Messages in this thread | | | Date | Mon, 6 Dec 2021 13:28:11 +0000 | Subject | Re: [PATCH 4/9] iommu/amd: Simplify pagetable freeing | From | Robin Murphy <> |
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On 2021-12-06 12:40, Joerg Roedel wrote: > On Tue, Nov 23, 2021 at 02:10:39PM +0000, Robin Murphy wrote: >> For reasons unclear, pagetable freeing is an effectively recursive >> method implemented via an elaborate system of templated functions that >> turns out to account for 25% of the object file size. Implementing it >> using regular straightforward recursion makes the code simpler, and >> seems like a good thing to do before we work on it further. As part of >> that, also fix the types to avoid all the needless casting back and >> forth which just gets in the way. > > Nice cleanup! The stack of functions came from the fact that recursion > was pretty much discouraged in the kernel. But in this case it looks > well bounded and should be fine.
I did wonder about explicitly clamping lvl to ensure that it couldn't possibly recurse any further than the multi-function version, but given that you'd need to craft a suitable bogus pagetable in addition to corrupting the arguments to be able to exploit it at all, that seemed perhaps a little too paranoid. Happy to add something like:
if (WARN_ON(lvl > PAGE_MODE_7_LEVEL)) return NULL;
if you like, though.
>> +static struct page *free_pt_lvl(u64 *pt, struct page *freelist, int lvl) >> +{ >> + u64 *p; >> + int i; >> + >> + for (i = 0; i < 512; ++i) { >> + /* PTE present? */ >> + if (!IOMMU_PTE_PRESENT(pt[i])) >> + continue; >> + >> + /* Large PTE? */ >> + if (PM_PTE_LEVEL(pt[i]) == 0 || >> + PM_PTE_LEVEL(pt[i]) == 7) >> + continue; >> + >> + p = IOMMU_PTE_PAGE(pt[i]); >> + if (lvl > 2) > > I thinkt this function deserves a couple of comments. It took me a while > to make sense of the 'lvl > 2' comparision. I think it is right, but if > I have think again I'd appreciate a comment :)
Heh, it's merely a direct transformation of the logic encoded in the existing "DEFINE_FREE_PT_FN(...)" cases - I assume that's just an optimisation, so I'll add a comment to that effect.
Thanks, Robin.
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