From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
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 17:35:22 -0600 [thread overview]
Message-ID: <794b4022-843b-32de-f382-be0a375659c7@deltatee.com> (raw)
In-Reply-To: <Yy4/f+s1jOCm7dFo@ziepe.ca>
On 2022-09-23 17:21, Jason Gunthorpe wrote:
> 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.
Most of the call sites don't apply though, with huge pages and gate pages...
>>> 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.
That's not what the documentation for the function says:
"Either FOLL_PIN or FOLL_GET (or neither) may be set... Return: true for success,
or if no action was required (if neither FOLL_PIN nor FOLL_GET was set, nothing
is done)."
https://elixir.bootlin.com/linux/v6.0-rc6/source/mm/gup.c#L194
Logan
next prev parent reply other threads:[~2022-09-23 23:35 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
2022-09-23 23:35 ` Logan Gunthorpe [this message]
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=794b4022-843b-32de-f382-be0a375659c7@deltatee.com \
--to=logang@deltatee.com \
--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=jgg@ziepe.ca \
--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=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).