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ANDROID: 16K: x86_64: Remove /dev/userfaultfd

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When emulating 16KB page size on x86_64, the kernel presents a 16KB page
size to userspace. However the kernel and hardware still operates at a
4KB page size granularity.

This mean that even though the mimumun size of memory that userspace can
request is 16384 (from mmap); faults still happen at a 4KB granularity
in the kernel.

This is inherently incompatible with UFFD, which is used by the default
ART GC in Android V. Since UFFD necessarily needs to operate on
PTEs (4096) but from the client's perspective in userspace the page size
is 16384.

Don't create /dev/userfaultfd if emulating 16KB (larger than 4KB) page
sizes on x86_64.

Bug: 409631148
Bug: 379001861
Bug: 384985178
Bug: 377361489
Test: atest vts_linux_kselftest_x86_64:mm_uffd_unit_tests_x86_64#mm_uffd_unit_tests_x86_64 -- --abi x86_64
Change-Id: Idce473108e2275c806eba3815a54f7753e0a1352
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
This commit is contained in:
Kalesh Singh
2025-05-19 19:49:51 -07:00
parent 0dac95bbe5
commit ed212f01bf
1 changed files with 3 additions and 0 deletions
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3
fs/userfaultfd.c
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@@ -2336,6 +2336,9 @@ static int __init userfaultfd_init(void)
{
int ret;
if (__PAGE_SIZE != PAGE_SIZE)
return 0;
ret = misc_register(&userfaultfd_misc);
if (ret)
return ret;