# git rev-parse -q --verify 80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6^{commit}
80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6
already have revision, skipping fetch
# git checkout -q -f -B kisskb 80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6
# git clean -qxdf
# < git log -1
# commit 80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Thu Feb 17 08:57:47 2022 -0800
# 
#     mm: don't try to NUMA-migrate COW pages that have other uses
#     
#     Oded Gabbay reports that enabling NUMA balancing causes corruption with
#     his Gaudi accelerator test load:
#     
#      "All the details are in the bug, but the bottom line is that somehow,
#       this patch causes corruption when the numa balancing feature is
#       enabled AND we don't use process affinity AND we use GUP to pin pages
#       so our accelerator can DMA to/from system memory.
#     
#       Either disabling numa balancing, using process affinity to bind to
#       specific numa-node or reverting this patch causes the bug to
#       disappear"
#     
#     and Oded bisected the issue to commit 09854ba94c6a ("mm: do_wp_page()
#     simplification").
#     
#     Now, the NUMA balancing shouldn't actually be changing the writability
#     of a page, and as such shouldn't matter for COW.  But it appears it
#     does.  Suspicious.
#     
#     However, regardless of that, the condition for enabling NUMA faults in
#     change_pte_range() is nonsensical.  It uses "page_mapcount(page)" to
#     decide if a COW page should be NUMA-protected or not, and that makes
#     absolutely no sense.
#     
#     The number of mappings a page has is irrelevant: not only does GUP get a
#     reference to a page as in Oded's case, but the other mappings migth be
#     paged out and the only reference to them would be in the page count.
#     
#     Since we should never try to NUMA-balance a page that we can't move
#     anyway due to other references, just fix the code to use 'page_count()'.
#     Oded confirms that that fixes his issue.
#     
#     Now, this does imply that something in NUMA balancing ends up changing
#     page protections (other than the obvious one of making the page
#     inaccessible to get the NUMA faulting information).  Otherwise the COW
#     simplification wouldn't matter - since doing the GUP on the page would
#     make sure it's writable.
#     
#     The cause of that permission change would be good to figure out too,
#     since it clearly results in spurious COW events - but fixing the
#     nonsensical test that just happened to work before is obviously the
#     CorrectThing(tm) to do regardless.
#     
#     Fixes: 09854ba94c6a ("mm: do_wp_page() simplification")
#     Link: https://bugzilla.kernel.org/show_bug.cgi?id=215616
#     Link: https://lore.kernel.org/all/CAFCwf10eNmwq2wD71xjUhqkvv5+_pJMR1nPug2RqNDcFT4H86Q@mail.gmail.com/
#     Reported-and-tested-by: Oded Gabbay <oded.gabbay@gmail.com>
#     Cc: David Hildenbrand <david@redhat.com>
#     Cc: Peter Xu <peterx@redhat.com>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-ld --version
# < git log --format=%s --max-count=1 80d47f5de5e311cbc0d01ebb6ee684e8f4c196c6
# < make -s -j 32 ARCH=xtensa O=/kisskb/build/linus_xtensa-allnoconfig_xtensa-gcc11 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-  allnoconfig
# < make -s -j 32 ARCH=xtensa O=/kisskb/build/linus_xtensa-allnoconfig_xtensa-gcc11 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-  help
# make -s -j 32 ARCH=xtensa O=/kisskb/build/linus_xtensa-allnoconfig_xtensa-gcc11 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-  olddefconfig
# make -s -j 32 ARCH=xtensa O=/kisskb/build/linus_xtensa-allnoconfig_xtensa-gcc11 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-11.1.0-nolibc/xtensa-linux/bin/xtensa-linux-  
Completed OK
# rm -rf /kisskb/build/linus_xtensa-allnoconfig_xtensa-gcc11
# Build took: 0:00:18.821344