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Patch series "Implement IOCTL to get and optionally clear info about PTEs", v33. *Motivation* The real motivation for adding PAGEMAP_SCAN IOCTL is to emulate Windows GetWriteWatch() and ResetWriteWatch() syscalls [1]. The GetWriteWatch() retrieves the addresses of the pages that are written to in a region of virtual memory. This syscall is used in Windows applications and games etc. This syscall is being emulated in pretty slow manner in userspace. Our purpose is to enhance the kernel such that we translate it efficiently in a better way. Currently some out of tree hack patches are being used to efficiently emulate it in some kernels. We intend to replace those with these patches. So the whole gaming on Linux can effectively get benefit from this. It means there would be tons of users of this code. CRIU use case [2] was mentioned by Andrei and Danylo: > Use cases for migrating sparse VMAs are binaries sanitized with ASAN, > MSAN or TSAN [3]. All of these sanitizers produce sparse mappings of > shadow memory [4]. Being able to migrate such binaries allows to highly > reduce the amount of work needed to identify and fix post-migration > crashes, which happen constantly. Andrei defines the following uses of this code: * it is more granular and allows us to track changed pages more effectively. The current interface can clear dirty bits for the entire process only. In addition, reading info about pages is a separate operation. It means we must freeze the process to read information about all its pages, reset dirty bits, only then we can start dumping pages. The information about pages becomes more and more outdated, while we are processing pages. The new interface solves both these downsides. First, it allows us to read pte bits and clear the soft-dirty bit atomically. It means that CRIU will not need to freeze processes to pre-dump their memory. Second, it clears soft-dirty bits for a specified region of memory. It means CRIU will have actual info about pages to the moment of dumping them. * The new interface has to be much faster because basic page filtering is happening in the kernel. With the old interface, we have to read pagemap for each page. *Implementation Evolution (Short Summary)* From the definition of GetWriteWatch(), we feel like kernel's soft-dirty feature can be used under the hood with some additions like: * reset soft-dirty flag for only a specific region of memory instead of clearing the flag for the entire process * get and clear soft-dirty flag for a specific region atomically So we decided to use ioctl on pagemap file to read or/and reset soft-dirty flag. But using soft-dirty flag, sometimes we get extra pages which weren't even written. They had become soft-dirty because of VMA merging and VM_SOFTDIRTY flag. This breaks the definition of GetWriteWatch(). We were able to by-pass this short coming by ignoring VM_SOFTDIRTY until David reported that mprotect etc messes up the soft-dirty flag while ignoring VM_SOFTDIRTY [5]. This wasn't happening until [6] got introduced. We discussed if we can revert these patches. But we could not reach to any conclusion. So at this point, I made couple of tries to solve this whole VM_SOFTDIRTY issue by correcting the soft-dirty implementation: * [7] Correct the bug fixed wrongly back in 2014. It had potential to cause regression. We left it behind. * [8] Keep a list of soft-dirty part of a VMA across splits and merges. I got the reply don't increase the size of the VMA by 8 bytes. At this point, we left soft-dirty considering it is too much delicate and userfaultfd [9] seemed like the only way forward. From there onward, we have been basing soft-dirty emulation on userfaultfd wp feature where kernel resolves the faults itself when WP_ASYNC feature is used. It was straight forward to add WP_ASYNC feature in userfautlfd. Now we get only those pages dirty or written-to which are really written in reality. (PS There is another WP_UNPOPULATED userfautfd feature is required which is needed to avoid pre-faulting memory before write-protecting [9].) All the different masks were added on the request of CRIU devs to create interface more generic and better. [1] https://learn.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-getwritewatch [2] https://lore.kernel.org/all/20221014134802.1361436-1-mdanylo@google.com [3] https://github.com/google/sanitizers [4] https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#64-bit [5] https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@redhat.com [6] https://lore.kernel.org/all/20220725142048.30450-1-peterx@redhat.com/ [7] https://lore.kernel.org/all/20221122115007.2787017-1-usama.anjum@collabora.com [8] https://lore.kernel.org/all/20221220162606.1595355-1-usama.anjum@collabora.com [9] https://lore.kernel.org/all/20230306213925.617814-1-peterx@redhat.com [10] https://lore.kernel.org/all/20230125144529.1630917-1-mdanylo@google.com This patch (of 6): Add a new userfaultfd-wp feature UFFD_FEATURE_WP_ASYNC, that allows userfaultfd wr-protect faults to be resolved by the kernel directly. It can be used like a high accuracy version of soft-dirty, without vma modifications during tracking, and also with ranged support by default rather than for a whole mm when reset the protections due to existence of ioctl(UFFDIO_WRITEPROTECT). Several goals of such a dirty tracking interface: 1. All types of memory should be supported and tracable. This is nature for soft-dirty but should mention when the context is userfaultfd, because it used to only support anon/shmem/hugetlb. The problem is for a dirty tracking purpose these three types may not be enough, and it's legal to track anything e.g. any page cache writes from mmap. 2. Protections can be applied to partial of a memory range, without vma split/merge fuss. The hope is that the tracking itself should not affect any vma layout change. It also helps when reset happens because the reset will not need mmap write lock which can block the tracee. 3. Accuracy needs to be maintained. This means we need pte markers to work on any type of VMA. One could question that, the whole concept of async dirty tracking is not really close to fundamentally what userfaultfd used to be: it's not "a fault to be serviced by userspace" anymore. However, using userfaultfd-wp here as a framework is convenient for us in at least: 1. VM_UFFD_WP vma flag, which has a very good name to suite something like this, so we don't need VM_YET_ANOTHER_SOFT_DIRTY. Just use a new feature bit to identify from a sync version of uffd-wp registration. 2. PTE markers logic can be leveraged across the whole kernel to maintain the uffd-wp bit as long as an arch supports, this also applies to this case where uffd-wp bit will be a hint to dirty information and it will not go lost easily (e.g. when some page cache ptes got zapped). 3. Reuse ioctl(UFFDIO_WRITEPROTECT) interface for either starting or resetting a range of memory, while there's no counterpart in the old soft-dirty world, hence if this is wanted in a new design we'll need a new interface otherwise. We can somehow understand that commonality because uffd-wp was fundamentally a similar idea of write-protecting pages just like soft-dirty. This implementation allows WP_ASYNC to imply WP_UNPOPULATED, because so far WP_ASYNC seems to not usable if without WP_UNPOPULATE. This also gives us chance to modify impl of WP_ASYNC just in case it could be not depending on WP_UNPOPULATED anymore in the future kernels. It's also fine to imply that because both features will rely on PTE_MARKER_UFFD_WP config option, so they'll show up together (or both missing) in an UFFDIO_API probe. vma_can_userfault() now allows any VMA if the userfaultfd registration is only about async uffd-wp. So we can track dirty for all kinds of memory including generic file systems (like XFS, EXT4 or BTRFS). One trick worth mention in do_wp_page() is that we need to manually update vmf->orig_pte here because it can be used later with a pte_same() check - this path always has FAULT_FLAG_ORIG_PTE_VALID set in the flags. The major defect of this approach of dirty tracking is we need to populate the pgtables when tracking starts. Soft-dirty doesn't do it like that. It's unwanted in the case where the range of memory to track is huge and unpopulated (e.g., tracking updates on a 10G file with mmap() on top, without having any page cache installed yet). One way to improve this is to allow pte markers exist for larger than PTE level for PMD+. That will not change the interface if to implemented, so we can leave that for later. Link: https://lkml.kernel.org/r/20230821141518.870589-1-usama.anjum@collabora.com Link: https://lkml.kernel.org/r/20230821141518.870589-2-usama.anjum@collabora.com Signed-off-by: Peter Xu <peterx@redhat.com> Co-developed-by: Muhammad Usama Anjum <usama.anjum@collabora.com> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com> Cc: Alex Sierra <alex.sierra@amd.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andrei Vagin <avagin@gmail.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Christian Brauner <brauner@kernel.org> Cc: Cyrill Gorcunov <gorcunov@gmail.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: David Hildenbrand <david@redhat.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Gustavo A. R. Silva <gustavoars@kernel.org> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Michal Miroslaw <emmir@google.com> Cc: Mike Rapoport (IBM) <rppt@kernel.org> Cc: Nadav Amit <namit@vmware.com> Cc: Pasha Tatashin <pasha.tatashin@soleen.com> Cc: Paul Gofman <pgofman@codeweavers.com> Cc: Shuah Khan <shuah@kernel.org> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Yang Shi <shy828301@gmail.com> Cc: Yun Zhou <yun.zhou@windriver.com> Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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bug-hunting.rst | ||
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clearing-warn-once.rst | ||
cpu-load.rst | ||
cputopology.rst | ||
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devices.rst | ||
devices.txt | ||
dynamic-debug-howto.rst | ||
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ext4.rst | ||
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iostats.rst | ||
java.rst | ||
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kernel-parameters.rst | ||
kernel-parameters.txt | ||
kernel-per-CPU-kthreads.rst | ||
lcd-panel-cgram.rst | ||
ldm.rst | ||
lockup-watchdogs.rst | ||
md.rst | ||
module-signing.rst | ||
mono.rst | ||
numastat.rst | ||
parport.rst | ||
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pnp.rst | ||
pstore-blk.rst | ||
quickly-build-trimmed-linux.rst | ||
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rapidio.rst | ||
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README.rst | ||
reporting-issues.rst | ||
reporting-regressions.rst | ||
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spkguide.txt | ||
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xfs.rst |
.. _readme: Linux kernel release 6.x <http://kernel.org/> ============================================= These are the release notes for Linux version 6. Read them carefully, as they tell you what this is all about, explain how to install the kernel, and what to do if something goes wrong. What is Linux? -------------- Linux is a clone of the operating system Unix, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance. It has all the features you would expect in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and multistack networking including IPv4 and IPv6. It is distributed under the GNU General Public License v2 - see the accompanying COPYING file for more details. On what hardware does it run? ----------------------------- Although originally developed first for 32-bit x86-based PCs (386 or higher), today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell, IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, and ARC architectures. Linux is easily portable to most general-purpose 32- or 64-bit architectures as long as they have a paged memory management unit (PMMU) and a port of the GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has also been ported to a number of architectures without a PMMU, although functionality is then obviously somewhat limited. Linux has also been ported to itself. You can now run the kernel as a userspace application - this is called UserMode Linux (UML). Documentation ------------- - There is a lot of documentation available both in electronic form on the Internet and in books, both Linux-specific and pertaining to general UNIX questions. I'd recommend looking into the documentation subdirectories on any Linux FTP site for the LDP (Linux Documentation Project) books. This README is not meant to be documentation on the system: there are much better sources available. - There are various README files in the Documentation/ subdirectory: these typically contain kernel-specific installation notes for some drivers for example. Please read the :ref:`Documentation/process/changes.rst <changes>` file, as it contains information about the problems, which may result by upgrading your kernel. Installing the kernel source ---------------------------- - If you install the full sources, put the kernel tarball in a directory where you have permissions (e.g. your home directory) and unpack it:: xz -cd linux-6.x.tar.xz | tar xvf - Replace "X" with the version number of the latest kernel. Do NOT use the /usr/src/linux area! This area has a (usually incomplete) set of kernel headers that are used by the library header files. They should match the library, and not get messed up by whatever the kernel-du-jour happens to be. - You can also upgrade between 6.x releases by patching. Patches are distributed in the xz format. To install by patching, get all the newer patch files, enter the top level directory of the kernel source (linux-6.x) and execute:: xz -cd ../patch-6.x.xz | patch -p1 Replace "x" for all versions bigger than the version "x" of your current source tree, **in_order**, and you should be ok. You may want to remove the backup files (some-file-name~ or some-file-name.orig), and make sure that there are no failed patches (some-file-name# or some-file-name.rej). If there are, either you or I have made a mistake. Unlike patches for the 6.x kernels, patches for the 6.x.y kernels (also known as the -stable kernels) are not incremental but instead apply directly to the base 6.x kernel. For example, if your base kernel is 6.0 and you want to apply the 6.0.3 patch, you must not first apply the 6.0.1 and 6.0.2 patches. Similarly, if you are running kernel version 6.0.2 and want to jump to 6.0.3, you must first reverse the 6.0.2 patch (that is, patch -R) **before** applying the 6.0.3 patch. You can read more on this in :ref:`Documentation/process/applying-patches.rst <applying_patches>`. Alternatively, the script patch-kernel can be used to automate this process. It determines the current kernel version and applies any patches found:: linux/scripts/patch-kernel linux The first argument in the command above is the location of the kernel source. Patches are applied from the current directory, but an alternative directory can be specified as the second argument. - Make sure you have no stale .o files and dependencies lying around:: cd linux make mrproper You should now have the sources correctly installed. Software requirements --------------------- Compiling and running the 6.x kernels requires up-to-date versions of various software packages. Consult :ref:`Documentation/process/changes.rst <changes>` for the minimum version numbers required and how to get updates for these packages. Beware that using excessively old versions of these packages can cause indirect errors that are very difficult to track down, so don't assume that you can just update packages when obvious problems arise during build or operation. Build directory for the kernel ------------------------------ When compiling the kernel, all output files will per default be stored together with the kernel source code. Using the option ``make O=output/dir`` allows you to specify an alternate place for the output files (including .config). Example:: kernel source code: /usr/src/linux-6.x build directory: /home/name/build/kernel To configure and build the kernel, use:: cd /usr/src/linux-6.x make O=/home/name/build/kernel menuconfig make O=/home/name/build/kernel sudo make O=/home/name/build/kernel modules_install install Please note: If the ``O=output/dir`` option is used, then it must be used for all invocations of make. Configuring the kernel ---------------------- Do not skip this step even if you are only upgrading one minor version. New configuration options are added in each release, and odd problems will turn up if the configuration files are not set up as expected. If you want to carry your existing configuration to a new version with minimal work, use ``make oldconfig``, which will only ask you for the answers to new questions. - Alternative configuration commands are:: "make config" Plain text interface. "make menuconfig" Text based color menus, radiolists & dialogs. "make nconfig" Enhanced text based color menus. "make xconfig" Qt based configuration tool. "make gconfig" GTK+ based configuration tool. "make oldconfig" Default all questions based on the contents of your existing ./.config file and asking about new config symbols. "make olddefconfig" Like above, but sets new symbols to their default values without prompting. "make defconfig" Create a ./.config file by using the default symbol values from either arch/$ARCH/defconfig or arch/$ARCH/configs/${PLATFORM}_defconfig, depending on the architecture. "make ${PLATFORM}_defconfig" Create a ./.config file by using the default symbol values from arch/$ARCH/configs/${PLATFORM}_defconfig. Use "make help" to get a list of all available platforms of your architecture. "make allyesconfig" Create a ./.config file by setting symbol values to 'y' as much as possible. "make allmodconfig" Create a ./.config file by setting symbol values to 'm' as much as possible. "make allnoconfig" Create a ./.config file by setting symbol values to 'n' as much as possible. "make randconfig" Create a ./.config file by setting symbol values to random values. "make localmodconfig" Create a config based on current config and loaded modules (lsmod). Disables any module option that is not needed for the loaded modules. To create a localmodconfig for another machine, store the lsmod of that machine into a file and pass it in as a LSMOD parameter. Also, you can preserve modules in certain folders or kconfig files by specifying their paths in parameter LMC_KEEP. target$ lsmod > /tmp/mylsmod target$ scp /tmp/mylsmod host:/tmp host$ make LSMOD=/tmp/mylsmod \ LMC_KEEP="drivers/usb:drivers/gpu:fs" \ localmodconfig The above also works when cross compiling. "make localyesconfig" Similar to localmodconfig, except it will convert all module options to built in (=y) options. You can also preserve modules by LMC_KEEP. "make kvm_guest.config" Enable additional options for kvm guest kernel support. "make xen.config" Enable additional options for xen dom0 guest kernel support. "make tinyconfig" Configure the tiniest possible kernel. You can find more information on using the Linux kernel config tools in Documentation/kbuild/kconfig.rst. - NOTES on ``make config``: - Having unnecessary drivers will make the kernel bigger, and can under some circumstances lead to problems: probing for a nonexistent controller card may confuse your other controllers. - A kernel with math-emulation compiled in will still use the coprocessor if one is present: the math emulation will just never get used in that case. The kernel will be slightly larger, but will work on different machines regardless of whether they have a math coprocessor or not. - The "kernel hacking" configuration details usually result in a bigger or slower kernel (or both), and can even make the kernel less stable by configuring some routines to actively try to break bad code to find kernel problems (kmalloc()). Thus you should probably answer 'n' to the questions for "development", "experimental", or "debugging" features. Compiling the kernel -------------------- - Make sure you have at least gcc 5.1 available. For more information, refer to :ref:`Documentation/process/changes.rst <changes>`. - Do a ``make`` to create a compressed kernel image. It is also possible to do ``make install`` if you have lilo installed to suit the kernel makefiles, but you may want to check your particular lilo setup first. To do the actual install, you have to be root, but none of the normal build should require that. Don't take the name of root in vain. - If you configured any of the parts of the kernel as ``modules``, you will also have to do ``make modules_install``. - Verbose kernel compile/build output: Normally, the kernel build system runs in a fairly quiet mode (but not totally silent). However, sometimes you or other kernel developers need to see compile, link, or other commands exactly as they are executed. For this, use "verbose" build mode. This is done by passing ``V=1`` to the ``make`` command, e.g.:: make V=1 all To have the build system also tell the reason for the rebuild of each target, use ``V=2``. The default is ``V=0``. - Keep a backup kernel handy in case something goes wrong. This is especially true for the development releases, since each new release contains new code which has not been debugged. Make sure you keep a backup of the modules corresponding to that kernel, as well. If you are installing a new kernel with the same version number as your working kernel, make a backup of your modules directory before you do a ``make modules_install``. Alternatively, before compiling, use the kernel config option "LOCALVERSION" to append a unique suffix to the regular kernel version. LOCALVERSION can be set in the "General Setup" menu. - In order to boot your new kernel, you'll need to copy the kernel image (e.g. .../linux/arch/x86/boot/bzImage after compilation) to the place where your regular bootable kernel is found. - Booting a kernel directly from a floppy without the assistance of a bootloader such as LILO, is no longer supported. If you boot Linux from the hard drive, chances are you use LILO, which uses the kernel image as specified in the file /etc/lilo.conf. The kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or /boot/bzImage. To use the new kernel, save a copy of the old image and copy the new image over the old one. Then, you MUST RERUN LILO to update the loading map! If you don't, you won't be able to boot the new kernel image. Reinstalling LILO is usually a matter of running /sbin/lilo. You may wish to edit /etc/lilo.conf to specify an entry for your old kernel image (say, /vmlinux.old) in case the new one does not work. See the LILO docs for more information. After reinstalling LILO, you should be all set. Shutdown the system, reboot, and enjoy! If you ever need to change the default root device, video mode, etc. in the kernel image, use your bootloader's boot options where appropriate. No need to recompile the kernel to change these parameters. - Reboot with the new kernel and enjoy. If something goes wrong ----------------------- If you have problems that seem to be due to kernel bugs, please follow the instructions at 'Documentation/admin-guide/reporting-issues.rst'. Hints on understanding kernel bug reports are in 'Documentation/admin-guide/bug-hunting.rst'. More on debugging the kernel with gdb is in 'Documentation/dev-tools/gdb-kernel-debugging.rst' and 'Documentation/dev-tools/kgdb.rst'.