Re: [PATCH v4] tools/mm: Add script to display page state for a given PID and VADDR

[Stephen Brennan <stephen.s.brennan@oracle.com>]

Hi Ye,

I just wanted to leave a code review related to the drgn/Python elements
of this patch. I'm no mm expert, and most of the things I'm flagging
here are small changes that I don't think are critical.

Ye Liu <ye.liu@linux.dev> writes:
> From: Ye Liu <liuye@kylinos.cn>
>
> Introduces a new drgn script, `show_page_info.py`, which allows users
> to analyze the state of a page given a process ID (PID) and a virtual
> address (VADDR). This can help kernel developers or debuggers easily
> inspect page-related information in a live kernel or vmcore.
>
> The script extracts information such as the page flags, mapping, and
> other metadata relevant to diagnosing memory issues.
>
> Output example:
> sudo ./show_page_info.py 1 0x7fb3eb1b2000
> PID: 1 Comm: systemd mm: 0xffff8d27279f9cc0
> Raw: 0017ffffc000416c fffff31105a61b08 fffff31105a63608 ffff8d27121326a8
> Raw: 0000000000000000 ffff8d271b9dcc40 0000002500000007 ffff8d2711f12700
> User Virtual Address: 0x7fb3eb1b2000
> Page Address:         0xfffff31106356a00
> Page Flags:           PG_referenced|PG_uptodate|PG_lru|PG_head|PG_active|
>                       PG_private|PG_reported|PG_has_hwpoisoned
> Page Size:            4096
> Page PFN:             0x18d5a8
> Page Physical:        0x18d5a8000
> Page Virtual:         0xffff8d274d5a8000
> Page Refcount:        37
> Page Mapcount:        7
> Page Index:           0x0
> Page Memcg Data:      0xffff8d2711f12700
> Memcg Name:           init.scope
> Memcg Path:           /sys/fs/cgroup/memory/init.scope
> Page Mapping:         0xffff8d27121326a8
> Page Anon/File:       File
> Page VMA:             0xffff8d26cac47600
> VMA Start:            0x7fb3eb1b2000
> VMA End:              0x7fb3eb1b6000
> This page is part of a compound page.
> This page is the head page of a compound page.
> Head Page:            0xfffff31106356a00
> Compound Order:       2
> Number of Pages:      4
>
> Signed-off-by: Ye Liu <liuye@kylinos.cn>
>
> Changes in v4:
> - Add error and exception handling.
> - Adjust the way to obtain PAGE_SIZE.
> - Fix the acquisition of memcg.
> - Link to v3:https://lore.kernel.org/all/20250423014850.344501-1-ye.liu@linux.dev/
>
> Changes in v3:
> - Adjust display style.
> - Link to v2:https://lore.kernel.org/all/20250421080748.114750-1-ye.liu@linux.dev/
>
> Changes in v2:
> - Move the show_page_info.py file to tools/mm.
> - Link to v1: https://lore.kernel.org/all/20250415075024.248232-1-ye.liu@linux.dev/
> ---
>  MAINTAINERS                |   5 ++
>  tools/mm/show_page_info.py | 152 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 157 insertions(+)
>  create mode 100755 tools/mm/show_page_info.py
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0cb9e55021cb..3cbd46bf1eab 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -18726,6 +18726,11 @@ F:	Documentation/mm/page_table_check.rst
>  F:	include/linux/page_table_check.h
>  F:	mm/page_table_check.c
>  
> +PAGE STATE DEBUG SCRIPT
> +M:	Ye Liu <liuye@kylinos.cn>
> +S:	Maintained
> +F:	tools/mm/show_page_info.py
> +
>  PANASONIC LAPTOP ACPI EXTRAS DRIVER
>  M:	Kenneth Chan <kenneth.t.chan@gmail.com>
>  L:	platform-driver-x86@vger.kernel.org
> diff --git a/tools/mm/show_page_info.py b/tools/mm/show_page_info.py
> new file mode 100755
> index 000000000000..5c46501e24f4
> --- /dev/null
> +++ b/tools/mm/show_page_info.py
> @@ -0,0 +1,152 @@
> +#!/usr/bin/env drgn
> +# SPDX-License-Identifier: GPL-2.0-only
> +# Copyright (C) 2025 Ye Liu <liuye@kylinos.cn>
> +
> +import argparse
> +from drgn import Object, FaultError
> +from drgn.helpers.linux import find_task, follow_page, page_size
> +from drgn.helpers.linux.mm import (
> +    decode_page_flags, page_to_pfn, page_to_phys, page_to_virt, vma_find,
> +    PageSlab, PageCompound, PageHead, PageTail, compound_head, compound_order, compound_nr
> +)
> +from drgn.helpers.linux.cgroup import cgroup_name, cgroup_path

Anything in "drgn.helpers.linux.*" can be imported from
"drgn.helpers.linux" instead, which would help if any helper moved
around from one module to another. I've recently started preferring
that, but I don't know if it's a huge improvement. EG:

from drgn.helpers.linux import (
    PageCompound, PageHead, PageSlab, PageTail, cgroup_name,
    cgroup_path, compound_head, compound_nr, compound_order,
    decode_page_flags, find_task, follow_page, page_size, page_to_pfn,
    page_to_phys, page_to_virt, vma_find,
)

Again, not sure it improves anything :)

> +DESC = """
> +This is a drgn script to show the page state.
> +For more info on drgn, visit https://github.com/osandov/drgn.
> +"""
> +
> +MEMCG_DATA_OBJEXTS = 1 << 0
> +MEMCG_DATA_KMEM = 1 << 1
> +__NR_MEMCG_DATA_FLAGS = 1 << 2

These are available as enums since commit 87944e2992bd2 ("mm: Introduce
page memcg flags"). So you can access them without redeclaring their
values like this:

prog.constant("MEMCG_DATA_OBJEXTS")

You can still save them as globals for efficiency.

> +def format_page_data(data):
> +    """Format raw page data into a readable hex dump."""
> +    try:
> +        chunks = [data[i:i+8] for i in range(0, len(data), 8)]
> +        hex_chunks = ["".join(f"{b:02x}" for b in chunk[::-1]) for chunk in chunks]
> +        lines = [" ".join(hex_chunks[i:i+4]) for i in range(0, len(hex_chunks), 4)]
> +        return "\n".join(f"Raw: {line}" for line in lines)
> +    except Exception as e:
> +        return f"Error formatting page data: {e}"

I'm thinking this would show big-endian addresses backwards, and on
32-bit architectures it would group the data into 8-byte words, when I
think it would probably be more valuable to show them in the native word
size.

You could resolve the endianness issue by using Program.read_word() to
read each individual word of memory in the correct byte order, and use
Program.platform.flags to distinguish a 32-bit architecture so that you
can compute each address. In fact, check print_annotated_memory() in
drgn which has very similar logic:

https://github.com/osandov/drgn/blob/main/drgn/helpers/common/memory.py

> +def get_memcg_info(page):
> +    """Retrieve memory cgroup information for a page."""
> +    try:
> +        memcg_data = page.memcg_data.value_()
> +        if memcg_data & MEMCG_DATA_OBJEXTS:
> +            slabobj_ext = Object(prog, "struct slabobj_ext *", address=memcg_data & ~(__NR_MEMCG_DATA_FLAGS - 1))

A slightly shorter and more idiomatic way to do it would be using the
cast() function:

slabobj_ext = cast("struct slabobj_ext *", memcg_data & ~(__NR_MEMCG_DATA_FLAGS - 1))

> +            memcg_value = slabobj_ext.objcg.memcg.value_()
> +        elif memcg_data & MEMCG_DATA_KMEM:
> +            objcg = Object(prog, "struct obj_cgroup *", address=memcg_data & ~(__NR_MEMCG_DATA_FLAGS - 1))
> +            memcg_value = objcg.memcg.value_()
> +        else:
> +            memcg_value = memcg_data & ~(__NR_MEMCG_DATA_FLAGS - 1)
> +
> +        if memcg_value == 0:
> +            return "none", "/sys/fs/cgroup/memory/"
> +
> +        memcg = Object(prog, "struct mem_cgroup *", address=memcg_value)
> +        cgrp = memcg.css.cgroup
> +        return cgroup_name(cgrp).decode(), f"/sys/fs/cgroup/memory{cgroup_path(cgrp).decode()}"
> +    except FaultError as e:
> +        return "unknown", f"Error retrieving memcg info: {e}"
> +    except Exception as e:
> +        return "unknown", f"Unexpected error: {e}"
> +
> +def show_page_state(page, addr, mm, pid, task):
> +    """Display detailed information about a page."""
> +    try:
> +        print(f'PID: {pid} Comm: {task.comm.string_().decode()} mm: {hex(mm)}')
> +        try:
> +            print(format_page_data(prog.read(page.value_(), 64)))

Rather than hard-code the size of struct page, you can use sizeof(page).
And in fact, all drgn Objects have a .bytes_() that will just give you
the bytes of the object directly, which would even avoid the sizeof().

Though in this case, I'd argue for just passing the page into
format_page_data() and letting it use Program.read_word() to read each
word in the correct endianness, like I said above.

> +        except FaultError as e:
> +            print(f"Error reading page data: {e}")
> +
> +        fields = {
> +            "User Virtual Address": hex(addr),
> +            "Page Address": hex(page.value_()),
> +            "Page Flags": decode_page_flags(page),
> +            "Page Size": prog["PAGE_SIZE"].value_(),
> +            "Page PFN": hex(page_to_pfn(page).value_()),
> +            "Page Physical": hex(page_to_phys(page).value_()),
> +            "Page Virtual": hex(page_to_virt(page).value_()),
> +            "Page Refcount": page._refcount.counter.value_(),
> +            "Page Mapcount": page._mapcount.counter.value_(),
> +            "Page Index": hex(page.__folio_index.value_()),
> +            "Page Memcg Data": hex(page.memcg_data.value_()),
> +        }
> +
> +        memcg_name, memcg_path = get_memcg_info(page)
> +        fields["Memcg Name"] = memcg_name
> +        fields["Memcg Path"] = memcg_path
> +        fields["Page Mapping"] = hex(page.mapping.value_())
> +        fields["Page Anon/File"] = "Anon" if page.mapping.value_() & 0x1 else "File"
> +
> +        try:
> +            vma = vma_find(mm, addr)
> +            fields["Page VMA"] = hex(vma.value_())
> +            fields["VMA Start"] = hex(vma.vm_start.value_())
> +            fields["VMA End"] = hex(vma.vm_end.value_())
> +        except FaultError as e:
> +            fields["Page VMA"] = "Unavailable"
> +            fields["VMA Start"] = "Unavailable"
> +            fields["VMA End"] = "Unavailable"
> +            print(f"Error retrieving VMA information: {e}")
> +
> +        # Calculate the maximum field name length for alignment
> +        max_field_len = max(len(field) for field in fields)
> +
> +        # Print aligned fields
> +        for field, value in fields.items():
> +            print(f"{field}:".ljust(max_field_len + 2) + f"{value}")
> +
> +        # Additional information about the page
> +        if PageSlab(page):
> +            print("This page belongs to the slab allocator.")
> +
> +        if PageCompound(page):
> +            print("This page is part of a compound page.")
> +            if PageHead(page):
> +                print("This page is the head page of a compound page.")
> +            if PageTail(page):
> +                print("This page is the tail page of a compound page.")
> +            print(f"{'Head Page:'.ljust(max_field_len + 2)}{hex(compound_head(page).value_())}")
> +            print(f"{'Compound Order:'.ljust(max_field_len + 2)}{compound_order(page).value_()}")
> +            print(f"{'Number of Pages:'.ljust(max_field_len + 2)}{compound_nr(page).value_()}")
> +        else:
> +            print("This page is not part of a compound page.")
> +    except FaultError as e:
> +        print(f"Error accessing page state: {e}")
> +    except Exception as e:
> +        print(f"Unexpected error: {e}")
> +
> +def main():
> +    """Main function to parse arguments and display page state."""
> +    parser = argparse.ArgumentParser(description=DESC, formatter_class=argparse.RawTextHelpFormatter)
> +    parser.add_argument('pid', metavar='PID', type=int, help='Target process ID (PID)')
> +    parser.add_argument('vaddr', metavar='VADDR', type=str, help='Target virtual address in hexadecimal format (e.g., 0x7fff1234abcd)')
> +    args = parser.parse_args()
> +
> +    try:
> +        vaddr = int(args.vaddr, 16)
> +    except ValueError:
> +        print(f"Error: Invalid virtual address format: {args.vaddr}")
> +        return

I find it quite useful to replace things like this with:

    sys.exit(f"Error: Invalid virtual address format: {args.vaddr}")

Which will result in the script exiting with a non-zero exit code, and
it will print the message to stderr, rather than stdout. All while being
one line shorter, for the code golfers :)

The actual logic looks excellent, and most of my suggestions are just
that: suggestions. Sorry for putting this review on v4, I should have
sat down and done this sooner.

Regards,
Stephen

> +    try:
> +        task = find_task(args.pid)
> +        mm = task.mm
> +        page = follow_page(mm, vaddr)
> +
> +        if page:
> +            show_page_state(page, vaddr, mm, args.pid, task)
> +        else:
> +            print(f"Address {hex(vaddr)} is not mapped.")
> +    except FaultError as e:
> +        print(f"Error accessing task or memory: {e}")
> +    except Exception as e:
> +        print(f"Unexpected error: {e}")
> +
> +if __name__ == "__main__":
> +    main()
> -- 
> 2.25.1