linux-kernel - Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap

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Date:   Fri, 1 Oct 2021 16:22:28 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Alistair Popple <apopple@...dia.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()




On 2021-10-01 4:14 p.m., Jason Gunthorpe wrote:
> On Fri, Oct 01, 2021 at 02:13:14PM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2021-10-01 11:45 a.m., Jason Gunthorpe wrote:
>>>> Before the invalidation, an active flag is cleared to ensure no new
>>>> mappings can be created while the unmap is proceeding.
>>>> unmap_mapping_range() should sequence itself with the TLB flush and
>>>
>>> AFIAK unmap_mapping_range() kicks off the TLB flush and then
>>> returns. It doesn't always wait for the flush to fully finish. Ie some
>>> cases use RCU to lock the page table against GUP fast and so the
>>> put_page() doesn't happen until the call_rcu completes - after a grace
>>> period. The unmap_mapping_range() does not wait for grace periods.
>>
>> Admittedly, the tlb flush code isn't the easiest code to understand.
>> But, yes it seems at least on some arches the pages are freed by
>> call_rcu(). But can't this be fixed easily by adding a synchronize_rcu()
>> call after calling unmap_mapping_range()? Certainly after a
>> synchronize_rcu(), the TLB has been flushed and it is safe to free those
>> pages.
> 
> It would close this issue, however synchronize_rcu() is very slow
> (think > 1second) in some cases and thus cannot be inserted here.

It shouldn't be *that* slow, at least not the vast majority of the
time... it seems a bit unreasonable that a CPU wouldn't schedule for
more than a second. But these aren't fast paths and synchronize_rcu()
already gets called in the unbind path for p2pdma a of couple times. I'm
sure it would also be fine to slow down the vma_close() path as well.

> I'm also not completely sure that rcu is the only case, I don't know
> how every arch handles its gather structure.. I have a feeling the
> general intention was for this to be asynchronous

Yeah, this is not clear to me either.

> My preferences are to either remove devmap from gup_fast, or fix it to
> not use special pages - the latter being obviously better.

Yeah, I rather expect DAX users want the optimization provided by
gup_fast. I don't think P2PDMA users would be happy about being stuck
with slow gup either.

Loga