linux-kernel - Re: [PATCH v4 01/10] iommu/vt-d: add wrapper functions for page allocations

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Date: Thu, 15 Feb 2024 20:05:55 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Robin Murphy <robin.murphy@....com>
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Subject: Re: [PATCH v4 01/10] iommu/vt-d: add wrapper functions for page allocations

> >>> + */
> >>> +static inline void *iommu_alloc_page_node(int nid, gfp_t gfp)
> >>> +{
> >>> +     return iommu_alloc_pages_node(nid, gfp, 0);
> >>> +}
> >>
> >> TBH I'm not entirely convinced that saving 4 characters per invocation
> >> times 11 invocations makes this wrapper worthwhile :/
> >
> > Let's keep them. After the clean-up that you suggested, there are
> > fewer functions left in this file, but I think that it is cleaner to
> > keep these remaining, as it is beneficial to easily distinguish when
> > exactly one page is allocated vs when multiple are allocated via code
> > search.
>
> But is it, really? It's not at all obvious to me *why* it would be
> significantly interesting to distinguish fixed order-0 allocations from
> higher-order or variable-order (which may still be 0) ones. After all,
> there's no regular alloc_page_node() wrapper, yet plenty more callers of
> alloc_pages_node(..., 0) :/

The pages that are allocated with order > 0 cannot be freed using
iommu_put_pages_list(), without messing up refcounts in the tail
pages. I think having a dedicated function that guarantees order = 0
pages allocation makes it easier for the reviewer to follow the code,
and ensures that only these pages are put on the freelist.

Even in the existing code, the order=0 allocation is wrapped in the
*alloc_pgtable_page() function.

Pasha


>
> Thanks,
> Robin.