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Sun, Nov 16, 2025 at 11:51:14AM +0000, Matthew Wilcox wrote: > On Sun, Nov 16, 2025 at 01:47:20AM +0000, Jiaqi Yan wrote: > > Introduce uniform_split_unmapped_folio_to_zero_order, a wrapper > > to the existing __split_unmapped_folio. Caller can use it to > > uniformly split an unmapped high-order folio into 0-order folios. > > Please don't make this function exist. I appreciate what you're trying > to do, but let's try to do it differently? > > When we have struct folio separately allocated from struct page, > splitting a folio will mean allocating new struct folios for every > new folio created. I anticipate an order-0 folio will be about 80 or > 96 bytes. So if we create 512 * 512 folios in a single go, that'll be > an allocation of 20MB. > > This is why I asked Zi Yan to create the asymmetrical folio split, so we > only end up creating log() of this. In the case of a single hwpoison page > in an order-18 hugetlb, that'd be 19 allocations totallying 1520 bytes. Oh god, I completely overlooked this aspect when discussing this with Jiaqi. Thanks for raising this concern. > But since we're only doing this on free, we won't need to do folio > allocations at all; we'll just be able to release the good pages to the > page allocator and sequester the hwpoison pages. [+Cc PAGE ALLOCATOR folks] So we need an interface to free only healthy portion of a hwpoison folio. I think a proper approach to this should be to "free a hwpoison folio just like freeing a normal folio via folio_put() or free_frozen_pages(), then the page allocator will add only healthy pages to the freelist and isolate the hwpoison pages". Oherwise we'll end up open coding a lot, which is too fragile. In fact, that can be done by teaching free_pages_prepare() how to handle the case where one or more subpages of a folio are hwpoison pages. How this should be implemented in the page allocator in memdescs world? Hmm, we'll want to do some kind of non-uniform split, without actually splitting the folio but allocating struct buddy? But... for now I think hiding this complexity inside the page allocator is good enough. For now this would just mean splitting a frozen page inside the page allocator (probably non-uniform?). We can later re-implement this to provide better support for memdescs. -- Cheers, Harry / Hyeonggon