Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 7 Dec 2023 16:01:56 +0100 | Subject | Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP | From | David Hildenbrand <> |
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On 07.12.23 15:45, Ryan Roberts wrote: > On 07/12/2023 13:28, David Hildenbrand wrote: >>>> >>>> Right, but you know from the first loop which order is applicable (and will be >>>> fed to the second loop) and could just pte_unmap(pte) + tryalloc. If that fails, >>>> remap and try with the next orders. >>> >>> You mean something like this? >>> >>> pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK); >>> if (!pte) >>> return ERR_PTR(-EAGAIN); >>> >>> order = highest_order(orders); >>> while (orders) { >>> addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order); >>> if (!pte_range_none(pte + pte_index(addr), 1 << order)) { >>> order = next_order(&orders, order); >>> continue; >>> } >>> >>> pte_unmap(pte); >>> >>> folio = vma_alloc_folio(gfp, order, vma, addr, true); >>> if (folio) { >>> clear_huge_page(&folio->page, vmf->address, 1 << order); >>> return folio; >>> } >>> >>> pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK); >>> if (!pte) >>> return ERR_PTR(-EAGAIN); >>> >>> order = next_order(&orders, order); >>> } >>> >>> pte_unmap(pte); >>> >>> I don't really like that because if high order folio allocations fail, then you >>> are calling pte_range_none() again for the next lower order; once that check has >>> succeeded for an order it shouldn't be required for any lower orders. In this >>> case you also have lots of pte map/unmap. >> >> I see what you mean. >> >>> >>> The original version feels more efficient to me. >> Yes it is. Adding in some comments might help, like >> >> /* >> * Find the largest order where the aligned range is completely prot_none(). Note >> * that all remaining orders will be completely prot_none(). >> */ >> ... >> >> /* Try allocating the largest of the remaining orders. */ > > OK added. > >> >>> >>>> >>>> That would make the code certainly easier to understand. That "orders" magic of >>>> constructing, filtering, walking is confusing :) >>>> >>>> >>>> I might find some time today to see if there is an easy way to cleanup all what >>>> I spelled out above. It really is a mess. But likely that cleanup could be >>>> deferred (but you're touching it, so ... :) ). >>> >>> I'm going to ignore the last 5 words. I heard the "that cleanup could be >>> deferred" part loud and clear though :) >> >> :) >> >> If we could stop passing orders into thp_vma_allowable_orders(), that would >> probably >> be the biggest win. It's just all a confusing mess. > > > > I tried an approach like you suggested in the other thread originally, but I > struggled to define exactly what "thp_vma_configured_orders()" should mean; > Ideally, I just want "all the THP orders that are currently enabled for this > VMA+flags". But some callers want to enforce_sysfs and others don't, so you > probably have to at least pass that flag. Then you have DAX which explicitly
Yes, the flags would still be passed. It's kind of the "context".
> ignores enforce_sysfs, but only in a page fault. And shmem, which ignores > enforce_sysfs, but only outside of a page fault. So it quickly becomes pretty > complex. It is basically thp_vma_allowable_orders() as currently defined.
Yeah, but moving the "can we actually fit a THP in there" check out of the picture.
> > If this could be a simple function then it could be inline and as you say, we > can do the masking in the caller and exit early for the order-0 case. But it is > very complex (at least if you want to retain the equivalent logic to what > thp_vma_allowable_orders() has) so I'm not sure how to do the order-0 early exit > without passing in the orders bitfield. And we are unlikely to exit early > because PMD-sized THP is likely enabled and because we didn't pass in a orders > bitfield, that wasn't filtered out. > > In short, I can't see a solution that's better than the one I have. But if you > have something in mind, if you can spell it out, then I'll have a go at tidying > it up and integrating it into the series. Otherwise I really would prefer to > leave it for a separate series.
I'm playing with some cleanups, but they can all be built on top if they materialize.
-- Cheers,
David / dhildenb
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