Arnd BergmannJul 23, 2024

I had a feeling that kernel compilation got slower recently and tried to find the slowest file across randconfig builds. It turned out to be arch/x86/xen/setup.c, which takes 15 seconds to preprocess on a reasonably fast Apple M1 Ultra.

This all comes from one line "extra_pages = min3(EXTRA_MEM_RATIO * min(max_pfn, PFN_DOWN(MAXMEM)), extra_pages, max_pages - max_pfn);" that expands to 47MB of preprocessor output after commits 80fcac55385c..867046cc70277.

Show threadKeyJJul 23, 2024
@arnd But ... how? Even if it expanded to 47 kilobytes, that would be excessive, but 47 *megabytes*? 🤯
Show threadArnd BergmannJul 23, 2024

@KeyJ it nests min() multiple levels deep with the use of min3(), and each one expands its argument 20 times times now (up from 6 back in linux-6.6). This gets 8000 expansions for each of the arguments, plus a lot of extra bits with each expansion. PFN_DOWN(MAXMEM) contributes a bit to the initial size as well.

See https://pastebin.com/MmfWH7TM for the first few pages of it.

unsigned long extra_pages = 0; extra_pages = __builtin_choose_expr((sizeof(in - Pastebin.com

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Show threadAstraLuma
@arnd @KeyJ that reads like XML entity attack
Jul 24, 2024 at 1:27amWeb