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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. > > Why not use __split_unmaped_folio(folio, /*new_order=*/0, > /split_at=*/HWPoisoned_page, > ..., /*uniform_split=*/ false)? > > If there are multiple HWPoisoned pages, just repeat. Using __split_unmapped_folio() is totally fine. I was just thinking that maybe we should hide the complexity inside the page allocator if we want to avoid allocating struct folio at all when handling this. > > 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