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.
- 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.