Re: [Patch v2] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp

["David Hildenbrand (arm)" <david@kernel.org>]

On 2/4/26 21:02, Zi Yan wrote:
> On 4 Feb 2026, at 14:36, David Hildenbrand (arm) wrote:
> 
>> Sorry for the late reply. I saw that I was CCed in v1 but I am only now catching up with mails ... slowly but steadily.
>>
>>> Without the above commit, we can successfully split to order 0.
>>> With the above commit, the folio is still a large folio.
>>>
>>> The reason is the above commit return false after split pmd
>>> unconditionally in the first process and break try_to_migrate().
>>>
>>> The tricky thing in above reproduce method is current debugfs interface
>>> leverage function split_huge_pages_pid(), which will iterate the whole
>>> pmd range and do folio split on each base page address. This means it
>>> will try 512 times, and each time split one pmd from pmd mapped to pte
>>> mapped thp. If there are less than 512 shared mapped process,
>>> the folio is still split successfully at last. But in real world, we
>>> usually try it for once.
>>
>> Ah, that explains magic number 513.
>>
>>>
>>> This patch fixes this by restart page_vma_mapped_walk() after
>>> split_huge_pmd_locked(). Because split_huge_pmd_locked() may fall back to
>>> (freeze = false) if folio_try_share_anon_rmap_pmd() fails and the PMD is
>>> just split instead of split to migration entry.
>>
>> Right, but folio_try_share_anon_rmap_pmd() should never fail on the folios that have already been shared? (above you write that it is shared with 512 children)
>>
>> The only case where  folio_try_share_anon_rmap_pmd() could fail would be if the folio would not be shared, and there would only be a single PMD then, so there is nothing you can do -> abort.
>>
>> Returning "false" from try_to_migrate_one() is the real issue, as it makes rmap_walk_anon() to just stop -> abort the walk.
>>
>>
>> So I suspect v1 was actually sufficient, or what am I missing where the restart would actually be required?
> 
> The explanation is not for the shared case mentioned above. It is for unshared
> folio. If an unshared folio’s PAE cannot be cleared, try_to_migrate_one() return
> true, indicating a success.

Oh. You mean that should be something like

"This patch fixes this by restart page_vma_mapped_walk() after 
split_huge_pmd_locked(). We cannot simply return "true" to fix the 
problem, as that would affect another case: 
split_huge_pmd_locked()->folio_try_share_anon_rmap_pmd() can failed and 
leave the folio mapped through PTEs; we would return "true" from 
try_to_migrate_one() in that case as well. While that is mostly 
harmless, we could end up walking the rmap, wasting some cycles.".


> Yeah, since it is an unshared folio, the return
> value of try_to_migrate_one() does not matter. This fix makes try_to_migrate_one()
> return false.

Right, it's not really problematic. We could end up walking the rmap and 
burn some cycles.

> 
>>
>>
>> (maybe we should get rid of the usage of booleans here at some point, an enum like abort/continue would have been much clearer)
>>
>>> Restart
>>> page_vma_mapped_walk() and let try_to_migrate_one() try on each PTE
>>> again and fail try_to_migrate() early if it fails.
>>>
>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>> Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()")
>>> Cc: Gavin Guo <gavinguo@igalia.com>
>>> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: Lance Yang <lance.yang@linux.dev>
>>> Cc: <stable@vger.kernel.org>
>>>
>>> ---
>>> v2:
>>>     * restart page_vma_mapped_walk() after split_huge_pmd_locked()
>>> ---
>>>    mm/rmap.c | 11 ++++++++---
>>>    1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 618df3385c8b..5b853ec8901d 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -2446,11 +2446,16 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>>>    			__maybe_unused pmd_t pmdval;
>>>     			if (flags & TTU_SPLIT_HUGE_PMD) {
>>> +				/*
>>> +				 * After split_huge_pmd_locked(), restart the
>>> +				 * walk to detect PageAnonExclusive handling
>>> +				 * failure in __split_huge_pmd_locked().
>>> +				 */
>>>    				split_huge_pmd_locked(vma, pvmw.address,
>>>    						      pvmw.pmd, true);
>>> -				ret = false;
>>> -				page_vma_mapped_walk_done(&pvmw);
>>> -				break;
>>> +				flags &= ~TTU_SPLIT_HUGE_PMD;
>>> +				page_vma_mapped_walk_restart(&pvmw);
>>> +				continue;
>>>    			}
>>
>> The change looks more consistent to what we have in try_to_unmap().
>>
>> But the explanation above is not quite right I think. And consequently the comment above as well.
>>
>> PAE being set implies "single PMD" -> unshared.
> 
> The commit message might be improved with some additional context. The comment
> above pairs with the comment in __split_huge_pmd_locked()
> “In case we cannot clear PageAnonExclusive(), split the PMD
> only and let try_to_migrate_one() fail later”. What is problem with it?

With your explanation it's much clearer, thanks.

I'd remove some details from the comments about PAE like:

"split_huge_pmd_locked() might leave the folio mapped through PTEs. 
Retry the walk so we can detect this scenario and properly abort the walk."


With some clarifications along those lines

Acked-by: David Hildenbrand (arm) <david@kernel.org>

-- 
Cheers,

David