lkml.org 
[lkml]   [2019]   [Mar]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions
    On Wed, Mar 20, 2019 at 12:33:20AM -0400, Jerome Glisse wrote:
    > On Tue, Mar 19, 2019 at 06:43:45PM -0700, John Hubbard wrote:
    > > On 3/19/19 5:08 PM, Jerome Glisse wrote:
    > > > On Wed, Mar 20, 2019 at 10:57:52AM +1100, Dave Chinner wrote:
    > > >> On Tue, Mar 19, 2019 at 06:06:55PM -0400, Jerome Glisse wrote:
    > > >>> On Wed, Mar 20, 2019 at 08:23:46AM +1100, Dave Chinner wrote:
    > > >>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
    > > >>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
    > > >>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
    > > >>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote:
    > > >>>>>>>> From: John Hubbard <jhubbard@nvidia.com>
    > > >>>>>> [...]
    > > >>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan
    > > >>>>> about set_page_dirty() and Andrea had the good idea of maybe doing
    > > >>>>> the set_page_dirty() at GUP time (when GUP with write) not when the
    > > >>>>> GUP user calls put_page(). We can do that by setting the dirty bit
    > > >>>>> in the pte for instance. They are few bonus of doing things that way:
    > > >>>>> - amortize the cost of calling set_page_dirty() (ie one call for
    > > >>>>> GUP and page_mkclean()
    > > >>>>> - it is always safe to do so at GUP time (ie the pte has write
    > > >>>>> permission and thus the page is in correct state)
    > > >>>>> - safe from truncate race
    > > >>>>> - no need to ever lock the page
    > > >>>>
    > > >>>> I seem to have missed this conversation, so please excuse me for
    > > >>>
    > > >>> The set_page_dirty() at GUP was in a private discussion (it started
    > > >>> on another topic and drifted away to set_page_dirty()).
    > > >>>
    > > >>>> asking a stupid question: if it's a file backed page, what prevents
    > > >>>> background writeback from cleaning the dirty page ~30s into a long
    > > >>>> term pin? i.e. I don't see anything in this proposal that prevents
    > > >>>> the page from being cleaned by writeback and putting us straight
    > > >>>> back into the situation where a long term RDMA is writing to a clean
    > > >>>> page....
    > > >>>
    > > >>> So this patchset does not solve this issue.
    > > >>
    > > >> OK, so it just kicks the can further down the road.
    > > >>
    > > >>> [3..N] decide what to do for GUPed page, so far the plans seems
    > > >>> to be to keep the page always dirty and never allow page
    > > >>> write back to restore the page in a clean state. This does
    > > >>> disable thing like COW and other fs feature but at least
    > > >>> it seems to be the best thing we can do.
    > > >>
    > > >> So the plan for GUP vs writeback so far is "break fsync()"? :)
    > > >>
    > > >> We might need to work on that a bit more...
    > > >
    > > > Sorry forgot to say that we still do write back using a bounce page
    > > > so that at least we write something to disk that is just a snapshot
    > > > of the GUPed page everytime writeback kicks in (so either through
    > > > radix tree dirty page write back or fsync or any other sync events).
    > > > So many little details that i forgot the big chunk :)
    > > >
    > > > Cheers,
    > > > Jérôme
    > > >
    > >
    > > Dave, Jan, Jerome,
    > >
    > > Bounce pages for periodic data integrity still seem viable. But for the
    > > question of things like fsync or truncate, I think we were zeroing in
    > > on file leases as a nice building block.
    > >
    > > Can we revive the file lease discussion? By going all the way out to user
    > > space and requiring file leases to be coordinated at a high level in the
    > > software call chain, it seems like we could routinely avoid some of the
    > > worst conflicts that the kernel code has to resolve.
    > >
    > > For example:
    > >
    > > Process A
    > > =========
    > > gets a lease on file_a that allows gup
    > > usage on a range within file_a
    > >
    > > sets up writable DMA:
    > > get_user_pages() on the file_a range
    > > start DMA (independent hardware ops)
    > > hw is reading and writing to range
    > >
    > > Process B
    > > =========
    > > truncate(file_a)
    > > ...
    > > __break_lease()
    > >
    > > handle SIGIO from __break_lease
    > > if unhandled, process gets killed
    > > and put_user_pages should get called
    > > at some point here
    > >
    > > ...and so this way, user space gets to decide the proper behavior,
    > > instead of leaving the kernel in the dark with an impossible decision
    > > (kill process A? Block process B? User space knows the preference,
    > > per app, but kernel does not.)
    >
    > There is no need to kill anything here ... if truncate happens then
    > the GUP user is just GUPing page that do not correspond to anything
    > anymore. This is the current behavior and it is what GUP always has
    > been. By the time you get the page from GUP there is no garantee that
    > they correspond to anything.
    >
    > If a device really want to mirror process address faithfully then the
    > hardware need to make little effort either have something like ATS/
    > PASID or be able to abide mmu notifier.
    >
    > If we start blocking existing syscall just because someone is doing a
    > GUP we are opening a pandora box. It is not just truncate, it is a
    > whole range of syscall that deals with either file or virtual address.
    >
    > The semantic of GUP is really the semantic of direct I/O and the
    > virtual address you are direct I/O-ing to/from and the rule there is:
    > do not do anything stupid to those virtual addresses while you are
    > doing direct I/O with them (no munmap, mremap, madvise, truncate, ...).
    >
    >
    > Same logic apply to file, when two process do thing to same file there
    > the kernel never get in the way of one process doing something the
    > other process did not expect. For instance one process mmaping the file
    > the other process truncating the file, if the first process try to access
    > the file through the mmap after the truncation it will get a sigbus.
    >
    > So i believe best we could do is send a SIGBUS to the process that has
    > GUPed a range of a file that is being truncated this would match what
    > we do for CPU acces. There is no reason access through GUP should be
    > handled any differently.

    I agree in sending SIGBUS but the fact is most "Process A"'s will not be
    handling SIGBUS and will then result in that process dying.

    Ira

    >
    > Cheers,
    > Jérôme
    >

    \
     
     \ /
      Last update: 2019-03-20 18:11    [W:3.459 / U:0.364 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site