Re: [PATCH v9 2/8] mm/huge_memory: add two new (not yet used) functions for folio_split()

[Zi Yan <ziy@nvidia.com>]

On 6 Mar 2025, at 4:19, David Hildenbrand wrote:

> On 05.03.25 22:08, Zi Yan wrote:
>> On 5 Mar 2025, at 15:50, Hugh Dickins wrote:
>>
>>> On Wed, 5 Mar 2025, Zi Yan wrote:
>>>> On 4 Mar 2025, at 6:49, Hugh Dickins wrote:
>>>>>
>>>>> I think (might be wrong, I'm in a rush) my mods are all to this
>>>>> "add two new (not yet used) functions for folio_split()" patch:
>>>>> please merge them in if you agree.
>>>>>
>>>>> 1. From source inspection, it looks like a folio_set_order() was missed.
>>>>
>>>> Actually no. folio_set_order(folio, new_order) is called multiple times
>>>> in the for loop above. It is duplicated but not missing.
>>>
>>> I was about to disagree with you, when at last I saw that, yes,
>>> it is doing that on "folio" at the time of setting up "new_folio".
>>>
>>> That is confusing: in all other respects, that loop is reading folio
>>> to set up new_folio.  Do you have a reason for doing it there?
>>
>> No. I agree your fix is better. Just point out folio_set_order() should
>> not trigger a bug.
>>
>>>
>>> The transient "nested folio" situation is anomalous either way.
>>> I'd certainly prefer it to be done at the point where you
>>> ClearPageCompound when !new_order; but if you think there's an issue
>>> with racing isolate_migratepages_block() or something like that, which
>>> your current placement handles better, then please add a line of comment
>>> both where you do it and where I expected to find it - thanks.
>>
>> Sure. I will use your patch unless I find some racing issue.
>>
>>>
>>> (Historically, there was quite a lot of difficulty in getting the order
>>> of events in __split_huge_page_tail() to be safe: I wonder whether we
>>> shall see a crop of new weird bugs from these changes. I note that your
>>> loops advance forwards, whereas the old ones went backwards: but I don't
>>> have anything to say you're wrong.  I think it's mainly a matter of how
>>> the first tail or two gets handled: which might be why you want to
>>> folio_set_order(folio, new_order) at the earliest opportunity.)
>>
>> I am worried about that too. In addition, in __split_huge_page_tail(),
>> page refcount is restored right after new tail folio split is done,
>> whereas I needed to delay them until all new after-split folios
>> are done, since non-uniform split is iterative and only the after-split
>> folios NOT containing the split_at page will be released. These
>> folios are locked and frozen after __split_folio_to_order() like
>> the original folio. Maybe because there are more such locked frozen
>> folios than before?
>
> What's the general concern here?
>
> A frozen folio cannot be referenced and consequently not trusted. For example, if we want to speculatively lookup a folio in the pagecache and find it to be frozen, we'll have to spin (retry) until we find a folio that is unfrozen.
>
> While a folio has a refcount of 0, there are no guarantees. It could change its size, it could be freed + reallocated (changed mapping etc) ...
>
> So whoever wants to grab a speculative reference -- using folio_try_get() -- must re-verify folio properties after grabbing the speculative reference succeeded. Including whether it is small/large, number of pages, mapping, ...
>
> The important part is to unfreeze a folio only once it was fully prepared (e.g., order set, compound pages links to head set up etc).
>
> I am not sure if the sequence in which you process folios during a split matters here when doing a split: only that, whatever new folio  is unfrozen, is properly initialized.

Got it. Thanks for the confirmation.

My worry came from that after I rebased on mm-everything-2025-03-05-03-54,
which does not have folio_split() patches, I see a crash saying a buddy
page is hit in __split_folio_to_order(). It turns out that I did not
add the changes from your “mm: let _folio_nr_pages overlay memcg_data in
first tail page” patch. With that fixed, no crash is observed so far.


Best Regards,
Yan, Zi