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Message-ID: <9536873f-08ea-4a60-bbec-3e7a832dc0e1@lucifer.local>
Date: Wed, 3 Sep 2025 21:34:36 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, kas@...nel.org,
willy@...radead.org, hughd@...gle.com, ziy@...dia.com,
baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs
On Wed, Sep 03, 2025 at 11:16:34AM +0530, Dev Jain wrote:
> Currently khugepaged does not collapse a region which does not have a
> single writable page. This is wasteful since non-writable VMAs mapped by
As discussed elsewhere in the thread, you really need to clarify that you
mean the PTE is writable. This is far too vague otherwise.
> the application won't benefit from THP collapse. Therefore, remove this
> restriction and allow khugepaged to collapse a VMA with arbitrary
> protections.
It's weird thie history of this, it looks like we were super conservative
at first, and then introduced this 'at least one PTE writable' thing in
commit 10359213d05a ("mm: incorporate read-only pages into transparent huge
pages"), but it doesn't really explain why you even need (at least) a
writable page.
Perhaps a pre-PAE thing... (David?) we already do the refcount stuff
though, so it's hard to understand.
It seems the main case for anon where it'd matter is swapped in pages
read-faulting for a R/W mapping (as read-faulting R/W mappings would just
get you the zero page which vm_normal_page() would exclude anyway).
But not sure why we'd be reticent to collapse those anyway... you'd just
cahnge R/W bit on PMD instead of PTE?
Yeah it's bizarre.
I can't really see why your change shouldn't be done...
>
> Along with this, currently MADV_COLLAPSE does not perform a collapse on a
> non-writable VMA, and this restriction is nowhere to be found on the
> manpage - the restriction itself sounds wrong to me since the user knows
I'm not sure why a man page would talk about PTE scanning implementation
details?
But I guess as you say you're thinking specifically of a read-only VMA that
naturally has read-only PTE's as as result...
> the protection of the memory it has mapped, so collapsing read-only
> memory via madvise() should be a choice of the user which shouldn't
> be overriden by the kernel.
NIT: overriden -> overridden.
>
> On an arm64 machine, an average of 5% improvement is seen on some mmtests
> benchmarks, particularly hackbench, with a maximum improvement of 12%.
Nice!
Is this on a raw metal machine, or a VM? I thik it's important to clarify
details like this.
Please state precisely what you tested this on.
>
> Signed-off-by: Dev Jain <dev.jain@....com>
Can't find any problem with this, and doesn't really seem like it'd be
problematic so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> RFC->v1:
> Drop writable references from tracepoints
>
> RFC:
> https://lore.kernel.org/all/20250901074817.73012-1-dev.jain@arm.com/
>
> I can see performance improvements on mmtests run on an arm64 machine
> comparing with 6.17-rc2. (I) denotes statistically significant improvement,
> (R) denotes statistically significant regression (Please ignore the
> numbers in the middle column):
Let's drop the numbers in the middle column then please, this is going into the
commit log, let's not put extranous information there.
>
> +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
> | mmtests/hackbench | process-pipes-1 (seconds) | 0.145 | -0.06% |
> | | process-pipes-4 (seconds) | 0.4335 | -0.27% |
> | | process-pipes-7 (seconds) | 0.823 | (I) -12.13% |
> | | process-pipes-12 (seconds) | 1.3538333333333334 | (I) -5.32% |
> | | process-pipes-21 (seconds) | 1.8971666666666664 | (I) -2.87% |
> | | process-pipes-30 (seconds) | 2.5023333333333335 | (I) -3.39% |
> | | process-pipes-48 (seconds) | 3.4305 | (I) -5.65% |
> | | process-pipes-79 (seconds) | 4.245833333333334 | (I) -6.74% |
> | | process-pipes-110 (seconds) | 5.114833333333333 | (I) -6.26% |
> | | process-pipes-141 (seconds) | 6.1885 | (I) -4.99% |
> | | process-pipes-172 (seconds) | 7.231833333333334 | (I) -4.45% |
> | | process-pipes-203 (seconds) | 8.393166666666668 | (I) -3.65% |
> | | process-pipes-234 (seconds) | 9.487499999999999 | (I) -3.45% |
> | | process-pipes-256 (seconds) | 10.316166666666666 | (I) -3.47% |
> | | process-sockets-1 (seconds) | 0.289 | 2.13% |
> | | process-sockets-4 (seconds) | 0.7596666666666666 | 1.02% |
> | | process-sockets-7 (seconds) | 1.1663333333333334 | -0.26% |
> | | process-sockets-12 (seconds) | 1.8641666666666665 | -1.24% |
> | | process-sockets-21 (seconds) | 3.0773333333333333 | 0.01% |
> | | process-sockets-30 (seconds) | 4.2405 | -0.15% |
> | | process-sockets-48 (seconds) | 6.459666666666666 | 0.15% |
> | | process-sockets-79 (seconds) | 10.156833333333333 | 1.45% |
> | | process-sockets-110 (seconds) | 14.317833333333333 | -1.64% |
> | | process-sockets-141 (seconds) | 20.8735 | (I) -4.27% |
> | | process-sockets-172 (seconds) | 26.205333333333332 | 0.30% |
> | | process-sockets-203 (seconds) | 31.298000000000002 | -1.71% |
> | | process-sockets-234 (seconds) | 36.104000000000006 | -1.94% |
> | | process-sockets-256 (seconds) | 39.44016666666667 | -0.71% |
> | | thread-pipes-1 (seconds) | 0.17550000000000002 | 0.66% |
> | | thread-pipes-4 (seconds) | 0.44716666666666666 | 1.66% |
> | | thread-pipes-7 (seconds) | 0.7345 | -0.17% |
> | | thread-pipes-12 (seconds) | 1.405833333333333 | (I) -4.12% |
> | | thread-pipes-21 (seconds) | 2.0113333333333334 | (I) -2.13% |
> | | thread-pipes-30 (seconds) | 2.6648333333333336 | (I) -3.78% |
> | | thread-pipes-48 (seconds) | 3.6341666666666668 | (I) -5.77% |
> | | thread-pipes-79 (seconds) | 4.4085 | (I) -5.31% |
> | | thread-pipes-110 (seconds) | 5.374666666666666 | (I) -6.12% |
> | | thread-pipes-141 (seconds) | 6.385666666666666 | (I) -4.00% |
> | | thread-pipes-172 (seconds) | 7.403000000000001 | (I) -3.01% |
> | | thread-pipes-203 (seconds) | 8.570333333333332 | (I) -2.62% |
> | | thread-pipes-234 (seconds) | 9.719166666666666 | (I) -2.00% |
> | | thread-pipes-256 (seconds) | 10.552833333333334 | (I) -2.30% |
> | | thread-sockets-1 (seconds) | 0.3065 | (R) 2.39% |
> +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
>
> +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
> | mmtests/sysbench-mutex | sysbenchmutex-1 (usec) | 194.38333333333333 | -0.02% |
> | | sysbenchmutex-4 (usec) | 200.875 | -0.02% |
> | | sysbenchmutex-7 (usec) | 201.23000000000002 | 0.00% |
> | | sysbenchmutex-12 (usec) | 201.77666666666664 | 0.12% |
> | | sysbenchmutex-21 (usec) | 203.03 | -0.40% |
> | | sysbenchmutex-30 (usec) | 203.285 | 0.08% |
> | | sysbenchmutex-48 (usec) | 231.30000000000004 | 2.59% |
> | | sysbenchmutex-79 (usec) | 362.075 | -0.80% |
> | | sysbenchmutex-110 (usec) | 516.8233333333334 | -3.87% |
> | | sysbenchmutex-128 (usec) | 593.3533333333334 | (I) -4.46% |
> +------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
This is nice, but is clearly hugely exceeding the column width we should have in commit messages.
Let me use emacs's nice features to make life easy for you :) -
+-------------------------+--------------------------------+---------------+
| mmtests/hackbench | process-pipes-1 (seconds) | -0.06% |
| | process-pipes-4 (seconds) | -0.27% |
| | process-pipes-7 (seconds) | (I) -12.13% |
| | process-pipes-12 (seconds) | (I) -5.32% |
| | process-pipes-21 (seconds) | (I) -2.87% |
| | process-pipes-30 (seconds) | (I) -3.39% |
| | process-pipes-48 (seconds) | (I) -5.65% |
| | process-pipes-79 (seconds) | (I) -6.74% |
| | process-pipes-110 (seconds) | (I) -6.26% |
| | process-pipes-141 (seconds) | (I) -4.99% |
| | process-pipes-172 (seconds) | (I) -4.45% |
| | process-pipes-203 (seconds) | (I) -3.65% |
| | process-pipes-234 (seconds) | (I) -3.45% |
| | process-pipes-256 (seconds) | (I) -3.47% |
| | process-sockets-1 (seconds) | 2.13% |
| | process-sockets-4 (seconds) | 1.02% |
| | process-sockets-7 (seconds) | -0.26% |
| | process-sockets-12 (seconds) | -1.24% |
| | process-sockets-21 (seconds) | 0.01% |
| | process-sockets-30 (seconds) | -0.15% |
| | process-sockets-48 (seconds) | 0.15% |
| | process-sockets-79 (seconds) | 1.45% |
| | process-sockets-110 (seconds) | -1.64% |
| | process-sockets-141 (seconds) | (I) -4.27% |
| | process-sockets-172 (seconds) | 0.30% |
| | process-sockets-203 (seconds) | -1.71% |
| | process-sockets-234 (seconds) | -1.94% |
| | process-sockets-256 (seconds) | -0.71% |
| | thread-pipes-1 (seconds) | 0.66% |
| | thread-pipes-4 (seconds) | 1.66% |
| | thread-pipes-7 (seconds) | -0.17% |
| | thread-pipes-12 (seconds) | (I) -4.12% |
| | thread-pipes-21 (seconds) | (I) -2.13% |
| | thread-pipes-30 (seconds) | (I) -3.78% |
| | thread-pipes-48 (seconds) | (I) -5.77% |
| | thread-pipes-79 (seconds) | (I) -5.31% |
| | thread-pipes-110 (seconds) | (I) -6.12% |
| | thread-pipes-141 (seconds) | (I) -4.00% |
| | thread-pipes-172 (seconds) | (I) -3.01% |
| | thread-pipes-203 (seconds) | (I) -2.62% |
| | thread-pipes-234 (seconds) | (I) -2.00% |
| | thread-pipes-256 (seconds) | (I) -2.30% |
| | thread-sockets-1 (seconds) | (R) 2.39% |
+-------------------------+--------------------------------+---------------+
+-------------------------+------------------------------------------------+
| mmtests/sysbench-mutex | sysbenchmutex-1 (usec) | -0.02% |
| | sysbenchmutex-4 (usec) | -0.02% |
| | sysbenchmutex-7 (usec) | 0.00% |
| | sysbenchmutex-12 (usec) | 0.12% |
| | sysbenchmutex-21 (usec) | -0.40% |
| | sysbenchmutex-30 (usec) | 0.08% |
| | sysbenchmutex-48 (usec) | 2.59% |
| | sysbenchmutex-79 (usec) | -0.80% |
| | sysbenchmutex-110 (usec) | -3.87% |
| | sysbenchmutex-128 (usec) | (I) -4.46% |
+-------------------------+--------------------------------+---------------+
>
> mm/khugepaged.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4ec324a4c1fe..a0f1df2a7ae6 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> writable = true;
> }
>
> - if (unlikely(!writable)) {
> - result = SCAN_PAGE_RO;
> - } else if (unlikely(cc->is_khugepaged && !referenced)) {
> + if (unlikely(cc->is_khugepaged && !referenced)) {
> result = SCAN_LACK_REFERENCED_PAGE;
> } else {
> result = SCAN_SUCCEED;
> @@ -1421,9 +1419,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> mmu_notifier_test_young(vma->vm_mm, _address)))
> referenced++;
> }
> - if (!writable) {
> - result = SCAN_PAGE_RO;
> - } else if (cc->is_khugepaged &&
> + if (cc->is_khugepaged &&
> (!referenced ||
> (unmapped && referenced < HPAGE_PMD_NR / 2))) {
> result = SCAN_LACK_REFERENCED_PAGE;
> @@ -2830,7 +2826,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> case SCAN_PMD_NULL:
> case SCAN_PTE_NON_PRESENT:
> case SCAN_PTE_UFFD_WP:
> - case SCAN_PAGE_RO:
> case SCAN_LACK_REFERENCED_PAGE:
> case SCAN_PAGE_NULL:
> case SCAN_PAGE_COUNT:
> --
> 2.30.2
>
I guess you delay the final cleanup so you can combine it with tracepoint
removal in next patch, not really sure why they're separate but meh not a
big deal.
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