[PATCH v2 00/11] Fixup page directory freeing

Peter Zijlstra
2020-07-17T11:10:05Z

Hi All,

While fixing a silly bug on SH (patch #1), I realized that even with the
trivial patch to restore prior behaviour, page directory freeing was still
broken.

The thing is, on anything SMP, freeing page directories should observe the
exact same order as normal page freeing:

1) unhook page/directory
2) TLB invalidate
3) free page/directory

Without this any concurrent page-table walk could end up with a Use-after-Free.
This is esp. trivial for anything that has software page-table walkers
(HAVE_FAST_GUP / software TLB fill) or the hardware caches partial page-walks
(ie. caches page directories).

Even on UP this might give issues, since mmu_gather is preemptible these days.
An interrupt or preempted task accessing user pages might stumble into the free
page if the hardware caches page directories.

So I've converted everything to always observe the above order, simply so we
don't have to worry about it.

If however I've been over zealous and your arch/mmu really doesn't need this
and you're offended by this potentially superfluous code, please let me know
and I'll replace the patch with one that adds a comment describing your
rationale for why it is not needed.

v1: https://lkml.kernel.org/r/20191211120713.360281197@infradead.org