mm/hugetlb: fix hugetlb_pmd_shared()

Patch series "mm/hugetlb: fixes for PMD table sharing (incl.  using
mmu_gather)".

One functional fix, one performance regression fix, and two related
comment fixes.

I cleaned up my prototype I recently shared [1] for the performance fix,
deferring most of the cleanups I had in the prototype to a later point. 
While doing that I identified the other things.

The goal of this patch set is to be backported to stable trees "fairly"
easily.  At least patch #1 and #4.

Patch #1 fixes hugetlb_pmd_shared() not detecting any sharing
Patch #2 + #3 are simple comment fixes that patch #4 interacts with.
Patch #4 is a fix for the reported performance regression due to excessive
IPI broadcasts during fork()+exit().

The last patch is all about TLB flushes, IPIs and mmu_gather.  Read:
complicated

I added as much comments + description that I possibly could, and I am
hoping for review from Jann.

There are plenty of cleanups in the future to be had + one reasonable
optimization on x86.  But that's all out of scope for this series.


This patch (of 4):

We switched from (wrongly) using the page count to an independent shared
count.  Now, shared page tables have a refcount of 1 (excluding
speculative references) and instead use ptdesc->pt_share_count to identify
sharing.

We didn't convert hugetlb_pmd_shared(), so right now, we would never
detect a shared PMD table as such, because sharing/unsharing no longer
touches the refcount of a PMD table.

Page migration, like mbind() or migrate_pages() would allow for migrating
folios mapped into such shared PMD tables, even though the folios are not
exclusive.  In smaps we would account them as "private" although they are
"shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the
pagemap interface.

Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared().

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/all/[email protected]/ [1]
Fixes: 59d9094 ("mm: hugetlb: independent PMD page table shared count")
Signed-off-by: David Hildenbrand (Red Hat) <[email protected]>
Tested-by: Laurence Oberman <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Reviewed-by: Lance Yang <[email protected]>
Cc: Liu Shixin <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Prakash Sangappa <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Uschakow, Stanislav" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: David Hildenbrand (Red Hat)

include/linux/hugetlb.h

@@ -1326,7 +1326,7 @@ static inline __init void hugetlb_cma_reserve(int order)
 #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
 static inline bool hugetlb_pmd_shared(pte_t *pte)
 {
-	return page_count(virt_to_page(pte)) > 1;
+	return ptdesc_pmd_is_shared(virt_to_ptdesc(pte));
 }
 #else
 static inline bool hugetlb_pmd_shared(pte_t *pte)