-09-23 23:01             ` Logan Gunthorpe
2022-09-23 23:07               ` Jason Gunthorpe
2022-09-23 23:14                 ` Logan Gunthorpe
2022-09-23 23:21                   ` Jason Gunthorpe [this message]
2022-09-23 23:35                     ` Logan Gunthorpe
2022-09-23 23:51                     ` Logan Gunthorpe
2022-09-26 22:57                       ` Jason Gunthorpe
2022-09-28 21:38                         ` Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 2/8] iov_iter: introduce iov_iter_get_pages_[alloc_]flags() Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 3/8] block: add check when merging zone device pages Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 4/8] lib/scatterlist: " Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 5/8] block: set FOLL_PCI_P2PDMA in __bio_iov_iter_get_pages() Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 6/8] block: set FOLL_PCI_P2PDMA in bio_map_user_iov() Logan Gunthorpe
2022-09-22 16:39 ` [PATCH v10 7/8] PCI/P2PDMA: Allow userspace VMA allocations through sysfs Logan Gunthorpe
2022-09-22 18:27   ` Bjorn Helgaas
2022-09-23  8:15   ` Greg Kroah-Hartman
2022-09-22 16:39 ` [PATCH v10 8/8] ABI: sysfs-bus-pci: add documentation for p2pmem allocate Logan Gunthorpe
2022-09-23  8:15   ` Greg Kroah-Hartman
2022-09-23  6:01 ` [PATCH v10 0/8] Userspace P2PDMA with O_DIRECT NVMe devices Christoph Hellwig
2022-09-23 15:25   ` Logan Gunthorpe
2022-09-23  8:16 ` Greg Kroah-Hartman

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=Yy4/f+s1jOCm7dFo@ziepe.ca \
    --to=jgg@ziepe.ca \
    --cc=christian.koenig@amd.com \
    --cc=ckulkarnilinux@gmail.com \
    --cc=dan.j.williams@intel.com \
    --cc=daniel.vetter@ffwll.ch \
    --cc=dave.b.minturn@intel.com \
    --cc=dave.hansen@linux.intel.com \
    --cc=ddutile@redhat.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=hch@lst.de \
    --cc=helgaas@kernel.org \
    --cc=ira.weiny@intel.com \
    --cc=jason@jlekstrand.net \
    --cc=jhubbard@nvidia.com \
    --cc=jianxin.xiong@intel.com \
    --cc=linux-block@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-nvme@lists.infradead.org \
    --cc=linux-pci@vger.kernel.org \
    --cc=logang@deltatee.com \
    --cc=martin.oliveira@eideticom.com \
    --cc=rcampbell@nvidia.com \
    --cc=robin.murphy@arm.com \
    --cc=sbates@raithlin.com \
    --cc=willy@infradead.org \
    /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).