[PATCH v4 0/7] mm/mprotect: Fix dax puds

Peter Xu
2024-08-07T19:48:18Z

[Based on mm-unstable, commit 98808d08fc0f, Aug 7th]

v4:
- Added tags
- Dropped patch "mm/mprotect: Remove NUMA_HUGE_PTE_UPDATES" [DavidH]
- Touched up comment in pgtable_split_needed() [James]

v1: https://lore.kernel.org/r/20240621142504.1940209-1-peterx@redhat.com
v2: https://lore.kernel.org/r/20240703212918.2417843-1-peterx@redhat.com
v3: https://lore.kernel.org/r/20240715192142.3241557-1-peterx@redhat.com

Dax supports pud pages for a while, but mprotect on puds was missing since
the start. This series tries to fix that by providing pud handling in
mprotect(). The goal is to add more types of pud mappings like hugetlb or
pfnmaps. This series paves way for it by fixing known pud entries.

Considering nobody reported this until when I looked at those other types
of pud mappings, I am thinking maybe it doesn't need to be a fix for stable
and this may not need to be backported. I would guess whoever cares about
mprotect() won't care 1G dax puds yet, vice versa. I hope fixing that in
new kernels would be fine, but I'm open to suggestions.

There're a few small things changed to teach mprotect work on PUDs. E.g. it
will need to start with dropping NUMA_HUGE_PTE_UPDATES which may stop
making sense when there can be more than one type of huge pte. OTOH, we'll
also need to push the mmu notifiers from pmd to pud layers, which might
need some attention but so far I think it's safe. For such details, please
refer to each patch's commit message.

The mprotect() pud process should be straightforward, as I kept it as
simple as possible. There's no NUMA handled as dax simply doesn't support
that. There's also no userfault involvements as file memory (even if work
with userfault-wp async mode) will need to split a pud, so pud entry
doesn't need to yet know userfault's existance (but hugetlb entries will;
that's also for later).

Tests
=====

What I did test:

- cross-build tests that I normally cover [1]

- smoke tested on x86_64 the simplest program [2] on dev_dax 1G PUD
mprotect() using QEMU's nvdimm emulations [3] and ndctl to create
namespaces with proper alignments, which used to throw "bad pud" but now
it'll run through all fine. I checked sigbus happens if with illegal
access on protected puds.

- vmtests.

What I didn't test:

- fsdax: I wanted to also give it a shot, but only until then I noticed it
doesn't seem to be supported (according to dax_iomap_fault(), which will
always fallback on PUD_ORDER). I did remember it was supported before, I
could miss something important there.. please shoot if so.

- userfault wp-async: I also wanted to test userfault-wp async be able to
split huge puds (here it's simply a clear_pud.. though), but it won't
work for devdax anyway due to not allowed to do smaller than 1G faults in
this case. So skip too.

- Power, as no hardware on hand.

Thanks,

[1] https://gitlab.com/peterx/lkb-harness/-/blob/main/config.json
[2] https://github.com/xzpeter/clibs/blob/master/misc/dax.c
[3] https://github.com/qemu/qemu/blob/master/docs/nvdimm.txt

Peter Xu (7):
mm/dax: Dump start address in fault handler
mm/mprotect: Push mmu notifier to PUDs
mm/powerpc: Add missing pud helpers
mm/x86: Make pud_leaf() only care about PSE bit
mm/x86: arch_check_zapped_pud()
mm/x86: Add missing pud helpers
mm/mprotect: fix dax pud handlings

arch/powerpc/include/asm/book3s/64/pgtable.h | 3 +
arch/powerpc/mm/book3s64/pgtable.c | 20 ++++++
arch/x86/include/asm/pgtable.h | 68 ++++++++++++++++---
arch/x86/mm/pgtable.c | 19 ++++++
drivers/dax/device.c | 6 +-
include/linux/huge_mm.h | 24 +++++++
include/linux/pgtable.h | 7 ++
mm/huge_memory.c | 56 ++++++++++++++-
mm/mprotect.c | 71 +++++++++++++-------
9 files changed, 236 insertions(+), 38 deletions(-)

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2.45.0