Date: Tue, 2 Jul 2024 17:33:29 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Lance Yang <ioworker0@...il.com>, Yang Shi <shy828301@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] mm: Fix khugepaged activation policy

On 02/07/2024 16:38, David Hildenbrand wrote:
> On 02.07.24 17:29, Ryan Roberts wrote:
>> On 02/07/2024 15:57, David Hildenbrand wrote:
>>> On 02.07.24 16:46, Ryan Roberts wrote:
>>>> Since the introduction of mTHP, the docuementation has stated that
>>>> khugepaged would be enabled when any mTHP size is enabled, and disabled
>>>> when all mTHP sizes are disabled. There are 2 problems with this; 1.
>>>> this is not what was implemented by the code and 2. this is not the
>>>> desirable behavior.
>>>>
>>>> Desirable behavior is for khugepaged to be enabled when any PMD-sized
>>>> THP is enabled, anon or file. (Note that file THP is still controlled by
>>>> the top-level control so we must always consider that, as well as the
>>>> PMD-size mTHP control for anon). khugepaged only supports collapsing to
>>>> PMD-sized THP so there is no value in enabling it when PMD-sized THP is
>>>> disabled. So let's change the code and documentation to reflect this
>>>> policy.
>>>>
>>>> Further, per-size enabled control modification events were not
>>>> previously forwarded to khugepaged to give it an opportunity to start or
>>>> stop. Consequently the following was resulting in khugepaged eroneously
>>>> not being activated:
>>>>
>>>>     echo never > /sys/kernel/mm/transparent_hugepage/enabled
>>>>     echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>>> Fixes: 3485b88390b0 ("mm: thp: introduce multi-size THP sysfs interface")
>>>> Closes:
>>>> https://lore.kernel.org/linux-mm/7a0bbe69-1e3d-4263-b206-da007791a5c4@redhat.com/
>>>> Cc: stable@...r.kernel.org
>>>> ---
>>>>
>>>> Hi All,
>>>>
>>>> Applies on top of today's mm-unstable (9bb8753acdd8). No regressions
>>>> observed in
>>>> mm selftests.
>>>>
>>>> When fixing this I also noticed that khugepaged doesn't get (and never has
>>>> been)
>>>> activated/deactivated by `shmem_enabled=`. I'm not sure if khugepaged knows how
>>>> to collapse shmem - perhaps it should be activated in this case?
>>>>
>>>
>>> Call me confused.
>>>
>>> khugepaged_scan_mm_slot() and madvise_collapse() only all
>>> hpage_collapse_scan_file() with ... IS_ENABLED(CONFIG_SHMEM) ?
>>
>> Looks like khugepaged_scan_mm_slot() was converted from:
>>
>>    if (shmem_file(vma->vm_file)) {
>>
>> to:
>>
>>    if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {

CONFIG_READ_ONLY_THP_FOR_FS depends on CONFIG_SHMEM in Kconfig, so I think this is all correct/safe. Although I'm not really sure what the need for the dependency is.

>>
>> By 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0 which adds THP collapse support for
>> non-shmem files. Clearly that looks wrong, but I guess never spotted in practice
>> because noone disables shemem?
>>
>> I guess madvise_collapse() was a copy/paste?
>>
> 
> Likely.
> 
>>>
>>> collapse_file() is only called by hpage_collapse_scan_file() ... and there we
>>> check "shmem_file(file)".
>>>
>>> So why is the IS_ENABLED(CONFIG_SHMEM) check in there if collapse_file() seems
>>> to "collapse filemap/tmpfs/shmem pages into huge one".
>>>
>>> Anyhow, we certainly can collapse shmem (that's how it all started IIUC).
>>
>> Yes, thanks for pointing me at it. Should have just searched "shmem" in
>> khugepaged.c :-/
>>
>>>
>>> Besides that, khugepaged only seems to collapse !shmem with
>>>    VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
>>
>> That makes sense. I guess I could use IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) to
>> tighen the (non-shmem) file THP check in hugepage_pmd_enabled() (currently I'm
>> unconditionally using the top-level enabled setting as a "is THP enabled for
>> files" check).

I'll do a v2 with this addition if you agree?

static inline bool hugepage_pmd_enabled(void)
{
	/*
	 * We cover both the anon and the file-backed case here; for
	 * file-backed, we must return true if globally enabled, regardless of
	 * the anon pmd size control status.
	 */
	return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && hugepage_global_enabled()) ||
	       test_bit(PMD_ORDER, &huge_anon_orders_always) ||
	       test_bit(PMD_ORDER, &huge_anon_orders_madvise) ||
	       (test_bit(PMD_ORDER, &huge_anon_orders_inherit) && hugepage_global_enabled());
}

>>
>> But back to my original question, I think hugepage_pmd_enabled() should also be
>> explicitly checking the appropriate shmem_enabled controls and ORing in the
>> result? Otherwise in a situation where only shmem is THP enabled (and file/anon
>> THP is disabled) khugepaged won't run.
> 
> I think so.

I'll do this part as a separate change since it's fixing a separate bug.

> 
>>
>>>
>>> The thp_vma_allowable_order() check tests if we are allowed to collapse a
>>> PMD_ORDER in that VMA.
>>
>> I don't follow the relevance of this statement.
> 
> On whatever VMA we indicate true, we will try to collapse in khugepaged.
> Regarding the question, what khugepaged will try to collapse.

Oh I see. Yes agreed. But we don't have a VMA when enabling/disabling khugepaged. We just want to know "are there vmas for which khugepaged would attempt to collapse to PMD size?"

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