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Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com Subject: Re: [PATCH v28 5/6] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Content-Language: en-US To: =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= References: <20230809061603.1969154-1-usama.anjum@collabora.com> <20230809061603.1969154-6-usama.anjum@collabora.com> From: Muhammad Usama Anjum In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: rxwuu9h54m6p7dtf1duqt3ahu6hkudut X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 5DDA040018 X-HE-Tag: 1691772741-992090 X-HE-Meta: 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 om4+v9D4 jLnl+Mzzx1MMu5LzexLTkzWcY/juqeIunakI+XB4FyN2wNnWNnDnm9Qz9ySy8nQK76w23aPfEiq6mK5Or2eyMQ/iH+bBPFXSst3cTq9Oov8kVM7VC0BMaJ0fg/kSFx0rr2/7HIXlTQTOodoFiIz1cR8OYhqT4HQonmQ3S5Wme1W8DSICUt4g+res+Iw4jae8p6egkr0H/ZGdwjvXSJoYbJwFyT0gJmJ2/NXc/EdVlvA1N7UbQ5DQqkoVdeXpLbG0nYZCqClCfbZvdxXLruHdSir5MHxgz5ijuh8op5h8VZdoslCB+8j1qATxI9A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/11/23 12:26 AM, Michał Mirosław wrote: > On Wed, 9 Aug 2023 at 08:16, Muhammad Usama Anjum > wrote: >> Add some explanation and method to use write-protection and written-to >> on memory range. > [...] >> --- a/Documentation/admin-guide/mm/pagemap.rst >> +++ b/Documentation/admin-guide/mm/pagemap.rst >> @@ -227,3 +227,67 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is >> always 12 at most architectures). Since Linux 3.11 their meaning changes >> after first clear of soft-dirty bits. Since Linux 4.2 they are used for >> flags unconditionally. >> + >> +Pagemap Scan IOCTL >> +================== >> + >> +The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally >> +clear the info about page table entries. The following operations are supported >> +in this IOCTL: >> +- Get the information if the pages have Async Write-Protection enabled >> + (``PAGE_IS_WPALLOWED``), have been written to (``PAGE_IS_WRITTEN``), file mapped >> + (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped (``PAGE_IS_SWAPPED``) >> + or page has pfn zero (``PAGE_IS_PFNZERO``). > > A recent addition -- PAGE_IS_HUGE -- is missing. > > BTW, it could be easier to understand if the page categories were > separated from the operation description and listed so that each has > its own line and maybe a longer description where needed. I've made 90% of changes you have asked in documentation. > >> +- Find pages which have been written to and/or write protect >> + (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages atomically. >> + The (``PM_SCAN_WP_MATCHING``) is used to WP the matched pages. The >> + (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if non-Async-Write-Protected >> + pages are found. > > The operation the IOCTL does now is: "scan the process page tables and > report memory ranges matching provided criteria '. > Flags extend the operation: "PM_SCAN_WP_MATCHING write protects the > memory reported" (it does it atomically, but this is just an > optimization, isn't it? A process could gather the ranges, WP them, > and then copy.) > "PM_SCAN_CHECK_WPASYNC" aborts the scan early if a non-WP-able > matching page is found. > >> +The ``struct pm_scan_arg`` is used as the argument of the IOCTL. >> + 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size`` >> + field. This field will be helpful in recognizing the structure if extensions >> + are done later. >> + 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_WP_MATCHING`` >> + and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get >> + operation is optionally performed depending upon if the output buffer is >> + provided or not. >> + 3. The range is specified through ``start`` and ``end``. >> + 4. The output buffer of ``struct page_region`` array and size is specified in >> + ``vec`` and ``vec_len``. >> + 5. The optional maximum requested pages are specified in the ``max_pages``. >> + 6. The masks are specified in ``category_mask``, ``category_anyof_mask``, >> + ``category_inverted`` and ``return_mask``. >> + 1. To find if ``PAGE_IS_WRITTEN`` flag is set for pages which have >> + ``PAGE_IS_FILE`` set and ``PAGE_IS_SWAPPED`` unset, ``category_mask`` >> + is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``, ``category_inverted`` is >> + set to ``PAGE_IS_SWAPPED`` and ``return_mask`` is set to ``PAGE_IS_WRITTEN``. >> + The output buffer in ``vec`` and length must be specified in ``vec_len``. >> + 2. To find pages which have either ``PAGE_IS_FILE`` or ``PAGE_IS_SWAPPED`` >> + set, ``category_anyof_mask`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``. >> + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is >> + specified in ``category_mask`` and ``return_mask``. In addition to >> + specifying the output buffer in ``vec`` and length in ``vec_len``, the >> + ``PM_SCAN_WP_MATCHING`` is specified in ``flags`` to perform write protect >> + on the range as well. > > Could this be rewritten as examples? E.g.: > > Finding dirty file-backed pages: > > struct pm_scan_arg arg = { > .size = sizeof(arg), > .flags = 0, > ... > .category_mask = ..., > .return_mask = ... > }; > ssize_t n = ioctl(..., &arg); > > Find dirty pages and write protect them in the same call: > > arg = { ... }; > do { > ... ioctl(...) > } while(...); > > (The code snippets heavily commented.) > >> +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct > > "as a better-performing alternative" > >> +alternative of soft-dirty flag. It doesn't get affected by housekeeping chores >> +(VMA merging) of the kernel and hence the user can find the true soft-dirty pages >> +only. > > This is still an optimization, e.g. in THP case there might be too > many pages reported? > >> + This IOCTL adds the atomic way to find which pages have been written and >> +write protect those pages again. This kind of operation is needed to efficiently >> +find out which pages have changed in the memory. > > This repeats the description of PM_SCAN_WP_MATCHING -- I suggest > removing this part. > >> +To get information about which pages have been written to or optionally write >> +protect the pages, following must be performed first in order: > > "PAGE_IS_WRITTEN" category is used with uffd write protect-enabled > ranges to implement memory dirty tracking in userspace: > >> + 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall. >> + 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features >> + are set by ``UFFDIO_API`` IOCTL. >> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode >> + through ``UFFDIO_REGISTER`` IOCTL. >> + 4. Then any part of the registered memory or the whole memory region must >> + be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_OP_WP`` >> + or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the >> + same operation. The former is better in terms of performance. > > I guess that the UFFD performance could be fixed? But this part refers > to the old PM_SCAN_OP_WP, so an updated example is needed. > >> + 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which >> + have been written to and/or optionally write protect the pages as well. > > "find the pages written to since they were last write protected", but > this sounds contradicting: we look for pages that were WP but written > anyway. (IOW: marking write-protected is an implementation detail - > the ioctl is to find pages that changed since they were last marked.) > Maybe we should call the operation "marking CLEAN" or alike? > > Best Regards > Michał Mirosław -- BR, Muhammad Usama Anjum