1d8476cc67
The idea comes from a discussion between Linus and Andrea [1].
Before this patch we only allow a page fault to retry once. We achieved
this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
handle_mm_fault() the second time. This was majorly used to avoid
unexpected starvation of the system by looping over forever to handle the
page fault on a single page. However that should hardly happen, and after
all for each code path to return a VM_FAULT_RETRY we'll first wait for a
condition (during which time we should possibly yield the cpu) to happen
before VM_FAULT_RETRY is really returned.
This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY
flag when we receive VM_FAULT_RETRY. It means that the page fault handler
now can retry the page fault for multiple times if necessary without the
need to generate another page fault event. Meanwhile we still keep the
FAULT_FLAG_TRIED flag so page fault handler can still identify whether a
page fault is the first attempt or not.
Then we'll have these combinations of fault flags (only considering
ALLOW_RETRY flag and TRIED flag):
- ALLOW_RETRY and !TRIED: this means the page fault allows to
retry, and this is the first try
- ALLOW_RETRY and TRIED: this means the page fault allows to
retry, and this is not the first try
- !ALLOW_RETRY and !TRIED: this means the page fault does not allow
to retry at all
- !ALLOW_RETRY and TRIED: this is forbidden and should never be used
In existing code we have multiple places that has taken special care of
the first condition above by checking against (fault_flags &
FAULT_FLAG_ALLOW_RETRY). This patch introduces a simple helper to detect
the first retry of a page fault by checking against both (fault_flags &
FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & FAULT_FLAG_TRIED) because now
even the 2nd try will have the ALLOW_RETRY set, then use that helper in
all existing special paths. One example is in __lock_page_or_retry(), now
we'll drop the mmap_sem only in the first attempt of page fault and we'll
keep it in follow up retries, so old locking behavior will be retained.
This will be a nice enhancement for current code [2] at the same time a
supporting material for the future userfaultfd-writeprotect work, since in
that work there will always be an explicit userfault writeprotect retry
for protected pages, and if that cannot resolve the page fault (e.g., when
userfaultfd-writeprotect is used in conjunction with swapped pages) then
we'll possibly need a 3rd retry of the page fault. It might also benefit
other potential users who will have similar requirement like userfault
write-protection.
GUP code is not touched yet and will be covered in follow up patch.
Please read the thread below for more information.
[1] https://lore.kernel.org/lkml/20171102193644.GB22686@redhat.com/
[2] https://lore.kernel.org/lkml/20181230154648.GB9832@redhat.com/
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Brian Geffon <bgeffon@google.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Link: http://lkml.kernel.org/r/20200220160246.9790-1-peterx@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 4064b982706375025628094e51d11cf1a958a5d3)
Bug: 176847924
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Change-Id: If0378d8ccbfc54a574b91103a6dc76e446f5f12e
OpenRISC Linux
==============
This is a port of Linux to the OpenRISC class of microprocessors; the initial
target architecture, specifically, is the 32-bit OpenRISC 1000 family (or1k).
For information about OpenRISC processors and ongoing development:
website http://openrisc.io
For more information about Linux on OpenRISC, please contact South Pole AB.
email: info@southpole.se
website: http://southpole.se
http://southpoleconsulting.com
---------------------------------------------------------------------
Build instructions for OpenRISC toolchain and Linux
===================================================
In order to build and run Linux for OpenRISC, you'll need at least a basic
toolchain and, perhaps, the architectural simulator. Steps to get these bits
in place are outlined here.
1) The toolchain can be obtained from openrisc.io. Instructions for building
a toolchain can be found at:
https://github.com/openrisc/tutorials
2) or1ksim (optional)
or1ksim is the architectural simulator which will allow you to actually run
your OpenRISC Linux kernel if you don't have an OpenRISC processor at hand.
git clone https://github.com/openrisc/or1ksim.git
cd or1ksim
./configure --prefix=$OPENRISC_PREFIX
make
make install
3) Linux kernel
Build the kernel as usual
make ARCH=openrisc defconfig
make ARCH=openrisc
4) Run in architectural simulator
Grab the or1ksim platform configuration file (from the or1ksim source) and
together with your freshly built vmlinux, run your kernel with the following
incantation:
sim -f arch/openrisc/or1ksim.cfg vmlinux
---------------------------------------------------------------------
Terminology
===========
In the code, the following particles are used on symbols to limit the scope
to more or less specific processor implementations:
openrisc: the OpenRISC class of processors
or1k: the OpenRISC 1000 family of processors
or1200: the OpenRISC 1200 processor
---------------------------------------------------------------------
History
========
18. 11. 2003 Matjaz Breskvar (phoenix@bsemi.com)
initial port of linux to OpenRISC/or32 architecture.
all the core stuff is implemented and seams usable.
08. 12. 2003 Matjaz Breskvar (phoenix@bsemi.com)
complete change of TLB miss handling.
rewrite of exceptions handling.
fully functional sash-3.6 in default initrd.
a much improved version with changes all around.
10. 04. 2004 Matjaz Breskvar (phoenix@bsemi.com)
alot of bugfixes all over.
ethernet support, functional http and telnet servers.
running many standard linux apps.
26. 06. 2004 Matjaz Breskvar (phoenix@bsemi.com)
port to 2.6.x
30. 11. 2004 Matjaz Breskvar (phoenix@bsemi.com)
lots of bugfixes and enhancments.
added opencores framebuffer driver.
09. 10. 2010 Jonas Bonn (jonas@southpole.se)
major rewrite to bring up to par with upstream Linux 2.6.36