Re: [PATCH v3] alloc_tag: expose profiling and compression states

On Mon, Aug 3, 2026 at 9:14 PM Hao Ge <[email protected]> wrote:
>
>
> On 2026/8/4 10:15, Hao Ge wrote:
> > Hi Abhishek
> >
> >
> > Subject says "profiling and compression states", but this
> >
> > patch only adds the compression sysctl. The profiling state was
> >
> > already exposed via mem_profiling and is untouched here.
> >
> > I think we should make this more precise, like:
> >
> > alloc_tag: expose boot-time compression configuration
> >
> >
> > On 2026/8/4 05:54, Abhishek Bapat wrote:
> >> Currently, userspace has limited visibility into the exact active
> >> runtime state of memory allocation profiling and its page extension
> >> compression ('sysctl.vm.mem_profiling={0|1|never}[,compressed]').
> >
> >
> > Profiling state is already readable via mem_profiling. The gap is
> >
> > only compression. Also, as we discussed, this sysctl reports
> >
> > what the user requested at boot rather than the actual runtime state.
> >
> >
> >> While reading the sysctl provides basic on/off status, it is currently
> >> impossible for userspace to natively determine whether page-tag
> >> compression was successfully enabled without scraping dmesg boot logs.
> >>
> >> Resolve this ambiguity by exposing the active compression state by
> >> adding a new read-only sysctl `vm.mem_profiling_compressed` to output
> >> the
> >> state.
> >>
> >> v3 change:
> >> - Added documentation about the behaviour details of the new sysctl.
> >>
> >> v2 change:
> >> - Moved from displaying the state in /proc/allocinfo to a new read-only
> >> sysctl.
> >
> >
> > As this is a standalone patch, please put the v2/v3 changelog below
> > the --- line
> >
> > instead of inside the commit message.
> >
> >
> > Thanks
> >
> > Best Regards
> >
> > Hao
> >
> >
> >> Signed-off-by: Abhishek Bapat <[email protected]>
> >> ---
> >>   Documentation/mm/allocation-profiling.rst | 13 +++++++++++++
> >>   mm/alloc_tag.c                            |  6 ++++++
> >>   2 files changed, 19 insertions(+)
> >>
> >> diff --git a/Documentation/mm/allocation-profiling.rst
> >> b/Documentation/mm/allocation-profiling.rst
> >> index c3a28467955f..3ad1e9aacb9a 100644
> >> --- a/Documentation/mm/allocation-profiling.rst
> >> +++ b/Documentation/mm/allocation-profiling.rst
> >> @@ -43,6 +43,19 @@ sysctl:
> >>     warnings produced by allocations made while profiling is disabled
> >> and freed
> >>     when it's enabled.
> >>   +  /proc/sys/vm/mem_profiling_compressed
> >> +
> >> +  1: Page extension compression is enabled.
> >> +
> >> +  0: Page extension compression is disabled.
> >> +
> >> +  This control is read-only and reflects the compression status
> >> initialized at boot.
> >> +  Note that, unlike `mem_profiling`, which represents the current
> >> state of profiling,
> >> +  `mem_profiling_compressed` represents the state configured at boot
> >> time.
>
>
> Sorry, I forgot to mention this earlier.
>
> I wonder if we could remove this section:
>
>
> >> Turning off
> >> +  profiling at runtime will implicitly make this sysctl effectively
> >> dormant. However, if
> >> +  profiling is toggled off and then toggled on again, it will resume
> >> with compression
> >> +  still enabled as long as the value of `mem_profiling_compressed`
> >> is 1.
>
>
> mem_profiling_compressed only selects the tag storage format (page flags
> vs page_ext).
>
> mem_profiling controls whether allocations are tagged at runtime.
>
> The two are independent: toggling mem_profiling on/off has no effect on
>
> compression. Only shutdown_mem_profiling() tears it down, but by then
>
> the entire allocation profiling subsystem is disabled anyway. There is
>
> no resume path.
>
>

I added this based on Suren's comment on the previous version asking
me to explicitly describe the behaviour in the Documentation. The
original version did not contain it. But personally I do think keeping
this here makes sense.

> >> +
> >>   Runtime info:
> >>     /proc/allocinfo
> >>   diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> >> index 52aece27b00e..877068241f06 100644
> >> --- a/mm/alloc_tag.c
> >> +++ b/mm/alloc_tag.c
> >> @@ -1303,6 +1303,12 @@ static const struct ctl_table
> >> memory_allocation_profiling_sysctls[] = {
> >>           .mode        = 0644,
> >>           .proc_handler    = proc_mem_profiling_handler,
> >>       },
> >> +    {
> >> +        .procname    = "mem_profiling_compressed",
> >> +        .data        = &mem_profiling_compressed,
> >> +        .mode        = 0444,
> >> +        .proc_handler    = proc_do_static_key,
> >> +    },
> >>   };
> >>     static void __init sysctl_init(void)
> >>
> >> base-commit: 94f9b3980dd446b56acf1dfed649e9b32a9f3813