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.
--
Cheers,
David / dhildenb
