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

[Hugh Dickins]

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?

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.

(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.)

> 
> >
> > 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.

Thanks,
Hugh