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wrote: > Document its motivation, userspace API, behaviors, and limitations. > > Reviewed-by: Jane Chu > Signed-off-by: Jiaqi Yan > --- > Documentation/userspace-api/index.rst | 1 + > .../userspace-api/mfd_mfr_policy.rst | 60 +++++++++++++++++++ > 2 files changed, 61 insertions(+) > create mode 100644 Documentation/userspace-api/mfd_mfr_policy.rst > > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst > index 8a61ac4c1bf19..6d8d94028a6cd 100644 > --- a/Documentation/userspace-api/index.rst > +++ b/Documentation/userspace-api/index.rst > @@ -68,6 +68,7 @@ Everything else > futex2 > perf_ring_buffer > ntsync > + mfd_mfr_policy > > .. only:: subproject and html > > diff --git a/Documentation/userspace-api/mfd_mfr_policy.rst b/Documentation/userspace-api/mfd_mfr_policy.rst > new file mode 100644 > index 0000000000000..c5a25df39791a > --- /dev/null > +++ b/Documentation/userspace-api/mfd_mfr_policy.rst > @@ -0,0 +1,60 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +================================================== > +Userspace Memory Failure Recovery Policy via memfd > +================================================== > + > +:Author: > + Jiaqi Yan > + > + > +Motivation > +========== > + > +When a userspace process is able to recover from memory failures (MF) > +caused by uncorrected memory error (UE) in the DIMM, especially when it is > +able to avoid consuming known UEs, keeping the memory page mapped and > +accessible is benifical to the owning process for a couple of reasons: > + > +- The memory pages affected by UE have a large smallest granularity, for > + example 1G hugepage, but the actual corrupted amount of the page is only > + several cachlines. Losing the entire hugepage of data is unacceptable to > + the application. > + > +- In addition to keeping the data accessible, the application still wants > + to access with a large page size for the fastest virtual-to-physical > + translations. > + > +Memory failure recovery for 1G or larger HugeTLB is a good example. With > +memfd userspace process can control whether the kernel hard offlines its > +hugepages that backs the in-RAM file created by memfd. > + > + > +User API > +======== > + > +``int memfd_create(const char *name, unsigned int flags)`` > + > +``MFD_MF_KEEP_UE_MAPPED`` > + > + When ``MFD_MF_KEEP_UE_MAPPED`` bit is set in ``flags``, MF recovery > + in the kernel does not hard offline memory due to UE until the > + returned ``memfd`` is released. IOW, the HWPoison-ed memory remains > + accessible via the returned ``memfd`` or the memory mapping created > + with the returned ``memfd``. Note the affected memory will be > + immediately isolated and prevented from future use once the memfd > + is closed. By default ``MFD_MF_KEEP_UE_MAPPED`` is not set, and > + kernel hard offlines memory having UEs. > + > +Notes about the behavior and limitations > + > +- Even if the page affected by UE is kept, a portion of the (huge)page is > + already lost due to hardware corruption, and the size of the portion > + is the smallest page size that kernel uses to manages memory on the > + architecture, i.e. PAGESIZE. Accessing a virtual address within any of > + these parts results in a SIGBUS; accessing virtual address outside these > + parts are good until it is corrupted by new memory error. > + > +- ``MFD_MF_KEEP_UE_MAPPED`` currently only works for HugeTLB, so > + ``MFD_HUGETLB`` must also be set when setting ``MFD_MF_KEEP_UE_MAPPED``. > + Otherwise ``memfd_create`` returns EINVAL. Reviewed-by: William Roche