Hello, On Mon, Jun 22, 2020 at 04:30:41PM +0100, Robin Murphy wrote: > On 2020-06-22 13:46, Joerg Roedel wrote: > > + Robin > > > > Robin, any idea on this? > > After a bit of archaeology, this dates back to the original review: > > https://lore.kernel.org/linux-arm-kernel/54C285D4.3070802@xxxxxxx/ > https://lore.kernel.org/linux-arm-kernel/54DA2666.9030003@xxxxxxx/ > > In summary: originally this inherited from other arch code that did > simply strip __GFP_COMP; that was deemed questionable because of the > nonsensical comment about CONFIG_HUGETLBFS that was stuck to it; the > current code is like it is because in 5 and a half years nobody said > that it's wrong :) > > If there actually *are* good reasons for stripping __GFP_COMP, then I've > certainly no objection to doing so. The main question is if there's any good reasons for not forbidding __GFP_COMP to be specified in the callers. The reason given in the comment isn't convincing. I don't see how a caller that gets a pointer can care about how the page structure looks like and in turn why it's asking for __GFP_COMP. As far as I can tell there are two orthogonal issues in play here: 1) The comment about __GFP_COMP facilitating the sound driver to do partial mapping doesn't make much sense. It's probably best to WARN_ON immediately in dma_alloc_coherent if __GFP_COMP is specified, not only down the call stack in the __iommu_dma_alloc_pages() path. Note: the CMA paths would already ignore __GFP_COMP if it's specified so that __GFP_COMP request can already be ignored. It sounds preferable to warn the caller it's asking something it can't get, than to silently ignore __GFP_COMP. On a side note: hugetlbfs/THP pages can only be allocated with __GFP_COMP because for example put_page() must work on all tail pages (you can't call compound_head() unless the tail page is part of a compound page). But for private driver pages mapped by remap_pfn_range, any full or partial mapping is done manually and nobody can call GUP on VM_PFNMAP|VM_IO anyway (there's not even the requirement of a page struct backing those mappings in fact). 2) __iommu_dma_alloc_pages cannot use __GFP_COMP if it intends to return an array of small pages, which is the only thing that the current sg_alloc_table_from_pages() supports in input. split_page will work as expected to generate small pages from non-compound order>0 pages, incidentally it's implement on mm/page_alloc.c, not in huge_memory.c. split_huge_page as opposed is not intended to be used on newly allocated compound page. Maybe we should renamed it to split_trans_huge_page to make it more explicit, since it won't even work on hugetlbfs (compound) pages. Thanks, Andrea
