Re: [PATCH mm-unstable RFC 00/26] mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all architectures with swap PTEs

David Hildenbrand
2022-12-14T11:23:16Z

On 06.12.22 15:47, David Hildenbrand wrote:
> This is the follow-up on [1]:
> [PATCH v2 0/8] mm: COW fixes part 3: reliable GUP R/W FOLL_GET of
> anonymous pages
>
> After we implemented __HAVE_ARCH_PTE_SWP_EXCLUSIVE on most prominent
> enterprise architectures, implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all
> remaining architectures that support swap PTEs.
>
> This makes sure that exclusive anonymous pages will stay exclusive, even
> after they were swapped out -- for example, making GUP R/W FOLL_GET of
> anonymous pages reliable. Details can be found in [1].
>
> This primarily fixes remaining known O_DIRECT memory corruptions that can
> happen on concurrent swapout, whereby we can lose DMA reads to a page
> (modifying the user page by writing to it).
>
> To verify, there are two test cases (requiring swap space, obviously):
> (1) The O_DIRECT+swapout test case [2] from Andrea. This test case tries
> triggering a race condition.
> (2) My vmsplice() test case [3] that tries to detect if the exclusive
> marker was lost during swapout, not relying on a race condition.
>
>
> For example, on 32bit x86 (with and without PAE), my test case fails
> without these patches:
> $ ./test_swp_exclusive
> FAIL: page was replaced during COW
> But succeeds with these patches:
> $ ./test_swp_exclusive
> PASS: page was not replaced during COW
>
>
> Why implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE for all architectures, even
> the ones where swap support might be in a questionable state? This is the
> first step towards removing "readable_exclusive" migration entries, and
> instead using pte_swp_exclusive() also with (readable) migration entries
> instead (as suggested by Peter). The only missing piece for that is
> supporting pmd_swp_exclusive() on relevant architectures with THP
> migration support.
>
> As all relevant architectures now implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE,,
> we can drop __HAVE_ARCH_PTE_SWP_EXCLUSIVE in the last patch.
>
>
> RFC because some of the swap PTE layouts are really tricky and I really
> need some feedback related to deciphering these layouts and "using yet
> unused PTE bits in swap PTEs". I tried cross-compiling all relevant setups
> (phew, I might only miss some power/nohash variants), but only tested on
> x86 so far.

As I was messing with sparc64 either way and got debian to boot under
QEMU, I verified that the sparc64 change also seems to work as expected
(under sun4u).

--
Thanks,

David / dhildenb