On Tue, Aug 01, 2023 at 02:48:37PM +0200, David Hildenbrand wrote:
Unfortunately commit 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") missed that follow_page() and follow_trans_huge_pmd() never implicitly set FOLL_NUMA because they really don't want to fail on PROT_NONE-mapped pages -- either due to NUMA hinting or due to inaccessible (PROT_NONE) VMAs.
As spelled out in commit 0b9d705297b2 ("mm: numa: Support NUMA hinting page faults from gup/gup_fast"): "Other follow_page callers like KSM should not use FOLL_NUMA, or they would fail to get the pages if they use follow_page instead of get_user_pages."
liubo reported [1] that smaps_rollup results are imprecise, because they miss accounting of pages that are mapped PROT_NONE. Further, it's easy to reproduce that KSM no longer works on inaccessible VMAs on x86-64, because pte_protnone()/pmd_protnone() also indictaes "true" in inaccessible VMAs, and follow_page() refuses to return such pages right now.
As KVM really depends on these NUMA hinting faults, removing the pte_protnone()/pmd_protnone() handling in GUP code completely is not really an option.
To fix the issues at hand, let's revive FOLL_NUMA as FOLL_HONOR_NUMA_FAULT to restore the original behavior for now and add better comments.
Set FOLL_HONOR_NUMA_FAULT independent of FOLL_FORCE in is_valid_gup_args(), to add that flag for all external GUP users.
Note that there are three GUP-internal __get_user_pages() users that don't end up calling is_valid_gup_args() and consequently won't get FOLL_HONOR_NUMA_FAULT set.
- get_dump_page(): we really don't want to handle NUMA hinting faults. It specifies FOLL_FORCE and wouldn't have honored NUMA hinting faults already.
- populate_vma_page_range(): we really don't want to handle NUMA hinting faults. It specifies FOLL_FORCE on accessible VMAs, so it wouldn't have honored NUMA hinting faults already.
- faultin_vma_page_range(): we similarly don't want to handle NUMA hinting faults.
To make the combination of FOLL_FORCE and FOLL_HONOR_NUMA_FAULT work in inaccessible VMAs properly, we have to perform VMA accessibility checks in gup_can_follow_protnone().
As GUP-fast should reject such pages either way in pte_access_permitted()/pmd_access_permitted() -- for example on x86-64 and arm64 that both implement pte_protnone() -- let's just always fallback to ordinary GUP when stumbling over pte_protnone()/pmd_protnone().
As Linus notes [2], honoring NUMA faults might only make sense for selected GUP users.
So we should really see if we can instead let relevant GUP callers specify it manually, and not trigger NUMA hinting faults from GUP as default. Prepare for that by making FOLL_HONOR_NUMA_FAULT an external GUP flag and adding appropriate documenation.
[1] https://lore.kernel.org/r/20230726073409.631838-1-liubo254@huawei.com [2] https://lore.kernel.org/r/CAHk-=wgRiP_9X0rRdZKT8nhemZGNateMtb366t37d8-x7VRs=...
Reported-by: liubo liubo254@huawei.com Closes: https://lore.kernel.org/r/20230726073409.631838-1-liubo254@huawei.com Reported-by: Peter Xu peterx@redhat.com Closes: https://lore.kernel.org/all/ZMKJjDaqZ7FW0jfe@x1n/ Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") Cc: stable@vger.kernel.org Signed-off-by: David Hildenbrand david@redhat.com
I agree that FOLL_REMOTE probably needs separate treatment but also agree that it's outside the context of this patch, particularly as a -stable candidate so
Acked-by: Mel Gorman mgorman@techsingularity.net
I've a minor nit below that would be nice to get fixed up, but not mandatory.
include/linux/mm.h | 21 +++++++++++++++------ include/linux/mm_types.h | 9 +++++++++ mm/gup.c | 29 +++++++++++++++++++++++------ mm/huge_memory.c | 2 +- 4 files changed, 48 insertions(+), 13 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c index 2493ffa10f4b..f463d3004ddc 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2240,6 +2244,12 @@ static bool is_valid_gup_args(struct page **pages, int *locked, gup_flags |= FOLL_UNLOCKABLE; }
- /*
* For now, always trigger NUMA hinting faults. Some GUP users like* KVM really require it to benefit from autonuma.*/- gup_flags |= FOLL_HONOR_NUMA_FAULT;
- /* FOLL_GET and FOLL_PIN are mutually exclusive. */ if (WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) == (FOLL_PIN | FOLL_GET)))
Expand on *why* KVM requires it even though I suspect this changes later in the series. Maybe "Some GUP users like KVM require the hint to be as the calling context of GUP is functionally similar to a memory reference from task context"?
Also minor nit -- s/autonuma/NUMA Balancing/ or numab. autonuma refers to a specific implementation of automatic balancing that operated similar to khugepaged but not merged. If you grep for it, you'll find that autonuma is only referenced in powerpc-specific code. It's not important these days but very early on, it was very confusing if AutoNUMA was mentioned when NUMAB was intended.