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SubjectRe: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
On Thu, Dec 20, 2018 at 09:33:12AM +1100, Dave Chinner wrote:
> On Wed, Dec 19, 2018 at 12:35:40PM +0100, Jan Kara wrote:
> > On Wed 19-12-18 21:28:25, Dave Chinner wrote:
> > > On Tue, Dec 18, 2018 at 08:03:29PM -0700, Jason Gunthorpe wrote:
> > > > On Wed, Dec 19, 2018 at 10:42:54AM +1100, Dave Chinner wrote:
> > > >
> > > > > Essentially, what we are talking about is how to handle broken
> > > > > hardware. I say we should just brun it with napalm and thermite
> > > > > (i.e. taint the kernel with "unsupportable hardware") and force
> > > > > wait_for_stable_page() to trigger when there are GUP mappings if
> > > > > the underlying storage doesn't already require it.
> > > >
> > > > If you want to ban O_DIRECT/etc from writing to file backed pages,
> > > > then just do it.
> > >
> > > O_DIRECT IO *isn't the problem*.
> >
> > That is not true. O_DIRECT IO is a problem. In some aspects it is easier
> > than the problem with RDMA but currently O_DIRECT IO can crash your machine
> > or corrupt data the same way RDMA can.
>
> It's not O_DIRECT - it's a ""transient page pin". Yes, there are
> problems with that right now, but as we've discussed the issues can
> be avoided by:
>
> a) stable pages always blocking in ->page_mkwrite;
> b) blocking in write_cache_pages() on an elevated map count
> when WB_SYNC_ALL is set; and
> c) blocking in truncate_pagecache() on an elevated map
> count.
>
> That prevents:
> a) gup pinning a page that is currently under writeback and
> modifying it while IO is in flight;
> b) a dirty page being written back while it is pinned by
> GUP, thereby turning it clean before the gup reference calls
> set_page_dirty() on DMA completion; and
> c) truncate/hole punch for pulling the page out from under
> the gup operation that is ongoing.
>
> This is an adequate solution for a short term transient pins. It
> doesn't break fsync(), it doesn't change how truncate works and it
> fixes the problem where a mapped file is the buffer for an O_DIRECT
> IO rather than the open fd and that buffer file gets truncated.
> IOWs, transient pins (and hence O_DIRECT) is not really the problem
> here.
>
> The problem with this is that blocking on elevated map count does
> not work for long term pins (i.e. gup_longterm()) which are defined
> as:
>
> * "longterm" == userspace controlled elevated page count lifetime.
> * Contrast this to iov_iter_get_pages() usages which are transient.
>
> It's the "userspace controlled" part of the long term gup pin that
> is the problem we need to solve. If we treat them the same as a
> transient pin, then this leads to fsync() and truncate either
> blocking for a long time waiting for userspace to drop it's gup
> reference, or having to be failed with something like EBUSY or
> EAGAIN.
>
> This is the problem revokable file layout leases solve. The NFS
> server is already using this for revoking delegations from remote
> clients. Userspace holding long term GUP references is essentially
> the same thing - it's a delegation of file ownership to userspace
> that the filesystem must be able to revoke when it needs to run
> internal and/or 3rd-party requested operations on that delegated
> file.
>
> If the hardware supports page faults, then we can further optimise
> the long term pin case to relax stable page requirements and allow
> page cleaning to occur while there are long term pins. In this case,
> the hardware will write-fault the clean pages appropriately before
> DMA is initiated, and hence avoid the need for data integrity
> operations like fsync() to trigger lease revocation. However,
> truncate/hole punch still requires lease revocation to work sanely,
> especially when we consider DAX *must* ensure there are no remaining
> references to the physical pmem page after the space has been freed.

truncate does not requires lease recovations for faulting hardware,
truncate will trigger a mmu notifier callback which will invalidate
the hardware page table. On next access the hardware will fault and
this will turn into a regular page fault from kernel point of view.

So truncate/reflink and all fs expectation for faulting hardware do
hold. It is exactly as the CPU page table. So if CPU page table is
properly updated then so will be the hardware one.

Note that such hardware also abive by munmap() so hardware mapping
does not outlive vma.


Cheers,
Jérôme

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