Re: [RFC PATCH] mm: Enable khugepaged to operate on non-writable VMAs
From: Dev Jain
Date: Mon Sep 01 2025 - 04:51:09 EST
On 01/09/25 2:02 pm, David Hildenbrand wrote:
On 01.09.25 09:48, Dev Jain wrote:
Currently khugepaged does not collapse a region which does not have a
single writable page. This is wasteful since, apart from any
non-writable
memory mapped by the application, there are a lot of non-writable VMAs
which will benefit from collapsing - the VMAs of the executable, those
of the glibc, vvar and vdso, which won't be unmapped during the lifetime
of the process, as opposed to other VMAs which maybe unmapped.
Are these anonymous folios? ("VMAs of the executable"), or is you
description
misleading?
Oops. I dropped writable everywhere and failed to notice that the
callsites are
all for anon collapse. So I'll only mention vvar, vdso and other
non-writable
VMAs in the description.
Therefore,
remove this restriction and allow khugepaged to collapse a VMA with
arbitrary protections.
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
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.
I dug into the history of this and couldn't find any concrete reason of
the current behaviour - [1] is the v1 of the original khugepaged patch
which required all ptes to be writable. [2] is the v1 of the patch which
changed this behaviour to require at least one pte to be writable. The
closest thing I could find was: in response to [2], Kirill says in [3] -
"As a side effect it will effectively allow collapse in PROT_READ vmas,
right? I'm not convinced it's a good idea." (Although Kirill realizes in
[4] that this was not the intention of the patch).
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):
I once dug into that myself as well as part of
commit 1bafe96e89f056cb6e25d47451fb16aee2c7c4d0
Author: David Hildenbrand <david@xxxxxxxxxx>
Date: Wed Apr 24 14:26:30 2024 +0200
mm/khugepaged: replace page_mapcount() check by
folio_likely_mapped_shared()
where I noted:
Interestingly, khugepaged will only collapse an anonymous THP if
at least
one PTE is writable. After fork(), that means that something
(usually a
page fault) populated at least a single exclusive anonymous THP in
that
PMD range.
The problem I was concerned with (also documented in that patch)
should no
longer apply ever since we changed how folio_maybe_mapped_shared()
operates.
So yes, I don't see a good reason to fail on R/O PTEs
+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
| 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% |
+------------------------------------+----------------------------------------------------------+-----------------------+--------------------------+
No regressions were observed with mm-selftests.
[1]
https://lore.kernel.org/all/679861e2e81b32a0ae08.1264054854@v2.random/
[2]
https://lore.kernel.org/all/1421999256-3881-1-git-send-email-ebru.akagunduz@xxxxxxxxx/
[3] https://lore.kernel.org/all/20150123113701.GB5975@xxxxxxxxxxxxxxxxx/
[4] https://lore.kernel.org/all/20150123155802.GA7011@xxxxxxxxxxxxxxxxx/
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
Based on mm-new.
Not very sure of the tracing parts which this patch changes. I have kept
the writable portion for the tracing to maintain backward compat, just
dropped it as a collapse condition.
include/trace/events/huge_memory.h | 2 +-
mm/khugepaged.c | 11 +++--------
2 files changed, 4 insertions(+), 9 deletions(-)
diff --git a/include/trace/events/huge_memory.h
b/include/trace/events/huge_memory.h
index 2305df6cb485..f2472c1c132a 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -19,7 +19,7 @@
EM( SCAN_PTE_NON_PRESENT, "pte_non_present") \
EM( SCAN_PTE_UFFD_WP, "pte_uffd_wp") \
EM( SCAN_PTE_MAPPED_HUGEPAGE, "pte_mapped_hugepage") \
- EM( SCAN_PAGE_RO, "no_writable_page") \
+ EM( SCAN_PAGE_RO, "no_writable_page") /* deprecated */ \
EM( SCAN_LACK_REFERENCED_PAGE, "lack_referenced_page") \
EM( SCAN_PAGE_NULL, "page_null") \
EM( SCAN_SCAN_ABORT, "scan_aborted") \
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4ec324a4c1fe..5ef8482597a9 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -39,7 +39,7 @@ enum scan_result {
SCAN_PTE_NON_PRESENT,
SCAN_PTE_UFFD_WP,
SCAN_PTE_MAPPED_HUGEPAGE,
- SCAN_PAGE_RO,
+ SCAN_PAGE_RO, /* deprecated */
Why can't we remove that completely.