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Date: Fri, 26 Aug 2022 11:45:35 +0500 From: Muhammad Usama Anjum <usama.anjum@...labora.com> To: Jonathan Corbet <corbet@....net>, Alexander Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org (open list:DOCUMENTATION), linux-kernel@...r.kernel.org (open list), linux-fsdevel@...r.kernel.org (open list:PROC FILESYSTEM), linux-mm@...ck.org (open list:MEMORY MANAGEMENT), linux-kselftest@...r.kernel.org (open list:KERNEL SELFTEST FRAMEWORK) Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>, kernel@...labora.com, Gabriel Krisman Bertazi <krisman@...labora.com>, David Hildenbrand <david@...hat.com>, Peter Enderborg <peter.enderborg@...y.com>, Greg KH <gregkh@...uxfoundation.org> Subject: [PATCH v3 4/4] mm: add documentation of the new ioctl on pagemap Add the explanation of the added ioctl on pagemap file. It can be used to get, clear or both soft-dirty PTE bit of the specified range. or both at the same time. Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com> --- Changes in v2: - Update documentation to mention ioctl instead of the syscall --- Documentation/admin-guide/mm/soft-dirty.rst | 42 ++++++++++++++++++++- 1 file changed, 41 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/mm/soft-dirty.rst b/Documentation/admin-guide/mm/soft-dirty.rst index cb0cfd6672fa..d3d33e63a965 100644 --- a/Documentation/admin-guide/mm/soft-dirty.rst +++ b/Documentation/admin-guide/mm/soft-dirty.rst @@ -5,7 +5,12 @@ Soft-Dirty PTEs =============== The soft-dirty is a bit on a PTE which helps to track which pages a task -writes to. In order to do this tracking one should +writes to. + +Using Proc FS +------------- + +In order to do this tracking one should 1. Clear soft-dirty bits from the task's PTEs. @@ -20,6 +25,41 @@ writes to. In order to do this tracking one should 64-bit qword is the soft-dirty one. If set, the respective PTE was written to since step 1. +Using IOCTL +----------- + +The IOCTL on the ``/proc/PID/pagemap`` can be can be used to find the dirty pages +atomically. The following commands are supported:: + + MEMWATCH_SD_GET + Get the page offsets which are soft dirty. + + MEMWATCH_SD_CLEAR + Clear the pages which are soft dirty. + + MEMWATCH_SD_GET_AND_CLEAR + Get and clear the pages which are soft dirty. + +The struct :c:type:`pagemap_sd_args` is used as the argument. In this struct: + + 1. The range is specified through start and len. The len argument need not be + the multiple of the page size, but since the information is returned for the + whole pages, len is effectively rounded up to the next multiple of the page + size. + + 2. The output buffer and size is specified in vec and vec_len. The offsets of + the dirty pages from start are returned in vec. The ioctl returns when the + whole range has been searched or vec is completely filled. The whole range + isn't cleared if vec fills up completely. + + 3. The flags can be specified in flags field. Currently only one flag, + PAGEMAP_SD_NO_REUSED_REGIONS is supported which can be specified to ignore + the VMA dirty flags for better performance. This flag shows only those pages + dirty which have been written to by the user. All new allocations aren't returned + to be dirty. + +Explanation +----------- Internally, to do this tracking, the writable bit is cleared from PTEs when the soft-dirty bit is cleared. So, after this, when the task tries to -- 2.30.2
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