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From: Dan Williams <dan.j.williams@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Joao Martins <joao.m.martins@oracle.com>,
	Linux MM <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Matthew Wilcox <willy@infradead.org>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ralph Campbell <rcampbell@nvidia.com>
Subject: Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps
Date: Tue, 23 Feb 2021 17:32:16 -0800	[thread overview]
Message-ID: <CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@mail.gmail.com> (raw)
In-Reply-To: <20210224010017.GQ2643399@ziepe.ca>

On Tue, Feb 23, 2021 at 5:00 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Feb 23, 2021 at 04:14:01PM -0800, Dan Williams wrote:
> > [ add Ralph ]
> >
> > On Tue, Feb 23, 2021 at 3:07 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Tue, Feb 23, 2021 at 02:48:20PM -0800, Dan Williams wrote:
> > > > On Tue, Feb 23, 2021 at 10:54 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > >
> > > > > On Tue, Feb 23, 2021 at 08:44:52AM -0800, Dan Williams wrote:
> > > > >
> > > > > > > The downside would be one extra lookup in dev_pagemap tree
> > > > > > > for other pgmap->types (P2P, FSDAX, PRIVATE). But just one
> > > > > > > per gup-fast() call.
> > > > > >
> > > > > > I'd guess a dev_pagemap lookup is faster than a get_user_pages slow
> > > > > > path. It should be measurable that this change is at least as fast or
> > > > > > faster than falling back to the slow path, but it would be good to
> > > > > > measure.
> > > > >
> > > > > What is the dev_pagemap thing doing in gup fast anyhow?
> > > > >
> > > > > I've been wondering for a while..
> > > >
> > > > It's there to synchronize against dax-device removal. The device will
> > > > suspend removal awaiting all page references to be dropped, but
> > > > gup-fast could be racing device removal. So gup-fast checks for
> > > > pte_devmap() to grab a live reference to the device before assuming it
> > > > can pin a page.
> > >
> > > From the perspective of CPU A it can't tell if CPU B is doing a HW
> > > page table walk or a GUP fast when it invalidates a page table. The
> > > design of gup-fast is supposed to be the same as the design of a HW
> > > page table walk, and the tlb invalidate CPU A does when removing a
> > > page from a page table is supposed to serialize against both a HW page
> > > table walk and gup-fast.
> > >
> > > Given that the HW page table walker does not do dev_pagemap stuff, why
> > > does gup-fast?
> >
> > gup-fast historically assumed that the 'struct page' and memory
> > backing the page-table walk could not physically be removed from the
> > system during its walk because those pages were allocated from the
> > page allocator before being mapped into userspace.
>
> No, I'd say gup-fast assumes that any non-special PTE it finds in a
> page table must have a struct page.
>
> If something wants to remove that struct page it must first remove all
> the PTEs pointing at it from the entire system and flush the TLBs,
> which directly prevents a future gup-fast from running and trying to
> access the struct page. No extra locking needed
>
> > implied elevated reference on any page that gup-fast would be asked to
> > walk, or pte_special() is there to "say wait, nevermind this isn't a
> > page allocator page fallback to gup-slow()".
>
> pte_special says there is no struct page, and some of those cases can
> be fixed up in gup-slow.
>
> > > Can you sketch the exact race this is protecting against?
> >
> > Thread1 mmaps /mnt/daxfile1 from a "mount -o dax" filesystem and
> > issues direct I/O with that mapping as the target buffer, Thread2 does
> > "echo "namespace0.0" > /sys/bus/nd/drivers/nd_pmem/unbind". Without
> > the dev_pagemap check reference gup-fast could execute
> > get_page(pte_page(pte)) on a page that doesn't even exist anymore
> > because the driver unbind has already performed remove_pages().
>
> Surely the unbind either waits for all the VMAs to be destroyed or
> zaps them before allowing things to progress to remove_pages()?

If we're talking about device-dax this is precisely what it does, zaps
and prevents new faults from resolving, but filesystem-dax...

> Having a situation where the CPU page tables still point at physical
> pages that have been removed sounds so crazy/insecure, that can't be
> what is happening, can it??

Hmm, that may be true and an original dax bug! The unbind of a
block-device from underneath the filesystem does trigger the
filesystem to emergency shutdown / go read-only, but unless that
process also includes a global zap of all dax mappings not only is
that violating expectations of "page-tables to disappearing memory",
but the filesystem may also want to guarantee that no further dax
writes can happen after shutdown. Right now I believe it only assumes
that mmap I/O will come from page writeback so there's no need to
bother applications with mappings to page cache, but dax mappings need
to be ripped away.

/me goes to look at what filesytems guarantee when the block-device is
surprise removed out from under them.

In any event, this accelerates the effort to go implement
fs-global-dax-zap at the request of the device driver.
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WARNING: multiple messages have this Message-ID
From: Dan Williams <dan.j.williams@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Joao Martins <joao.m.martins@oracle.com>,
	Linux MM <linux-mm@kvack.org>,  Ira Weiny <ira.weiny@intel.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	 Matthew Wilcox <willy@infradead.org>,
	Jane Chu <jane.chu@oracle.com>,
	 Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	 Andrew Morton <akpm@linux-foundation.org>,
	Ralph Campbell <rcampbell@nvidia.com>
Subject: Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps
Date: Tue, 23 Feb 2021 17:32:16 -0800	[thread overview]
Message-ID: <CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@mail.gmail.com> (raw)
In-Reply-To: <20210224010017.GQ2643399@ziepe.ca>

On Tue, Feb 23, 2021 at 5:00 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Feb 23, 2021 at 04:14:01PM -0800, Dan Williams wrote:
> > [ add Ralph ]
> >
> > On Tue, Feb 23, 2021 at 3:07 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Tue, Feb 23, 2021 at 02:48:20PM -0800, Dan Williams wrote:
> > > > On Tue, Feb 23, 2021 at 10:54 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > >
> > > > > On Tue, Feb 23, 2021 at 08:44:52AM -0800, Dan Williams wrote:
> > > > >
> > > > > > > The downside would be one extra lookup in dev_pagemap tree
> > > > > > > for other pgmap->types (P2P, FSDAX, PRIVATE). But just one
> > > > > > > per gup-fast() call.
> > > > > >
> > > > > > I'd guess a dev_pagemap lookup is faster than a get_user_pages slow
> > > > > > path. It should be measurable that this change is at least as fast or
> > > > > > faster than falling back to the slow path, but it would be good to
> > > > > > measure.
> > > > >
> > > > > What is the dev_pagemap thing doing in gup fast anyhow?
> > > > >
> > > > > I've been wondering for a while..
> > > >
> > > > It's there to synchronize against dax-device removal. The device will
> > > > suspend removal awaiting all page references to be dropped, but
> > > > gup-fast could be racing device removal. So gup-fast checks for
> > > > pte_devmap() to grab a live reference to the device before assuming it
> > > > can pin a page.
> > >
> > > From the perspective of CPU A it can't tell if CPU B is doing a HW
> > > page table walk or a GUP fast when it invalidates a page table. The
> > > design of gup-fast is supposed to be the same as the design of a HW
> > > page table walk, and the tlb invalidate CPU A does when removing a
> > > page from a page table is supposed to serialize against both a HW page
> > > table walk and gup-fast.
> > >
> > > Given that the HW page table walker does not do dev_pagemap stuff, why
> > > does gup-fast?
> >
> > gup-fast historically assumed that the 'struct page' and memory
> > backing the page-table walk could not physically be removed from the
> > system during its walk because those pages were allocated from the
> > page allocator before being mapped into userspace.
>
> No, I'd say gup-fast assumes that any non-special PTE it finds in a
> page table must have a struct page.
>
> If something wants to remove that struct page it must first remove all
> the PTEs pointing at it from the entire system and flush the TLBs,
> which directly prevents a future gup-fast from running and trying to
> access the struct page. No extra locking needed
>
> > implied elevated reference on any page that gup-fast would be asked to
> > walk, or pte_special() is there to "say wait, nevermind this isn't a
> > page allocator page fallback to gup-slow()".
>
> pte_special says there is no struct page, and some of those cases can
> be fixed up in gup-slow.
>
> > > Can you sketch the exact race this is protecting against?
> >
> > Thread1 mmaps /mnt/daxfile1 from a "mount -o dax" filesystem and
> > issues direct I/O with that mapping as the target buffer, Thread2 does
> > "echo "namespace0.0" > /sys/bus/nd/drivers/nd_pmem/unbind". Without
> > the dev_pagemap check reference gup-fast could execute
> > get_page(pte_page(pte)) on a page that doesn't even exist anymore
> > because the driver unbind has already performed remove_pages().
>
> Surely the unbind either waits for all the VMAs to be destroyed or
> zaps them before allowing things to progress to remove_pages()?

If we're talking about device-dax this is precisely what it does, zaps
and prevents new faults from resolving, but filesystem-dax...

> Having a situation where the CPU page tables still point at physical
> pages that have been removed sounds so crazy/insecure, that can't be
> what is happening, can it??

Hmm, that may be true and an original dax bug! The unbind of a
block-device from underneath the filesystem does trigger the
filesystem to emergency shutdown / go read-only, but unless that
process also includes a global zap of all dax mappings not only is
that violating expectations of "page-tables to disappearing memory",
but the filesystem may also want to guarantee that no further dax
writes can happen after shutdown. Right now I believe it only assumes
that mmap I/O will come from page writeback so there's no need to
bother applications with mappings to page cache, but dax mappings need
to be ripped away.

/me goes to look at what filesytems guarantee when the block-device is
surprise removed out from under them.

In any event, this accelerates the effort to go implement
fs-global-dax-zap at the request of the device driver.


  reply	other threads:[~2021-02-24  1:32 UTC|newest]

Thread overview: 147+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-08 17:28 Joao Martins
2020-12-08 17:28 ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-09  5:59   ` John Hubbard
2020-12-09  5:59     ` John Hubbard
2020-12-09  6:33     ` Matthew Wilcox
2020-12-09  6:33       ` Matthew Wilcox
2020-12-09 13:12       ` Joao Martins
2020-12-09 13:12         ` Joao Martins
2021-02-20  1:43     ` Dan Williams
2021-02-20  1:43       ` Dan Williams
2021-02-22 11:24       ` Joao Martins
2021-02-22 11:24         ` Joao Martins
2021-02-22 20:37         ` Dan Williams
2021-02-22 20:37           ` Dan Williams
2021-02-23 15:46           ` Joao Martins
2021-02-23 15:46             ` Joao Martins
2021-02-23 16:50             ` Dan Williams
2021-02-23 16:50               ` Dan Williams
2021-02-23 17:18               ` Joao Martins
2021-02-23 17:18                 ` Joao Martins
2021-02-23 18:18                 ` Dan Williams
2021-02-23 18:18                   ` Dan Williams
2021-03-10 18:12           ` Joao Martins
2021-03-10 18:12             ` Joao Martins
2021-03-12  5:54             ` Dan Williams
2021-03-12  5:54               ` Dan Williams
2021-02-20  1:24   ` Dan Williams
2021-02-20  1:24     ` Dan Williams
2021-02-22 11:09     ` Joao Martins
2021-02-22 11:09       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 2/9] sparse-vmemmap: Consolidate arguments in vmemmap section populate Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-09  6:16   ` John Hubbard
2020-12-09  6:16     ` John Hubbard
2020-12-09 13:51     ` Joao Martins
2020-12-09 13:51       ` Joao Martins
2021-02-20  1:49   ` Dan Williams
2021-02-20  1:49     ` Dan Williams
2021-02-22 11:26     ` Joao Martins
2021-02-22 11:26       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given mhp_params::align Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 17:38   ` Joao Martins
2020-12-08 17:38     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given page size Joao Martins
2020-12-08 17:28   ` Joao Martins
2021-02-20  3:34   ` Dan Williams
2021-02-20  3:34     ` Dan Williams
2021-02-22 11:42     ` Joao Martins
2021-02-22 11:42       ` Joao Martins
2021-02-22 22:40       ` Dan Williams
2021-02-22 22:40         ` Dan Williams
2021-02-23 15:46         ` Joao Martins
2021-02-23 15:46           ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 4/9] mm/page_alloc: Reuse tail struct pages for compound pagemaps Joao Martins
2020-12-08 17:28   ` Joao Martins
2021-02-20  6:17   ` Dan Williams
2021-02-20  6:17     ` Dan Williams
2021-02-22 12:01     ` Joao Martins
2021-02-22 12:01       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 5/9] device-dax: Compound pagemap support Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 6/9] mm/gup: Grab head page refcount once for group of subpages Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 19:49   ` Jason Gunthorpe
2020-12-09 11:05     ` Joao Martins
2020-12-09 11:05       ` Joao Martins
2020-12-09 15:15       ` Jason Gunthorpe
2020-12-09 16:02         ` Joao Martins
2020-12-09 16:02           ` Joao Martins
2020-12-09 16:24           ` Jason Gunthorpe
2020-12-09 17:27             ` Joao Martins
2020-12-09 17:27               ` Joao Martins
2020-12-09 18:14             ` Matthew Wilcox
2020-12-09 18:14               ` Matthew Wilcox
2020-12-09 19:08               ` Jason Gunthorpe
2020-12-10 15:43               ` Joao Martins
2020-12-10 15:43                 ` Joao Martins
2020-12-09  4:40   ` John Hubbard
2020-12-09  4:40     ` John Hubbard
2020-12-09 13:44     ` Joao Martins
2020-12-09 13:44       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 7/9] mm/gup: Decrement head page " Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 19:34   ` Jason Gunthorpe
2020-12-09  5:06     ` John Hubbard
2020-12-09  5:06       ` John Hubbard
2020-12-09 13:43       ` Jason Gunthorpe
2020-12-09 12:17     ` Joao Martins
2020-12-09 12:17       ` Joao Martins
2020-12-17 19:05     ` Joao Martins
2020-12-17 19:05       ` Joao Martins
2020-12-17 20:05       ` Jason Gunthorpe
2020-12-17 22:34         ` Joao Martins
2020-12-17 22:34           ` Joao Martins
2020-12-18 14:25           ` Jason Gunthorpe
2020-12-19  2:06         ` John Hubbard
2020-12-19  2:06           ` John Hubbard
2020-12-19 13:10           ` Joao Martins
2020-12-19 13:10             ` Joao Martins
2020-12-08 17:29 ` [PATCH RFC 8/9] RDMA/umem: batch page unpin in __ib_mem_release() Joao Martins
2020-12-08 17:29   ` Joao Martins
2020-12-08 19:29   ` Jason Gunthorpe
2020-12-09 10:59     ` Joao Martins
2020-12-09 10:59       ` Joao Martins
2020-12-19 13:15       ` Joao Martins
2020-12-19 13:15         ` Joao Martins
2020-12-09  5:18   ` John Hubbard
2020-12-09  5:18     ` John Hubbard
2020-12-08 17:29 ` [PATCH RFC 9/9] mm: Add follow_devmap_page() for devdax vmas Joao Martins
2020-12-08 17:29   ` Joao Martins
2020-12-08 19:57   ` Jason Gunthorpe
2020-12-09  8:05     ` Christoph Hellwig
2020-12-09  8:05       ` Christoph Hellwig
2020-12-09 11:19     ` Joao Martins
2020-12-09 11:19       ` Joao Martins
2020-12-09  5:23   ` John Hubbard
2020-12-09  5:23     ` John Hubbard
2020-12-09  9:38 ` [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps David Hildenbrand
2020-12-09  9:38   ` David Hildenbrand
2020-12-09  9:52 ` [External] " Muchun Song
2020-12-09  9:52   ` Muchun Song
2021-02-20  1:18 ` Dan Williams
2021-02-20  1:18   ` Dan Williams
2021-02-22 11:06   ` Joao Martins
2021-02-22 11:06     ` Joao Martins
2021-02-22 14:32     ` Joao Martins
2021-02-22 14:32       ` Joao Martins
2021-02-23 16:28   ` Joao Martins
2021-02-23 16:28     ` Joao Martins
2021-02-23 16:44     ` Dan Williams
2021-02-23 16:44       ` Dan Williams
2021-02-23 17:15       ` Joao Martins
2021-02-23 17:15         ` Joao Martins
2021-02-23 18:15         ` Dan Williams
2021-02-23 18:15           ` Dan Williams
2021-02-23 18:54       ` Jason Gunthorpe
2021-02-23 22:48         ` Dan Williams
2021-02-23 22:48           ` Dan Williams
2021-02-23 23:07           ` Jason Gunthorpe
2021-02-24  0:14             ` Dan Williams
2021-02-24  0:14               ` Dan Williams
2021-02-24  1:00               ` Jason Gunthorpe
2021-02-24  1:32                 ` Dan Williams [this message]
2021-02-24  1:32                   ` Dan Williams

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