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

[Zi Yan <ziy@nvidia.com>]

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?


>>
>>>
>>> 2. Why is swapcache only checked when folio_test_anon? I can see that
>>>    you've just copied that over from the old __split_huge_page(), but
>>>    it seems wrong to me here and there - I guess a relic from before
>>>    shmem could swap out a huge page.
>>
>> Yes, it is a relic, but it is still right before I change another relic
>> in __folio_split() or split_huge_page_to_list_to_order() from mainline,
>> if (!mapping) { ret = -EBUSY; goto out; }. It excludes the shmem in swap
>> cache case. I probably will leave it as is in my next folio_split() version
>> to avoid adding more potential bugs, but will come back later in another
>> patch.
>
> I agree.  The "Truncated ?" check.  Good.  But I do prefer that you use
> that part of my patch, referring to mapping and swap_cache instead of anon,
> rather than rely on that accident of what's done at the higher level.

Definitely.


Best Regards,
Yan, Zi