Shu Wang opened bugzilla 211287 [1] saying,
When a memory region is mapped with huge pages, the softdirty bit is set
only right after the huge pages is mapped. After the memory mapping, if
the softdirty bit is cleared and the memory is written again, the softdirty
bit is not set when reading from the process's pagemap.
Their use case for hugetlb soft dirty is,
We maps both PMEM and hugetlb DRAM to process’s address space to provide
large amount of memory to the application. Periodically, we clear the soft
dirty bit by writing ‘4’ to the clear_refs. Then, we check the page’s soft
dirty bit from pagemap and use this information to track write activity to
memory. We migrate the data between DRAM and PMEM based the write activity
for better performance.
A quick check of the code revealed,
The /proc clear_refs code handles THP pages and 'normal' pages. There is
no check/processing for hugetlb pages. All the code will do is clear the
VM_SOFTDIRTY vma flag and vma_set_page_prot. None of the individual ptes
or page flags are touched.
The only soft dirty checking done by the /proc pagemap code is at the vma
level. It checks the VM_SOFTDIRTY flag. It does not look for soft dirty
on the individual ptes.
That explains the behavior described in the bugzilla report.
This series adds soft dirty support for hugetlb. It has only passed
some very basic testing and likely needs more work. However, Axel
Rasmussen's "userfaultfd: add minor fault handling" and Peter Xu's
"userfaultfd-wp: Support shmem and hugetlbfs" work touch the same areas
and have similiar issues. Therefore, code is being sent earlier than
normal so that efforts in common areas can be coordinated.
[1] https://bugzilla.kernel.org/show_bug.cgi?id=211287
Mike Kravetz (5):
hugetlb: add hugetlb helpers for soft dirty support
hugetlb: enhance hugetlb fault processing to support soft dirty
mm proc/task_mmu.c: add soft dirty pte checks for hugetlb
hugetlb: don't permit pmd sharing if soft dirty in use
mm proc/task_mmu.c: add hugetlb specific routine for clear_refs
arch/s390/include/asm/hugetlb.h | 30 +++++++++
fs/proc/task_mmu.c | 114 ++++++++++++++++++++++++++++++++
include/asm-generic/hugetlb.h | 30 +++++++++
include/linux/hugetlb.h | 1 +
mm/hugetlb.c | 40 ++++++-----
5 files changed, 199 insertions(+), 16 deletions(-)
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2.29.2