Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled
From: Baolin Wang
Date: Mon Jun 23 2025 - 21:44:31 EST
On 2025/6/23 19:08, Barry Song wrote:
On Mon, Jun 23, 2025 at 8:28 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag is not
specified, we will ignore the THP sysfs settings. Whilst it makes sense for the
callers who do not specify this flag, it creates a odd and surprising situation
where a sysadmin specifying 'never' for all THP sizes still observing THP pages
being allocated and used on the system.
The motivating case for this is MADV_COLLAPSE. The MADV_COLLAPSE will ignore
the system-wide Anon THP sysfs settings, which means that even though we have
disabled the Anon THP configuration, MADV_COLLAPSE will still attempt to collapse
into a Anon THP. This violates the rule we have agreed upon: never means never.
Should we update the man page for madv_collapse ?
https://man7.org/linux/man-pages/man2/madvise.2.html
MADV_COLLAPSE is independent of any sysfs (see sysfs(5))
setting under /sys/kernel/mm/transparent_hugepage, both in
terms of determining THP eligibility, and allocation
semantics. See Linux kernel source file
Documentation/admin-guide/mm/transhuge.rst for more
information. MADV_COLLAPSE also ignores huge= tmpfs mount
when operating on tmpfs files. Allocation for the new
hugepage may enter direct reclaim and/or compaction,
regardless of VMA flags (though VM_NOHUGEPAGE is still
respected).
So this effectively changes the uABI, right?
Good point. Will update the man page.
Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there is only
one other instance where TVA_ENFORCE_SYSFS is not set, which is in the
collapse_pte_mapped_thp() function, but I believe this is reasonable from its
comments:
"
/*
* If we are here, we've succeeded in replacing all the native pages
* in the page cache with a single hugepage. If a mm were to fault-in
* this memory (mapped by a suitably aligned VMA), we'd get the hugepage
* and map it by a PMD, regardless of sysfs THP settings. As such, let's
* analogously elide sysfs THP settings here.
*/
if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER))
"
Another rule for madvise, referring to David's suggestion: “allowing for
collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
To address this issue, the current strategy should be:
If no hugepage modes are enabled for the desired orders, nor can we enable them
by inheriting from a 'global' enabled setting - then it must be the case that
all desired orders either specify or inherit 'NEVER' - and we must abort.
Meanwhile, we should fix the khugepaged selftest for MADV_COLLAPSE by enabling
THP.
It’s a bit odd that the old test case expects collapsing to succeed
even when we’ve set it
to ‘never’.
Setting it to ‘always’ doesn’t seem to test anything as a counterpart.
I assume the goal is to test that setting it to ‘never’ prevents collapsing?
The original logic will prevent khugepaged by setting THP_NEVER,
allowing only madvise_collapse() to perform THP collapse. And this is
the logic this patchset tries to fix, which is to also prevent
madvise_collapse() from performing THP collapse when system-wide THP
sysfs settings are disabled.
Therefore, it should be changed to THP_ALWAYS here to allow
madvise_collapse() to perform THP collapse.
Of course, the current logic cannot completely disable khugepaged, but I
haven't found a better way to modify it. As David suggested, changing to
MADVISE mode would cause some test cases to fail because some tests
previously set MADV_NOHUGEPAGE, and now there is no other way to clear
the MADV_NOHUGEPAGE flag except for setting MADV_HUGEPAGE. As a result,
khugepaged cannot be completely disabled either.
So I think we should introduce a new method to clear MADV_NOHUGEPAGE
flag without setting MADV_HUGEPAGE in the future.